July 13, 2024

Gas Engines: Parts, Operation, Maintenance

The gas engine is a machine, in which combustion of the fuel takes place in a confined space, producing expanding gases that are used directly to provide mechanical power.


Paragraph                                                                                                                                Page


1.0     OBJECTIVES.                                                                                                                  2

2.0     INTRODUCTION.                                                                                                             3

3.0     BASIC THEORY OF GAS  ENGINE.                                                                               3

4.0     WORKING CYCLE.                                                                                                          5

5.0     MAIN COMPONENTS OF GAS ENGINES.                                                                  11

6.0     FUEL SUPPLY SYSTEM                                                                                               19

7.0      IGNITION SYSTEM                                                                                                        27

8.0     LUBRICATION SYSTEM.                                                                                              29

9.0     STARTING SYSTEM.                                                                                                    31

10.0     GOVERNING AND SPEED CONTROL.                                                                       34

11.0     COOLING SYSTEM.                                                                                                      35

12.0     AIR INTAKE SYSTEM.                                                                                                   37

13.0     EXHAUST SYSTEM.                                                                                                      38

14.0     ALARM AND SHUTDOWN SYSTEM.                                                                           38

15.0     ENGINE TUNE-UP                                                                                                         39

16.0     SERVICING OF INTERNAL COMBUSTION ENGINES                                               42

17.0     INSPECTION AND ROUTINE MAINTENANCE.                                                          47

18.0     OPERATION.                                                                                                                 48

19.0     TROUBLE SHOOTING.                                                                                                 51



1.0       OBJECTIVES.

The trainee will be able to:

  • State the definition of an engine, internal combustion engine and gas engine.
  • Demonstrate an understanding of the basic gas engine theory.
  • Demonstrate an understanding of two and four cycle gas engine.
  • Identify the main components of a gas engine and state its functions.
  • Demonstrate an understanding of Fuel supply system.
  • Demonstrate an understanding of starting system.
  • Demonstrate an understanding of governing and speed control system.
  • Demonstrate an understanding of lubrication system.
  • Demonstrate an understanding of engine cooling system.
  • Demonstrate an understanding of fuel supply system.
  • Demonstrate an understanding of air intake system.
  • Demonstrate an understanding of exhaust system.
  • Demonstrate an understanding of alarm and shutdown system.
  • Demonstrate a basic understanding of gas engine operation.
  • Demonstrate a sound understanding of inspection and maintenance of gas engines.
  • Acquire generic knowledge of trouble shooting a gas engine.


  •             The gas engine is a machine, in which combustion of the fuel takes place in a confined space, producing expanding gases that are used directly to provide mechanical power.

Such engines are classified as reciprocating or rotary, spark ignition or compression ignition, and two-stroke or four-stroke. The most familiar combination, used from large industrial engines to lawn mowers, is the reciprocating, spark-ignited, four-stroke gasoline engine and where such fuels are available natural gas driven spark ignition engine. Other types of internal-combustion engines include the reaction engine such as jet propulsion, rocket and the gas turbine.

Engines are rated by their maximum horsepower, which is usually reached a little below the speed at which undue mechanical stresses are developed.

In this module, we will learn the basic gas combustion theory, its components, auxiliary systems, operation, maintenance and trouble shooting.



In order to understand how the gas engines work, we have to first know the fuels used in a gas engine. In addition, we also have to know how they are built. In the next few paragraphs we will try and learn about these.

The most commonly used fuels in internal combustion engines are natural gas, gasoline, light oil fuels, and heavy oil fuels. Propane is increasingly used as a fuel for internal combustion engines, but mainly in automobiles rather than for stationary power generation.

Natural Gas: Engines burning this fuel are used in locations where natural gas is plentiful such as gas producing fields. They are generally used as the drive units for gas compression machinery. These engines work on a two-stroke or four-stroke cycle with ignition supplied by a hot ignition tube or electric spark.

Gasoline: Most commonly found fuel in automobiles and in industry. They can be either two stroke or four stroke type and are high speed engines. The fuel is in liquid form and is vaporized by being drawn through fine jets by the powerful suction of the engine during the intake stroke and ignited by spark. Governing takes place by throttling the resulting air/fuel mixture.

Light Oil Fuel: Engines using light oils such as kerosene are not much in use today. The cycle used may be two-stroke or four-stroke. The fuel is sprayed into a vaporizer heated by the exhaust gases and then drawn into the cylinder together with air. Governing is by throttling of this air/fuel mixture. However refined kerosene is used as aviation fuel in jet engines

Heavy Oil Fuel: This is an important type of modem high compression engine. It may be low speed or high speed and it may be two-stroke cycle or four-stroke cycle. Compression is carried out on a charge of air and then fuel is injected in atomized liquid form near the end of the compression stroke. Ignition of the fuel occurs by virtue of the high temperature developed during the compression of the air. Hence the name “compression ignition engine”. Governing is carried out by varying the fuel quantity injected at each stroke, reduced fuel quantity giving reduced engine output.


Thermodynamics of Combustion Process

Figure 2.0 shows the p-V diagram of Otto Thermodynamic Cycle which is used in all spark ignited internal combustion engines.


Figure: 1.0  p-V diagram. Otto Thermodynamic Cycle

Using the engine stage numbering system, we begin at the lower left with Stage 1 being the beginning of the intake stroke of the engine. The pressure is near atmospheric pressure and the gas volume is at a minimum.

Between Stage 1 and Stage 2 the piston is pulled out of the cylinder with the intake valve open. The pressure remains constant, and the gas volume increases as fuel/air mixture is drawn into the cylinder through the intake valve.

At Stage 2 begins the compression stroke of the engine with the closing of the intake valve. Between Stage 2 and Stage 3, the piston moves back into the cylinder, the gas volume decreases, and the pressure increases because of work done on the gas by the piston.

Stage 3 is the beginning of the combustion of the fuel/air mixture. The combustion occurs very quickly and the volume remains constant. Heat is released during combustion which increases both the temperature and the pressure.

Stage 4 begins the power stroke of the engine. Between Stage 4 and Stage 5, the piston is driven towards the crankshaft, the volume is increased, and the pressure falls as work is done by the combustion gas on the piston.

At Stage 5 the exhaust valve is opened and the residual heat in the gas is exchanged with the surroundings. The volume remains constant and the pressure adjusts back to atmospheric conditions.

Stage 6 begins the exhaust stroke of the engine during which the piston moves back into the cylinder, the volume decreases and the pressure remains constant. At the end of the exhaust stroke, conditions have returned to Stage 1 and the process repeats itself.


Combustion Process

Gasoline and the natural gas fuel is mainly a mixture of hydrocarbons in liquid or gas form. A hydrocarbon is a chemical compound composed of hydrogen and carbon. When hydrogen combines with oxygen the following reaction takes place:

2H2 + O2 = 2 H2O

Carbon burns with Oxygen in two proportions. First it burns with available oxygen and forms carbon monoxide. If plenty of oxygen is available complete combustion takes place according to following equations:

2C + O2 = 2CO

Carbon monoxide, CO, is poisonous gas. It readily combines with oxygen to form carbon dioxide COaccording to equation

2CO + O= 2CO2

If sufficient oxygen is available carbon readily combines with oxygen to form carbon dioxide CO2 according to equation

C +O2 = CO2

Another combustible but undesirable element found in fuels in small amount is sulfur. Sulfur, S, burns with oxygen to form sulfur dioxide SOas follows:

S +O2 = SO2

Sulfur dioxide is a corrosive gas in the presence of water. It is responsible for the corrosion found inside the exhaust pipes.



Internal combustion engines can be classified according to the number of strokes that constitute a working cycle. Working cycles for gas engines include two-stroke and four-stroke cycle spark ignition. We will be looking at the two stroke and four stroke spark ignition or “Otto Cycle” engines in this module.


Not all piston engines use the four‑stroke process. Many small piston engines convert energy in a two‑stroke process. These engines are called two‑stroke engines.

In two‑stroke engines, intake and compression happen during the same piston stroke. Combustion and exhaust happen during a second piston stroke. In this way, two‑stroke engines convert fuel into motion each time the piston reciprocates. With each 360° of crankshaft rotation, the engine converts energy.


Figure 2.0. Intake Phase


Inside the two‑stroke engine, the piston rises during the first stroke. As it rises, the piston compresses the fuel‑air mixture above it. The volume of the mixture is reduced. Pressure in the combustion chamber rises. This compression phase is the first part of the first stroke. (see figure 2.0).

As the piston continues to rise it Uncovers the intake port. The ports in two‑stroke engines are not located in the combustion chamber. They are located in the cylinder wall. When the piston is up, the port is uncovered. There is, therefore, no need for valves. When the intake port is uncovered, fuel mixture enters the cylinder below the piston. Now there is fuel both above and below the piston. The first stroke ends with this intake phase. The piston is now at the top of the cylinder. (see figure 3.0).


Figure 3.0.  Intake Phase – Compression

During the combustion phase the sparking plug ignites the compressed mixture. The explosion pushes the piston down. At this point, the engine produces mechanical energy. As the piston descends, it ‘ uncovers the exhaust port.‑ The exhaust gases begin to escape from the engine. At the same time, the new mixture below the piston is compressed. (see figure, 4.0).


Figure 4.0.  Combustion Phase

In the last phase, the piston continuous its‑downward movement. As the piston descends, it uncovers the transfer port. The new fuel mixture travels through the transfer port into the combustion chamber. This new mixture pushes the remaining exhaust gases through the exhaust port. This is the end of the transfer‑exhaust phase. Then the piston rises, compression takes place and the two‑stroke process begins again. (see figure 5.0).


Figure 5.0. Transfer –  Exhaust phase

Because they do not have valves or valve‑timing systems, two‑stroke engines are lighter and simpler than four‑stroke engines. Therefore, two‑stroke engines cost less to manufacture. They are normally used for small boats, motorcycles, pumps etc.

Large two‑stroke engines are not economical to operate. This is because fuel is lost in the transfer‑exhaust phase. During this phase, new mixture escapes through the exhaust port. Heat is another problem with two‑stroke engines. In these engines, combustion occurs more frequently than in four‑stroke engines. Therefore two stroke engines operate at higher temperatures. As a result, they wear, or get old with use, more quickly.


Four-Stroke Spark Ignition Cycle

The four-cycle engine, more properly called the “four-stroke cycle” engine is more efficient and used all over the oil industry. One “stroke” requires a complete movement of the piston in one direction, which is half revolution of the crankshaft. The four strokes comprising the working cycle are called:

  • Intake stroke,
  • Compression,
  • Power and
  • Exhaust

Intake stroke: is the first step in the cycle and commences when the piston is in the top most position, known as top dead centre (TDC). As the piston moves downward a vacuum is created in the cylinder above piston and the air-fuel mixture enters the cylinder from carburettor through the open intake valve. When the piston reaches the bottom of the stroke, the intake valve closes, sealing the air fuel mixture in the cylinder. See figure 6.0


Figure: 6.0 Intake Stroke


Compression stroke: Begins when the piston is at the bottom dead centre and both the intake and exhaust valves are closed. As the piston moves upward the air-fuel charge is compressed about one-fifteenth of original volume. See figure 7.0


Figure: 7.0 Compression Stroke


Power Stroke: This stroke contributes to the engine output. When the piston reaches the TDC, the air-fuel mixture is ignited by the electric spark generated at the spark plug gap. Both the valves are closed. The heat of combustion causes the burning gases to expand and force the piston to move downwards. This turns the crankshaft and produces mechanical energy. See figure 8.0


Figure: 8.0 Power Stroke

Exhaust stroke: The function of exhaust stroke is to expel the burned gases in preparation for the commencement of another cycle. The piston travels from BDC to TDC with the exhaust valve open. This stroke completes the cycle and the crankshaft has turned two revolutions by the end of exhaust stroke. The exhaust valve closes at the TDC and intake valve opens for another suction stroke.


Figure: 9.0 Exhaust Stroke

The four-cycle engine gives good fuel economy, good control at all speeds and high torque at low speeds, though it is more mechanically complicated and heavier than a two-stroke engine of the same power rating.



The design of a gas engine varies greatly due to the fuels used and operating principles but generally, they remain similar. The main components, which make up the gas engine are listed below:

  • The stationary parts:
  • Cylinders
  • Crankcase block
  • Cylinder head
  • Manifolds
  • The moving parts:
  • Pistons
  • Connecting rods
  • Crankshaft
  • Flywheel
  • Valves
  • Valve gear


In addition to these main parts, various systems are added to the engine as follows:

  • Fuel system
  • Ignition system
  • Lubrication system
  • Cooling system


Stationary Parts

The gas engine consists of the following main stationary parts the crankcase or the engine structure which holds the cylinders, crankshaft and main bearings in firm relation to each other.

This structure usually includes all the fixed parts i.e.

  1. bed plate, base or pan and frame that hold the engine together,
  2. the cylinder casting, which houses the piston and contains the action of combustion,
  • the cylinder head that encloses the top end of the cylinder so as to make a confined space in which to hold and compress the fuel-air mixture and to confine the gases while they are burning or expanding. Cylinder head carries inlet and exhaust valves, spark plug and sometimes the air starting valve.


Moving Parts

Pistons: Piston is the most important part in an engine. Generally, a cylindrical casting, closed at the top and connects to the connecting rod and through to crankshaft at the bottom. The main functions of the piston is to transform heat energy into mechanical energy and to carry off some of the excessive heat generated in the combustion chamber. Figure: 10.0 shows various parts of a piston

The high combustion heat inside the cylinder requires that sufficient clearance is maintained between the piston and cylinder to function effectively. This clearance depends upon the piston design and the operating temperature. The clearance is generally between 0.001” (low speed water cooled engine) to 0.010” (in air cooled high speed engines).


Figure: 10.0  Various parts of a piston

The materials generally used in a piston are (i) cast iron and (ii) aluminium alloy.  Aluminium alloy is the generally preferred piston material in modern engines but has a higher coefficient of expansion compared to the steel pistons.

Piston rings: Piston rings provide seal between piston and cylinder, permitting the gases to be compressed inside the cylinder. They are made of steel as well as cast iron, heat treated and plated with other metals. Two type of rings are generally used; (i) compression rings and (ii)oil control rings. Figure 11.0  shows an example of piston rings used in gas engines



Figure: 11.0  Various type of piston rings

Compression rings prevent compression loss (blow-by). They could be either (i) plain or (ii) grooved.

Oil control rings wipe off the excess oil on the cylinder walls and drain the oil back to the crank case. For this purpose the oil wiper rings are  provided with channels and slots  in the rings. In operation, the ring rides over the oil on the upstroke and scrapes oil into the groove in the down stroke of the piston.  Oil thus passes through the oil passages in the ring and then through holes in the piston. It is important that the holes in the piston and slots in the ring are clear to ensure proper drain off of excess oil from cylinder wall.

Crankshaft: The function of a crankshaft is to convert the reciprocating motion of the piston and connecting rod into rotary motion and to transmit the resulting torque to the flywheel.

Crankshafts are generally made of forged steel or cast from an alloy steel, machined, heat treated and the bearing area’s are finished by grinding.

Modern crankshafts have built in counterweights to balance the weight of piston, connecting rod, crank arm and crank pin so that assembly of moving parts will be in static equilibrium for all points of stroke.  In addition the counterweights reduce vibration due to constant acceleration and retardation during each stroke.


Figure: 12.0  Crankshaft assembly


Connecting rods and wrist pins: Wrist pins or piston pins connect the piston and the upper end of the connecting rod. They are made of alloy steel with precision finish and are case hardened and some times chromium plated to increase their wearing qualities. They form a pivot connecting one end of the connecting rod to the piston and thus allow lateral oscillating motion of the rod with reciprocating motion of the piston.


Figure: 13.0  Connecting rod assembly


Connecting rod is the connecting link between the piston and the crankshaft. It serves to transform the reciprocating motion of the piston into rotary motion at the crankshaft. Connecting rods are drop forged from a steel alloy and can withstand heavy loads without bending or twisting. The upper end of the connecting rod is connected to the piston by piston pin with a solid bearing or bushing of bronze or similar material. Lower end of the connecting rod is split to permit clamping on the crankshaft and is attached by two or more connecting rod bolts. A separate split shell bearing is provided between the connecting rod and crankshaft.

Main Bearings: Crankshaft of an engine rotates on main bearings. These bearings are located at both ends and at a few intermediate points along the crankshaft. They are lubricated with crankcase oil by pressure through drilled passages or by splash and have channels for oil lubrication.

Flywheel: The purpose of flywheel is to secure momentum necessary to keep the crankshaft turning when it is not receiving power impulses from piston. The flywheel will thus permit the engine to idle smoothly through intake, compression, exhaust strokes, and keep the engine turning at nearly uniform rate of rotation. Small engines with electrical starters, the rim of the flywheel caries a ring gear which meshes with the starter driving gear for cranking the engine at starting.

Valves and valve gear: On every four stroke engine cylinder, there must be a minimum of one intake and one exhaust valve fitted. Intake valve permits the fuel-air mixture to enter cylinder and the exhaust valve allows the spent gases to escape. These valves could be either poppet or mushroom and are generally made in one piece from special alloy steel suitable to work under very high temperatures.


Figure: 14.0 Valve, valve lifter and rocker arm assembly


Both intake and exhaust valves rest on valve seats. A minimum of one intake and exhaust valve seats are fitted in each cylinder and the intake and exhaust manifolds are connected to the valve ports. Since valve seats are subjected to intense heat, valve grinding and reseating are usually necessary from time to time to ensure proper sealing surfaces between the valve seat and the valve.

Valve stem guide guides the valve stem during the reciprocating motion. The guides may be an integral part with cylinder block or cylinder head or may be removable and replaceable sleeves when worn out.

The valve stems are ground finished to fit the guides in which they operate and the reamed hole in the guide must be aligned and square with the valve seat to ensure proper seating of the valve face.

Valve operating mechanism: Valve operating mechanism generally consists of  a cam, valve lifter assembly, retainer cup and spring.

The camshaft is a straight shaft on which eccentric lobes or cams are forged as an integral part. In multiple cylinder engines there are as many cams as there are valves to be operated.



Figure: 15.0  Camshaft and related parts


Cam transmits motion to lifter which works in a closely fitting guide to prevent any lateral motion. The lifter is also called as tappet, follower etc.

Forming part of the plunger is the clearance adjustment mechanism consisting of case hardened screw and lock nut or similar arrangement.

Valve spring exerts enough force to seat the valve firmly to ensure tightness and give sufficient acceleration in closing the valve quickly. Various type of locking devices are used to retain the spring in a partly compressed position on the valve stem.

Modern engines have overhead cams thus eliminating the push rods and rocker arms allowing higher engine RPM and softer valve springs.


Valve timing: In gas engines, timing of the valve applies to both the valves and ignition system.  Valves must be timed to open and close at precise instant in a cycle to prevent engine malfunction. Correct valve timing is determined by the camshaft and the position of the piston by the crankshaft. Timing is usually expressed in degrees and marks are placed on the flywheel and the camshaft gear by the engine manufacturer. Valve timing procedure differs from engine to engine and manufacturer to manufacturer. Always follow the engine manufacturer’s maintenance manual for setting correct valve timing to get the maximum efficiency from the engine. In most cases, there are markings on the flywheel and a pointer provided to locate the engine on dead centre. In addition there are marks on the timing gears to determine the correct position of the camshaft. See figure: 16.0


Figure: 16.0  Typical timing gear marks

In addition there are a number of auxiliary parts that form a complete engine such as, piping to supply air and remove exhaust gases, muffler to dampen the exhaust noise, lubricating system to lubricate the moving parts, water jackets to cool the cylinders, starters to start the engine, fuel supply system (tank, strainer, fuel pump, fuel piping), engine cooling system and instrumentation for engine control, safeguarding, alarm and shutdown.


6.0       FUEL SYSTEM

Two type of gas-engine fuel system are in use:

  • A carburettor system
  • A fuel injection system

Gaseous hydrocarbon fuels have an inherent advantage over diesel fuels in that they are

relatively clean burning fuels, which generate minimal exhaust emissions. However, the

wide variety of fuel sources and composition which one may encounter, coupled with the

demands of the application, require that the fuel system be configured to meet the needs

of each site and each fuel.

This section discusses the primary components in the gas engine fuel system, the function of

each, and how they must adapt to the needs of specialized fuels.


Fuel Systems

Caterpillar’s gas engines contain either a mechanical carburetted fuel system or an

electronic air/fuel ratio control system.G3500, G3400, and G3300 engines have standard carburetted systems with the G3500 having an option for air/fuel ratio control. The G3600 features an air/fuel specific level of NOx emissions even when there are changes in load, fuel

heating value, or ambient conditions. G3500 air/fuel ratio control is described later in

this manual. High and low pressure carburetted fuel systems contain the same basic components that will be described in the following sections.


Figure 17 Naturally aspirated system



Carburetted Fuel System


The following sections describe the main components of the carburetted fuel system.

Figure 16 shows the layout of a typical carburetted system.


Gas Shut-off Valve

Most engine models are shipped with a gas shut-off valve. However a few models require

a customer supplied gas shut-off valve. These shut-off valves are tied into the engine start/stop logic and safety system and are an integral part of the fuel system. There are two types of gas shut-off valves; self-powered (powered by magneto voltage) and powered (usually with 24 volt supply). If power is not available at the site, the self-powered shut-off valve must be used.

The self powered shut-off valves require voltage to shut off (energized to shutdown)

and are reset manually. Powered shut-off valves require power to stay open (energized

to run). In normal operation, gas shut-off valves open and close when starting and stopping the

engine with the start/stop switch. When the valves are closed due to a normal shutdown,

the ignition system is still active and fires the spark plugs. This allows all the fuel left in the

fuel lines downstream of the shut-off valve to be burned and therefore, prevents raw fuel from being pumped into the exhaust system. In an emergency shutdown, the shut-off valve is closed and the ignition system is grounded immediately. This can leave unburned fuel in the engine and exhaust system.

Caution: Always purge the exhaust system

after an emergency shutdown to avoid potential

exhaust system explosions due to unburned fuel

in the exhaust stack. This can be done by

cranking the engine while keeping the gas shutoff

valve closed and ignition system inactive..


Gas Differential Pressure Regulator

The gas differential pressure regulator maintains the proper gas pressure to the carburettor-mixer relative to the air supply pressure. As air pressure to the carburettor increases, fuel pressure is maintained equal to air pressure plus the gas differential pressure. The gas differential pressure is typically set to 1.0-1.3 kPa (4-5 inches of water) by adjustment of the spring force. Gas differential pressure regulators have six basic items common to all models – the body, internal orifice, spring, balance line, sensing line and diaphragms (see Figure 18).

Basic operation is as follows.

Fuel passes through the inlet (12), main orifice (6), valve disc (5), and the outlet (4). Fuel outlet

pressure is felt in the chamber (8) on the lever side of diaphragm (7). This model regulator has internal sensing. Some models have an external line to connect the outlet pressure to the diaphragm cavity. Carburettor air pressure is sensed in chamber (1) via the balance line (14).

As gas pressure in chamber (8) becomes higher than the force of the spring (3) plus air pressure in chamber (1) (pressure to the carburettor-mixer), the diaphragm is pushed against the spring. This rotates the lever (9) at pin (10) and causes the valve stem (11) to close the inlet orifice.

With the inlet orifice closed, gas is pulled by the carburettor-mixer from the lever side of chamber (8) through the outlet (4). This reduces the pressure in the chamber (8) below that of chamber (1). As a result, the force of the spring and air pressure in the chamber on the spring side moves the

diaphragm toward the lever. This pivots the lever and opens the valve stem, permitting additional gas flow to the carburettor-mixer. When the forces on both sides of the diaphragm are the same, the regulator sends gas to the carburettor at a constant rate. The balance line between the regulator and carburettor must be in place to maintain the proper force balance. A turbocharged engine will not develop full power with the balance line disconnected. With proper adjustment of the spring

pressure, gas pressure to the carburettor will always be greater than carburettor inlet air pressure, regardless of load conditions or turbocharger boost pressure. Gas differential pressure regulators have flow capacities based on the supply pressure to the regulator, the body size and the internal

orifice size  The gas supply pressure requirements for each engine family are shown in the section on Fuel System Considerations.



Figure  18 Gas Differential Pressure Regulator


Load Adjustment Valve


The load adjustment valve is a variable orifice in the fuel line between the carburettor-mixer

and the differential pressure regulator. The function of the load adjustment

valve is to make the air-fuel ratio non-linear; that is to lean the air-fuel ratio as the load

increases. The gas differential pressure regulator is used in combination with the load

adjustment valve to adjust the air-fuel ratio. The gas differential pressure effects the air fuel

ratio at lower load ranges. Raising the gas differential pressure richens the air-fuel ratio,

while reducing the gas differential pressure leans the air-fuel ratio. The load adjustment

valve effects the air-fuel ratio near full load operation. Opening the load screw richens the

air-fuel ratio and closing the load screw leans the air-fuel ratio. Larger changes in air-fuel

ratio are accomplished by changing the gas jets in the Impco carburettor-mixer or the

venturi in the Deltec carburettor-mixer



The carburettor-mixer’s main function is metering and mixing the fuel and air prior to

entering the combustion chamber. This can be done in one of two ways:

Figure 18 Carburetor



600 Series Varifuel Mixer



Figure 13. (1) Spring side chamber, (2) Adjustment screw, (3) Spring, (4) Outlet, (5) Valve disc, (6) Main orifice,

(7) Main diaphragm, (8) Lever side chamber, (9) Lever, (10) Pin, (11) Valve stem, (12) Inlet, (13) Regulator body,

(14) Balance line.


Figure 18 is a schematic of a fuel system using an Impco carburettor. Figure 14 is a cross section of a typical Impco carburettor. This system is used on all high pressure carburetted gas applications and some low pressure carburetted gas applications. As air flows past the carburettor diaphragm vacuum port, a vacuum is created. This vacuum is sensed by the air valve diaphragm which in

turn raises or lowers the gas valve as the air flow increases or decreases accordingly. This

allows the carburettor to adjust the fuel flow in proportion to air flow. The gas valve and jet

are sized for specific fuel and operating condition ranges. For example, a carburettor containing a gas valve and jet sized for natural gas, would not operate properly on landfill gas. Likewise, operation with a 3-way catalyst requires a different valve and jet than operation with no catalyst.


The air-fuel ratio is adjusted by setting the regulator differential pressure and the load adjustment valve. Instructions for correctly adjusting the air-fuel ratio can be found in the service manuals. The second type of carburetted system used on Caterpillar gas engines is aventuri type carburettor as shown in Figure 4. The venture carburettors are manufactured by Deltec and are used on some low pressure gas engines. Venturi carburettors operate on the venture effect which, simply stated, says that as air flows through a venturi its pressure is lower in the venturi (P2) than it is upstream (P1).The higher the air flow, the greater the differential pressure will be. If, at the same time, the gas pressure to the carburettor (P3) is held constant with respect to P1, the pressure differential P3-P2 will increase as air flow increases. Any increase or decrease in this differential pressure will cause a corresponding change in fuel flow. The gas pressure regulator is used to keep the

pressure difference between P3 and P1 constant.



Figure 19 Deltec Mixer


Engine power and emissions setting are determined by the mass air-fuel ratio entering the combustion chamber. A carburetted system can only maintain a fixed volume ratio of air and fuel. Therefore, as air temperature, fuel temperature, and heading value of the fuel change, so will the mass air-fuel ratio entering the engine. This is particularly important in applications where low exhaust emissions are a necessity since emissions will change with changes in mass air-fuel ratio.

Depending on carburettor design, emissions can vary throughout the load range.


Throttle Body

The throttle body is an adjustable orifice, typically a movable plate, in the air-fuel intake passage. The movable plate regulates the pressure of the air-fuel mixture in the intake manifold and ultimately the cylinders. The pressure in the cylinders has a direct relationship to the engine power. The movable plate is controlled by the governor. On high pressure gas arrangements,

the throttle body is physically bolted to the carburettor-mixer and both are located

downstream of the turbocharger. On low pressure gas arrangements, the carburettor mixer

is located upstream of the turbocharger and the throttle body is located downstream

of the turbocharger


Air-Fuel Ratio Control


Air-fuel ratio controlled devices seek to maintain a desired air/fuel ratio as operating conditions change. This is done by either measuring and/or calculating the actual airfuel

ratio and then adjusting either the air flow or the fuel flow to maintain the desired air-fuel

ratio. These devices are closed-loop and typically measure the amount of free oxygen in the exhaust, which is proportional to the actual air/fuel ratio. Figure 20 shows a basic air/fuel ratio control system. An oxygen sensor is used to measure the excess oxygen in the exhaust. This

information is used to determine if the air-fuel ratio is correct for the desired emissions. If it is incorrect, an appropriate correction can be made to the fuel flow by an actuator controlled butterfly valve.

Figure 20


Air-fuel ratio controlled engines provide several advantages over engines without airfuel ratio control. One of the primary functions of air-fuel ratio control is to maintain constant emissions for varying conditions of ambient air temperature, fuel quality, speed and load. In addition, when using air-fuel ratio control, engines can operate at leaner air-fuel ratio settings without misfire problems. This is due to the precise control that eliminates the small air-fuel ratio fluctuations present in all carburetted systems. Some high compression ratio, lean burn engines operate in a vary narrow air-fuel ratio band between lean misfire and detonation. The air-fuel ratio control system helps these engines stay within this operating band. Air-fuel ratio control is not only used for lean burn engines. It is also necessary when using a three-way catalytic converters. In three-way catalyst applications, the NOx and CO emissions must be approximately equal in order for the catalyst to operate as designed. This emissions setting is achieved by operating the engine at

a stoichiometric air-fuel ratio which results in about 0.5% oxygen in the exhaust. The air-fuel

ratio control will adjust air flow or fuel flow to maintain this exhaust oxygen level and therefore, allow the catalyst to provide optimum emissions reduction. Caterpillar does not offer air-fuel ratio control systems for use with stoichiometric engines operating with a three-way catalytic converters, however these control systems are widely available. Note that when using a 3-way catalyst with the Impco fuel systems, the carburettor valve and jet must be changed to match the type of air/fuel ratio control device you have selected.



Gas Pressure Regulator


The regulator maintains constant pressure to downstream equipment by controlling the fuel pressure at varying flow rates and supply pressures. Large fluctuations in the supply pressure can cause the gas regulator to fluctuate. This fluctuation can cause engine surge. For sites that expect to see more than a 10% fluctuation in the gas lines upstream from the regulator, a second regulator upstream from the first is required. Operation with supply gas pressures below the minimum values may prevent an engine from delivering rated power or maximum load acceptance. Pressure above the maximum may cause unstable engine operation and damage the gas shutoff valve. Supply pressure to the pilot is supplied by the customer directly from the inlet side of the main regulator body, thus requiring no upstream pilot supply line on installations.


Figure 21 Gas Pressure Regulator





The function of an ignition system is to produce the spark required to ignite the air-fuel mixture inside the cylinder. In addition, the spark should be produced in correct sequence. There are two general types of ignition systems (i) battery system and (ii) magneto system.

Battery system is used generally in vehicle engines and the magneto ignition is generally preferred in industrial and small gasoline engines. Each type of system produces high voltage (about 15,000 volts) current to jump the gap between two electrodes of a spark plug to create spark and ignite the air-fuel mixture inside the cylinder. Figure 19.0 shows the wiring for a 12 volt battery with coil resistor and starter solenoid.


Figure 19.0  A 12 volt battery with coil resistor and starter

The magneto is a simple generator using permanent magnets to produce magnetic fields. Either the magnet or the armature is rotated by the engine to produce a current flow. When the points are closed, a current flows in the primary winding of the induction coil. When the points open, a very high voltage is induced in the secondary winding and this high voltage is directed to the spark plug by the distributor.


Figure 20.0. Rotating Conductor Magneto Ignition


The magneto ignition system is lighter than the battery ignition system and has a further advantage in that the magneto being driven by the engine produces a hotter spark at higher speeds when this is needed. It is used in industrial engines, high performance racing engines, and aircraft engines.


Spark Plugs


Figure 21.0 Spark Plug

Fig. 21.0 shows a spark plug. The central electrode is connected to the secondary winding of the induction coil and the other electrode is grounded. When very high voltage is produced in the coil, a spark jumps from the central electrode to the grounded electrode.

The spark plug gap depends upon engine specifications, and is commonly set between 0.020” to 0.080”. The size of the gap depends upon the engine compression ratio, the ignition system and the characteristics of the combustion chamber. Higher gap permits additional fuel air mixture to be ignited for better ignition.

Spark plugs are threaded to facilitate removal for inspection and replacement.  General thread sizes are 10mm, 14mm, 18mm and 7/8” to fit the size of threads in cylinder head.

Plugs that are in good condition, except for carbon or oxide deposits may be thoroughly cleaned, adjusted and put back in service.  When adjusting the spark gaps, use round wire feeler gauges to check the gap between spark plug electrodes as flat feeler gauges may not give correct measurement. Round feeler gauge should be of the same diameter as the plug gap recommended by the manufacturer. Adjust by bending the side electrode only, bending the centre electrode will crack the insulator.

When installing a spark plug make sure that the seat in the cylinder head is clean and use a new gasket each time you replace a spark plug.



The primary function of engine  lubricating system is to reduce the friction between moving parts. Lubrication also assists in carrying heat away from the engine, cleans the engine parts as they lubricate and forms a seal between piston rings and cylinder walls to prevent blow-by of combustion gases.

Various types of lubrication employed in internal combustion engines are:

  • Forced lubrication,
  • Splash lubrication,
  • Oil feed with fuel

Forced lubrication by pump pressure is the preferred method in large high-speed industrial engines but some engines use combination of splash and force lubrication. In two stroke engines generally lubrication is generally done by mixing oil with fuel in certain specific amounts and as the fuel circulates in the form of vapour in the crank case, the heavier oil separates from fuel and is carried to the working parts in the form of an oily mist.


Figure: 22.0  Gear type lube oil pump

In forced lubrication, engine parts are lubricated with oil delivered by a gear-type oil pump (see figure 22.0). This pump takes suction through a filter from an oil pan or sump. From the pump, the oil is forced through the oil filter and the oil cooler into the main oil gallery.  The oil is fed from the main gallery, through individual passages, to the main crank-shaft bearings and one end of the hollow camshaft. All the other moving parts and bearings are lubricated by oil drawn from these two sources. The cylinder walls and the teeth of many of the gears are lubricated by oil spray thrown off by the rotating crankshaft. After the oil has served its purpose, it drains back to the sump to be used again. The oil pressure in the line leading from the pump to the engine is indicated on a pressure gauge.  A temperature gauge in the return line provides   an indirect method   for indicating variations in the temperature of the engine parts.


Figure 23.0  Elementary diagram of a full-pressure lubrication system



There are three basic types of starting systems used in   gas engines — electric, hydraulic and compressed air. While the electrical drive is common in all automobiles, the air starting system is extensively used in large industrial type machines.


Figure: 24.0  Bendix Starter

A typical electrical starting system is the Bendix drive. Bendix drive consists of a drive motor, a pinion gear which drives the engine flywheel and the Bendix spring as shown in figure: 24.0.

When the starter motor is not running the pinion is out of mesh and entirely away from the flywheel gear. When the starting switch is closed, the motor starts to rotate and drives the pinion to mesh with the flywheel teeth. When the pinion gear is fully meshed with the flywheel gear, the motor drives the flywheel and cranks the engine.

When engine fires and runs on its own power, the flywheel drives pinion at higher speed than the starting motor causing the pinion to turn in opposite direction on the threaded sleeve and disengage from the flywheel. This prevents the engine from driving the starting motor.

Hydraulic Starting System:

Hydraulic starting systemconsists  of  a  hydraulic starting motor,  a piston-type accumulator, a manually operated hydraulicpump,  or an engine-driven hydraulic  pump,  and  areservoir  for  the  hydraulic  fluid.Hydraulic pressure is provided in the accumulator by the manually operated hand pump orfrom the engine-driven pump when the engine isoperating.When  the starting lever  is operated,  thecontrol valve allows hydraulic oil (under pressureof nitrogen gas) from the  accumulator  to  passthrough  the  hydraulic  starting  motor, therebycranking the engine. When the starting lever is released, spring action disengages the startingpinion and closes the control valve.  This stops theflow of hydraulic oil from the accumulator.  Thestarter is protected from the high speeds of theengine by the action of an overrunning clutch.



Air Starting System:

Large gas engines are started when compressed air is admitted directly into the engine cylinders.  Compressed air   is directed into the cylinders to force the piston down and thereby, turn the crankshaft of the engine. This air admission process continues until  the  engine starts and develops idle power.


Figure: 25.0  Vane type Air Starter

Some engines are equipped with air or gas driven turbo-starters with inertially engaged drive. Generally, these turbo-drives do not need lubrication like the vane type starters. Figure 25.0 shows a turbine type starter drive with inertially engaged gear and spring.



 Governor is a device that controls the engine speed automatically. They can be mechanical, hydraulic or electronic.


Figure 26.0 Simple Governor

The main components in a governor are;

1)   Speed sensing mechanism, usually a fly-ball assembly for mechanical hydraulic governor and a frequency transducer for electro-hydraulic unit,

2)   Control mechanism of either mechanical linkages connecting to the fuel control unit in a mechanical unit or a hydraulic unit with linkages in a hydraulic system.

In centrifugal flyweight governors (fig. 26.0), two forces oppose each other. One of these forces is tension spring (or springs) which may be varied either by an adjusting device or by movement of the manual throttle. The engine produces the other force. Weights, attached to the governor drive shaft, are rotated, and a centrifugal force is created when the engine drives the shaft. The centrifugal force varies with the speed of the engine. Transmitted to the fuel system through a connecting linkage, the tension of the spring (or springs) tends to increase the amount of fuel delivered to the cylinders. On the other hand, the centrifugal force of the rotating weights, through connecting linkage, tends to reduce the quantity of fuel. When the two opposing forces are equal, or balanced, the speed of the engine remains constant.

In hydraulic governors (fig. 27.0), the power, which moves the engine throttle, does NOT come from thespeed-measuring device, but instead comes from ahydraulic power piston, or servomotor. This is a pistonthat is acted upon by fluid pressure, generally oil underthe pressure of a pump. By using appropriate piston sizeand oil pressure, the power of the governor at its outputshaft (work capacity) can be made sufficient to operatethe fuel-changing mechanism of the largest engines.


Figure 27.0. Hydraulic Governor (schematic)


Flywheels are used to regulate speed. We know that kinetic energy of a rotating part is proportional to its inertia. So, heavier the flywheel and larger its diameter, the greater its inertia. Flywheels thus absorb a given amount of surplus power depending on its weight and diameter and make the engine run steadier and release the stored power during the suction, compression and exhaust cycle. Flywheels thus assist in the speed control of the engine.


When fuel burns in the cylinders, only a third of the fuel’s heat enrgy is converted to mechanical energy. Rest of the heat shows up in exhaust gases and heat the metal walls which include combustion chamber, the cylinder head, cylinder and piston. To avoid overheating of metal from the combustion process, all internal combustion engines must be equipped with some type of cooling system. The cooling systems job is to remove this unwanted heat from these metal parts.

There are four methods of cooling gas engines:

  • By water circulation
  • By water and oil circulation
  • By air cooling
  • By air and oil circulation


Air cooled engines employ blades generally built in the flywheel which acts as a fan to circulate air over fins cast integrally with the cylinders. Air cooling is generally used in very small engines powering the small appliances such as lawn mowers and motor bikes. In addition to air cooling of cylinders, some engines are equipped with air cooling of oil to overcome some design constraints such as space and position limitations.


Figure 28.0 Simple diagram showing water circulation cooling

Water is the most widely used coolant for liquid cooled engines. Common components in a water cooled system is the radiator or cooler, water circulating pump, cooling jackets in the engine and a temperature control system (thermostat). In large units, the radiators are replaced by external cooling systems consisting of cooling towers to remove a large amount of combustion heat produced by the industrial gas engines. Figure 29.0 shows the shcematic arrangements of different cooling water systems.


Figure 29.0 Schematic of common cooling systems

Due to impurities in water, corrosive scales and mineral deposits tend to coat the cooling surfaces resulting in poor cooling. Chemical additives such as corrosion inhibitor, oxygen scavenger and biocide should be added to the water to prevent corrosion in the cooling system and the cooling system should be cleaned at regular frequency depending upon the condition of water used for cooling.



Air intake system provides the air required for the combustion of the fuel. Intake piping should be as short as possible and the bends should have a long sweep to reduce resistance to air flow. An air filter is installed in the intake system to prevent ingestion of solid particles such as sand and dust. The common type of filters used on a gas engine is dry type filter.


Figure 30.0 Turbocharger

Dry type filter is made from cloth, felt, fiberglass or synthetic such as polyester. The surface dirt may be blown off with air when clogged but generally the cloth or felt filter element should be replaced when the pores become permanently clogged or when the differential pressure is higher than specified in engine operations manual. Regular maintenance of the filter and cleaning/replacement of the filter element is critical as negligence may cause loss of power due to reduced air flow and damage to engine from solid particles entering the engine and damaging the moving parts.

In the normally aspirated gas engine, the atmospheric air passes through a filter and in a turbocharged engine; a fan, blower or compressor provides compressed air to the engine. (see figure 30.0)

Turbochargers are a type of forced induction system consisting of an exhaust gas turbine and a centrifugal blower mounted on a common shaft. The blower draws in atmospheric air and forces it under pressure into the engine intake manifold.  They compress the air flowing into the engine. The advantage of compressing the air is that it lets the engine squeeze more air into a cylinder and proportionately more fuel. This produces more power from each explosion in each cylinder. A turbocharged engine produces more power overall than the same engine without the charging. Some turbochargers are fitted with intercoolers on the blower outlet. Cooling of the air reduces the possibility of detonation.

Intake silencers are installed at the entrance to the intake system when the noise produced by the engine is objectionable.

Turbochargers bearings are pressure lubricated from the engine lubricating oil system and turbine housing is cooled by using the cooling water from the engine jacket water system.



Exhaust systems function to i) carry the products of combustion to a safe point for discharge, ii) Reduce noise produced by discharge pulsation and iii) impose minimum backpressure on the engine. Back pressure reduces engine power.

Steel or cast piping is used for exhaust system and the piping is minimized to reduce back pressure. An exhaust muffler is provided to absorb the noise. Insulation for personal protection is installed in the operator access area to prevent personal injury. In hazardous areas, a spark arrestor is installed in the exhaust line to remove nearly all dangerous sparks from the exhaust gas.



Alarm and shutdown systems are safeguarding systems and fall into two classes: Alarms to warn the operator and shutdown devices to stop the engine before damage results. The following are some of the common safety devices installed on the engine:

  1. Cooling water flow (high water outlet temperature and low water pressure)
  2. Lubricating oil flow (low oil pressure and high oil temperature to indicate oil pump failure or to indicate any oil leaks in the system.)
  • Engine speed (Regular governor to control the engine speed and an over speed trip to shutdown the engine)
  1. Bearing temperature (Either as alarm or trip when the bearings run hot) generally installed on larger machines
  2. Exhaust temperature (exhaust thermo couples are installed either to give visual indications or as alarms or to shutdown the engine if the cylinder temperature runs abnormally high)
  3. Day tank level ( high and low level alarms to indicate problems with fuel storage). In air/gas starters, all valves and controls.
  • Suction filter blockage (To indicate the suction filter element blockage)

Engine safety devices and trip systems should receive regular inspection, testing and maintenance to ensure reliable engine operation.



The term tune-up as applied to a gas engine is defined as the testing and servicing of the engine’s various mechanisms upon whose proper functioning satisfactory and efficient operation of the engine depends. These various mechanisms are the starting, ignition, carburettor and cooling systems in addition to the valves and valve gears.

There are two kinds of tune-up, termed minor and major. A minor tune-up is confined principally to the ignition system, whereas a major tune-up comprises a complete engine diagnosis or overall check and servicing where necessary.

Note: The tuning method explained below is a generic method and gives an idea of tuning generally done in a gas engine. This particular method may not be suitable for the engine you work on. Before doing a tuning of a specific engine, refer to the engine service manual and conduct the tune-up as specified in the manual.


Minor Engine Tune-Up

A minor engine tune-up is intended as a preventive measure for engines that are in fairly normal condition. This tune-up should be performed frequently in order to maintain the standard performance originally built into the engine. If the engine does not perform satisfactorily after a minor tune-up, a major tune-up including a compression test may be necessary.

A minor tune-up includes tests and servicing of:

  • Battery—check and add water.
  • Spark plugs—service or replace.
  • Distributor—check and adjust.
  • Magneto (when used).
  • Wiring circuits—check.
  • Ignition timing—check and adjust.
  • Carburetion—check and adjust.
  • Fuel and air filters.


Many of the minor and major tune-up activities pertain to engine electrical system and must be carried out by an electrician.


Inspect the battery cable and ground strap for broken insulation, corroded or broken strands and loose or corroded terminals. Repair broken or chafed insulation with tape and replace the cable as required. If cable strands are broken, corroded, or loose in the terminals, the cables should be replaced with new cable.

Clean and tighten all connections. Test for weak or discharged battery. Make a voltage test of the battery cells. Add distilled water if necessary. Tighten all primary and high-tension wire connections, particularly at the ignition starter switch, ammeter and fuel gauge behind the instruments.

On gas or air starter equipped engines, carry out tuning of the starter system (air/gas inlet lines and control system  including valves, starter turbine or vane motor) as specified in the vendors operations and maintenance manual

Spark Plugs

Probably more fuel is wasted by faulty spark plugs than from any other cause. Simple screwdriver test or laying the plug on top of the cylinder only show if the plug is dead. While a faulty plug may spark on very low compression, it will cease sparking with increasing compression, as when a load is put on the engine.

Reasons spark plugs fail:

  • Fouled with brown or oily carbon due to excessive oil consumption
  • Black residue due to rich fuel mixture.
  • Metallic specks on insulator due to spark advance or low octane fuel
  • Insulator cracks due to high engine temperature
  • Erosion or molten electrodes due to high engine temperature
  • Mechanical damage resulting in physical damage to plug

When normal wear occurs in a spark plug, electrode appears rounded and the gap increases over a period. If you find a spark plug with worn, cracked or broken insulator, replace the plug. Similarly, if the electrodes are burnt or rounded replace them. Fouled or coated plugs may be cleaned and reinstalled after checking the gap.


Electrical tuning

Every tune-up must include a complete distributor, condenser, magneto or battery and the complete wiring system (starting and ignition circuits) check. Get electricians help to carryout these checks and tune-up.


In addition to the tests and servicing included in a minor tune-up, a major tune-up includes such items as:

  • Battery.
  • Valve adjustment.
  • Bench distributor test.
  • Compression test.
  • Vacuum gauge test.
  • Complete ignition test.
  • Cooling system test.



A specific gravity reading of the electrolyte must be taken before adding water, as water will not mix with the electrolyte immediately and a true reading will not be obtained. Add pure distilled water to bring level of electrolyte to ¼ in. above the plates in each cell.

Valve Adjustment

Values for valve clearances are usually given with the engine hot, that is, at running temperature. Follow manufacturer’s recommendations for the particular engine to be serviced. A typical method used to measure clearance is shown in Fig. 34.0 on page 41. Here the clearance and thickness of feeler stock is greatly exaggerated so that they may be clearly visible. The adjustment is made with the valve tappet or lifter in the fully closed position to achieve a slight drag on the feeler gauge.

Compression Test

Satisfactory engine operation depends on adequate and uniform compression in all cylinders. Loss of compression results in loss of power, and non uniform compression in cylinders causes unsatisfactory or jerky operation. The compression test is therefore important. When making the test it is essential that the engine be at operating temperature and that the engine oil be of the proper grade. To make the compression test, proceed as follows:



Figure: 31.0  Typical Compression Testing Gauge

Remove spark plug of each cylinder to be tested and with engine warmed to working temperature, throttle open, ignition switch off and choke open. Apply compression test gauge to spark plug hole and crank engine. A check valve in the tester holds the compression in the gauge until released by the operator, permitting an accurate reading to be made. See Fig. 31.0.

Test each cylinder and record each reading. Check the manufacturers maintenance manual for permitted variation. If the reading exceeds the permitted compression, cause for low compression should be found and remedied.  A pressure variation of 2 to 4 percent in a high-compression engine is generally permissible

Fuel Pump

Check the fuel pump pressure with a low-reading pressure gauge.

Carburettor Adjustment

When adjusting the carburettor, be sure the engine is at normal operating temperature. Use combustion analyzer for accurate adjustment. To ensure normal engine performance, ensure that gas-air mix is within the manufacturers specified range.

Cooling System

In order to get the maximum efficiency from the cooling system, it must be kept clean. There is a tendency toward corrosion of parts due to electrolytic action of water containing minerals.

Both the corrosive scale and the mineral deposits tend to coat the cooling surfaces, reducing radiation, and in time will clog the radiator and cooler tubes unless special steps are taken to prevent these deposits.

The cooling system should be cleaned at least every two years or as specified by the engine manufacturer. Standard inhibitors or use of de-mineralized water would greatly reduce the formation of rust, scale and corrosion.




Oil and Gas industry uses so many models of gasengines that it is not possible to describe the various overhaul procedures used. For detailed repair procedures, you must always consult the vendors operation and maintenance manual beforestarting any type of repair work.Pay particularattention to assembly tolerances, wear limits,adjustments, and safety procedures. Also be sureto follow the general rules, listed below, whichapply to all engines.

Observe the highest degree of cleanlinessin handling engine parts. Engines have been completely damaged by the presence of abrasives, cotton rags, tools and tackles, blocks of wood, andvarious objects, which have been carelessly left inthe engines after overhaul. Make sure that anyengine assembled for post-repair running isscrupulously free of foreign matter prior to running.

When overhaulor repair of precision parts and surfaces is required, the parts and the surface must be cleaned thoroughly. Cleaned parts should be wrapped in clean cloth or suitable paper and stored in a dry and clean place until required for use. When reusing these parts, they must be wiped by a lint free cloth, and smeared with  light oil where applicable and then reassembled in position.

When removing or installing major rotating elements such as pistons, connecting rods, camshafts, and cylinder liners, make sure that these parts are not banged and damaged. Take precautions to keep dirt and other foreign material in the surrounding atmosphere from entering the engine while it is being overhauled.

You must proceed with a proper plan for the repairs and overhaul to ensure successful and expeditious completion of the work. Check and ensure that all required tools and spare parts are available onsite before starting work.

Wear suitable PPE and comply with the OXY/Ipedex HES requirements fully.

Keep detailed records of repairs, including measurements of worn parts (with hours in use), and the new parts installed. This will help you to know the service life of each component part and replacement time.

Even when the manufacturer supplies the parts, measurements of new parts are needed to determine whether they come within the tolerances listed in the manufacturers’ instruction books or the wear limit charts. In addition, before installation, all replacement parts should be compared with removed parts to ensure that they are suitable.

Never test the serviced engine at full load, slowly bring the engine in, starting from ‘No Load’.

Manufacturers’  maintenance manuals discuss repair procedures in detail, this chapter will be limited to general information on some of the troubles  encountered during overhaul,  the  causes of such troubles, and the methods of repair.

Cylinders and Heads: In operation, the cylinder of a gas engine wears out of true because of the angularity, or various angular positions, the connecting rods pass through during the compression and power strokes. The cause of wear is the lateral thrusts of the piston against the cylinder walls, due to compression and power impulses. See figure 32.0

This wear or scoring is either removed by honing or by reboring the cylinder. Alternately, if the cylinder is fitted with a liner, you can replace the scored liner with a new one.

When cylinder walls and valve chambers have become coated with carbon during operation, the carbon must be removed to avoid pre-ignition and the resulting unsatisfactory operation.

To remove carbon, it is scraped off with a hard, sharp-edged tool. For cleaning out the ring grooves, a special tool should be used made to fit so closely as to leave no deposit under the end or by the edges. Keeping the deposits moist with kerosene will facilitate their removal; soaking with kerosene for hours or even days will be even better. Take care that the wearing parts are not damaged during the carbon removal process.


Figure: 32.0   Typical Wear in a Cylinder.

Seizing and sticking of pistons in the cylinders is commonly due to overheating or lack of lubrication or both. In almost every case of this sort, piston rings get damaged and cylinders are scored. Measures for these problems are explained later.

Pistons are subject to such forces as gas pressure, side thrust, inertia, and friction resulting in piston wear, cracks, piston seizure, and piston pin bushing wear. This results in increased clearance between piston and cylinder, excessive oil consumption and piston slap. In addition, the cylinder wear results in cylinder taper, causing the piston rings to flex on each stroke and wear out prematurely. This results in oil accumulating in engine exhaust and burning in cylinder.

Some piston wear is normal in any engine; the amount and rate depends on several controllable factors. The causes of excessive piston wear are also the causes of other piston troubles. Replace the pistons with excessive wear.

One of the factors controlling wear is lubrication. An adequate supply of oil is essential to provide the film necessary to cushion the piston andother parts within the cylinder and prevent metal-to-metal contact.  Inadequate lubrication will notonly cause piston wear but increase friction causing piston seizure, land breakage, andpiston pin bushing wear.

Lack of lubrication is caused either by lack of lube oil pressure or by restricted oil passages. You must regularly watch the pressure instruments on the engine which warn you of low lube oil pressure and clean the piston and connecting rod oil passages regularly.

The second factor that creates piston problems is engine cooling; as higher engine temperature results in lubrication problems. High cylinder surface temperatures will reduce the viscosity of the oil. As the cylinder lubricant thins, it will run off the surfaces resulting in lack of lubrication. Lack of lubrication leads to excessive piston and liner wear.

Similarly if the engine is run at reduced temperature, oil viscosity increases and the oil will not readily reach the parts requiring lubrication.

Oil plays an important part in the cooling ofthe piston crown. If the oil flow to the undersideof the crown is restricted, deposits caused byoxidation of the oil will accumulate and lower therate of heat transfer. For this reason, the under-side of each piston crown should be thoroughlycleaned whenever pistons are removed.

While insufficient lubrication and unevencooling may cause ring land failure, excessive oiltemperatures may cause piston seizure. Anincrease in the rate of oxidation of the oil mayresult in clogged oil passages or damage to pistonpin bushings.


Figure: 33.0  Typical Compression Testing Gauge

Improper fit of piston causes seizure and excessive wear. Check all pistons and cylinders or liners for clearance before installation and ensure they meet the manufacturers recommended clearance. Method of checking the piston and cylinder clearance is shown in the figure: 33.0 and 34.0



Figure: 34.0  Checking piston with micrometer

If clearance is insufficient, a piston will NOT wearin and will probably bind. The resulting excesssurface temperatures may lead to seizure orbreakage.Binding increases wear and shortens piston lifeby scuffing the liner or galling the piston skirt.Scuffing roughens the liner so that an abrasiveaction takes place on the piston skirt, thusgenerating additional heat which may distort orcrack the piston or liner. Galling, especially onaluminium pistons, causes the metal to be wipedin such a manner that the rings bind in thegrooves.

A loose fitting piston may cause similar problems like a tight piston such as dragging and cocking of piston resulting in broken or cracked ring groove lands.

Excessive wear on the piston and piston pin bushing may be caused due to  overload or due to unbalanced load. Overloading an engine increases the forces on the pistons and subjects them to higher temperatures, thus increasing their rate of wear.

All cylinders should be balanced. You can check this by measuring the

  • exhaust gas temperature at each cylinder,
  • firing pressure and
  • compression pressure

Common piston problems and their causes are listed below:

TroublesPossible   Causes
Undue piston wear; crown and land draggingInsufficient lubrication,

Improper cooling watertemperaturesOverload

Unbalanced load

Improper fit

Dirty intake air cleaner

Dirty oil

Improper starting procedures

Cracks in CrownFaulty cooling

Loose piston

Obstruction in cylinder

Cracks in LandsInsufficient lubrication

Cocked piston

Insufficient ring groove clearance

Excessive wear of pistonring grooves

Broken rings

Improper installation or removal

Piston seizureInadequate lubrication

Excessive temperatures

Improper cleaning

Piston pin bushing wearInsufficient lubrication

Excessive temperatures

Overload and unbalanced load

PISTON RINGS: For correct piston ring operation, proper clearance must be maintained between the ring and the land, and also between the ends of the ring. This is necessary in order that the ring may be free to flex at all temperatures of operation. The clearance depends upon the ring and the materials involved. After installing a ring, check the clearance between the ring and the land. This check is made with a thickness gage, and must be made completely around the piston as shown in figure 35.0.


Figure: 35.0  Checking the piston and ring gap with feeler gauge


In addition to incorrect clearance the other major problem of ring failure is carbon build-up on the rings and in grooves due to burnt oil. Carbon build-up limits movement and expansion of the rings, prevents the rings from following the cylinder contour and sealing the cylinder, and may cause sticking, excessive wear, or breakage.

The other problem with the rings is insufficient ring pressure due to faulty material. The ring must return to its shape and push against the wall of the cylinder aided by the compression and combustion stroke. Extended use of these rings will weaken the rings due to overheating resulting in binding of rings inside grooves. Ensure that correct replacement rings are supplied by the engine manufacturers and maintain correct end gap and clearance when installing new rings.


Figure:36.0  Checking the piston ring end-gap with feeler gauge


Worn cylinders damage the rings quickly. When changing the rings or during servicing, check the cylinder diameter using a inside micrometer.  Also check the surface condition, amount of taper and out of roundness. It is ideal to replace a liner when changing the piston rings.

Some lubricating oils cause a resinous gum like deposit to form on engine parts. Trouble of this nature can be avoided by using oil recommended by the manufacturer.

The ring is in the best position to make allowance for cylinder wear if the ring gaps are in line with the piston bosses. Always stagger the gaps of adjacent rings 180 to reduce gas leakage.

With the wearing away of material near the top of a cylinder liner, a ridge will gradually be formed. When a piston is removed, this ridge must also be removed, even though it has caused no damage to the old set of rings. The new rings will travel higher in the bore by an amount equal to the wear of the old rings, and the replacement of the connecting rod bearing inserts will also increase piston travel. As the top piston ring will strike the ridge because of this increase in travel, breakage of the ring and perhaps of the land is almost certain if the ridge is not removed.

Old piston rings should never be refitted to the piston, as it takes much longer for old rings to seat in again than it would for new rings. This is because old rings can never be mounted in the same position as they were while in the engine. It is better always to install new rings. Use care in ordering correct size and type rings for each job. Whenever the new piston rings are installed, drain the crankcase and fill with the correct grade of oil.


PISTON  PINS  AND  PIN  BEARINGS Piston pins are made of hardened steel alloy, and their surfaces are precision finished. Piston sleeve bearings or bushings are made of bronze or a similar material. These pins and pin bearings require very little service and total failure rarely  occurs.

Wear, pitting, and scoring are the usual problems encountered with piston pins and piston pin bearings. Wear of a pin or bearing is normal, but the rate of wear can be increased by such factors as inadequate and improper lubrication, overloading, misalignment of parts, or failure of adjacent parts.

Every time a piston assembly is removed from an engine, the piston pin and bearing assembly should be checked for wear using a micrometer. If the clearance is in excess of recommended (refer to manufacturers service manual) you must replace both the pin and bearing.

The main reason for failure of piston pin and bearing is due to inadequate lubrication. This could be due to failure of pressure lubrication, incorrect alignment of lubrication holes or obstructions blocking the oil passage in connecting rod.


CONNECTING RODS Connecting rod problems usually involve either the connecting rod bearing or the piston pin bearing. Some of these problems, such as misalignment, defective bolts, cracks, or plugged oil passages, can be avoided by performing proper maintenance and by following instructions in the manufacturer’s technical manual.

Misalignment causes damage to the piston, piston pin, and the connecting rod journal bearing. This damage is likely to result in breakage and in increased wear of the parts, leading to total failure and possible damage to the entire engine structure.

Connecting rods must be checked for proper alignment before being installed in an engine, and after any derangement involving the piston, cylinder, or crankshaft. Defective bolts are often the result of over- tightening.

Connecting rod bolts should be tightened by using a torque wrench, or an elongated gage to ensure that a predetermined turning force is applied to the nut. Defective threads can cause considerable trouble by allowing the connecting rod to be loosened and cause serious damage to the engine. Whenever rod bolts are removed they should be carefully inspected for stripped or damaged threads and elongation. Cracked rods are usually the result of overstressing caused by overloading or over-speeding or because defective material was used at the time of manufacture.

It is of prime importance to discover the cracks before they have developed to the point where the failure of the rod will take place. No attempts should be made to repair cracked rods. They should be replaced; serious damage may result if breakage occurs during operation.

Restricted oil passages are often the result of improper assembly of the bushing and the connecting rod bearing inserts. They may also be due to foreign matter lodging in the oil passages


SHAFTS AND BEARINGS The crankshaft, cam-shaft and associated bearings of an internal combustion engine are all subject to several types of problems. While some problems are common others are not.

The most common problem is the metal fatigue, operation at critical speed and inadequate lubrication. Fatigue caused by cyclic peak loads leads to failure of shafts and journal bearings.

Over-speeding of an engine is a common reason of fatigue failure. This can be identified by the failure of bearings. Overloading of the engine will cause failure of the lower halves of main journal bearings, while over-speeding may cause either the upper or the lower halves to fail.

Crankshaft or camshaft failure does not occur too often but  when it occurs, it may be due to metal fatigue developed over long period of time. This failure may be caused by incorrect manufacturing process (incorrect quenching and balancing), or by torsional vibration.

Correct lubrication of all bearings is the key. Correct lubricant in sufficient quantity and at proper pressure must be maintained at all times during the operation of engine. Improper lubrication may result in damaged cams and camshaft  bearing failure, scored or out-of-round crankshaft journals, and journal bearing failure.

Contamination of lubricants is another major cause for bearing and journal failure. You must ensure that contamination does not enter the lubrication system either during replenishing oil or through seals and ports.

To prevent lubrication difficulties, you should watch for low lube oil pressure, high temperatures, and lube oil contamination by water, fuel, and foreign particles.

Operation of an engine at critical torsional speeds and in excess of the rated speed will lead to engine shaft and bearing problems. Each multi-cylinder engine has one or several critical speeds which must be avoided in order to prevent possible breakage of the crankshaft, camshaft, and gear train.

Over-speeding of an engine must be avoided. If the rated speed is exceeded for any extended period of time, the increase in inertia forces may cause excessive wear of the journal bearings and other engine parts, and in uneven wear of the journals.



Crankshaft journals are damaged due to

  • Poor lubrication
  • journal bearing failure
  • improper and careless handling during overhaul.

Journal bearing failures may cause scoring on the journals and in extreme cases may result in broken or bent crankshafts and out-of-round journals.

Broken or bent crankshafts may be caused by:

  • the improper functioning of a torsional vibration damper.
  • excessive bearing clearances
  • off-centre and out- of-round  journals
  • Excessive shaft deflection


Vibration dampers are mounted on the crankshafts of some engines to reduce the torsional vibrations set up within the crankshaft and to ensure a smoother running engine. If a damper functions improperly, torsional vibrations may rupture the internal structure of the shaft.


Figure: 37.0   Vibration Damper

A damper must be fastened securely to the crankshaft at all times during engine operation; otherwise, the damper will not control the crankshaft vibrations. Small dampers are usually grease-packed, while larger ones frequently receive lubrication from the main oil system. Dampers that are grease lubricated must have the grease  changed periodically, as specified in the manufacturer’s instructions. If the assembly is of the elastic type, it must be protected from fuel, lube oil, grease, and excessive heat, all of which are detrimental to the rubber.


   Figure 38.0. Cracked crank web.

Excessive bearing clearance may result in crankshaft breakage or bending. Excessive clearance in one main bearing may place practically all of the load on other main bearings. Flexing of the crankshaft under load may result in fatigue and eventual fracture of the crank web. (See figure 38.0)

Furthermore, off-centre and out-of-round journals tend to scrape off bearing material. This leads to excessive wear and increased clearance between the shaft and bearing. You can minimize the possibility of journal out-of-roundness by preventing incorrect lubrication, over-speeding or overloading of the engine, excessive crankshaft deflection, and misalignment of parts.

Excessive shaft deflection, caused by improper alignment between the driven unit and the engine, over-speeding may result in a broken or bent shaft along with considerable other damage to bearings, connecting rods, and other parts.

The amount of deflection of a crankshaft may be determined by a  dial-reading inside micrometer used to measure the variation in the distance between adjacent crank webs when the engine shaft is barred over. See figure 39.0 for a deflection gauge installed on crank webs


Figure 39.0. Measuring crank deflection.


When installing the gage, or indicator, between the webs of a crank throw, place the gage as far as possible from the axis of the crankpin. The ends of the indicator should rest in the prick-punch marks in the crank webs. Readings are generally taken at the four crank positions: top dead centre, inboard, near or at bottom dead centre, and outboard.

Once the indicator has been placed in position for the first deflection reading, do NOT touch the gage until all four readings have been taken and recorded. Variations in the readings obtained at the four crank positions will indicate distortion of the crank.

Distortion may be caused by several factors, such as a bent crankshaft, worn bearings, or improper engine alignment. The maximum allowable deflection can be obtained from the manufacturer’s technical manual. If the deflection exceeds the specified limit, take steps to determine the cause of the distortion and to correct the trouble.

Deflection readings are also employed to determine correct alignment between the engine and the generator, or between the engine and the coupling. When alignment is being determined, a set of deflection readings are usually taken at the crank nearest to the generator or the coupling.

CAMSHAFTS may be damaged as a result of improper tappet  adjustment, loose or broken tappet adjustment screw, worn  or stuck cam followers, or failure of the camshaft gear.

Valves must be timed correctly to ensure proper operation of the engine and also to prevent possible damage to the engine parts. You should inspect frequently the valve actuating linkage during operation to determine if it is operating properly. Such inspections should include taking tappet clearances (see figure 40.0) and adjusting at correct temperatures, if necessary; checking for broken, chipped, or improperly seated valve springs; inspecting push rod end fittings for proper seating; and inspecting cam follower surfaces for grooves or scoring.


Figure 40.0. Adjustment of tappet by feeler gauge.


JOURNAL BEARINGS The most  common journal bearing failures may be due to one or to a combination of the following  causes:

  • Corrosion of bearing materials
  • Surface pitting of bearings
  • Inadequate bond between the bearing metaland the bearing shell
  • Out-of-round journals due to excessivebearing wear
  • Rough spots. burrs or ridges
  • Misalignment of parts.
  • Faulty installation
  • Improper lube oil

Corrosion of bearing materials occurs due to oxidized lubricating oils resulting in small pits covering the bearing surface.  Since the small pits caused by corrosion are so closely spaced that they form channels, the oil film is not continuous and the load-carrying area of the bearing is reduced below the point of safe operation.

Oxidation of oil may be minimized by changing oil at the designated intervals, and by keeping engine temperatures within recommended limits.

Surface pitting of bearings due to high localized temperatures that cause the lead to melt. This is generally the result of very close oilclearances and the use of an oil having a viscosity higher than recommended. Early stages of theloss of lead, due to melting, will be evidenced byvery small streaks of lead on the bearing surface.

Inadequate bond between the bearing metal and the bearing shell. A poor bond may becaused by incorrect manufacturing procedures or due to fatigue in bearing.Babbitt lifts off the shell completely if the bonding is poor as shown in figure 41.0.


Figure 41.0. Bearing failure due to inadequate bonding.

Out-of-round journals In service, bearings wear resulting in increase in bearing clearance. Increased clearance results in oil leakage through bearing and reduced flow to other components This leads to increased oil temperature resulting in melting of bearing lining.

To prevent bearing wear, regularly check journals for out of roundness. This can be done using a micrometer and measuring the journal over the entire surface of bearing contact area. Maintain record of reading for future reference.

Rough spots. Burrs or ridges causes grooves in the bearings leading to bearing failure. Use a super-fine emery stone to remove rough spots. Ensure that the abrasives do not enter the lubricant by sealing the oil ports and cleaning off thoroughly after the job.

Misalignment of parts. Misalignment causes warping or bending in crankshaft resulting in bearing failure. Misalignment of crankshaft causes the main bearing failure and the misalignment of the connecting rod leads to connecting rod bearing.

Faulty installation of journals is mainly due to lack of cleanliness while installing the bearing, installation of incorrect bearings and lack of experience. Dirt or hard particles lodged between the bearing shell and the connecting rod bore when installing bearing, creates an air space. This space retards the normal flow of heat and causes localized high temperatures resulting in bearing failure as shown in figure 42.0.

Figure 42.0 Failed bearing due to poor installation

Another source of trouble during installation is due to the interchanging of the upper and lower shells. The installation of a plain upper shell in place of a lower shell, which contains an oil groove, completely stops the oil flow and leads to early bearing failure. The resulting damage not only may ruin the bearing but may also extend to other parts, such as the crankshaft connecting rod, piston, and wrist pin.

Failure to follow recommended procedures in the care of lubricating oil. Lack of proper amount of lubricating oil will cause the overheating of a bearing, causing its failure (see figure 43.0).

Figure 43.0    Failed bearing due to lack of oil

In large engines, the volume of the lubricating oil passages is so great that the time required to fill them when starting an engine could be sufficient to permit damage to the bearings. To prevent this, separately driven lubricating oil priming pumps are installed to circulate oil to the bearings before starting.

Correct oil temperature is key to uninterrupted engine operation. Normally,   the manufacturer’s technical manual should be followed as to the correct lubricating oil temperature to maintain. Oil must be analyzed at recommended intervals to determine its suitability for further use. In addition, regular service of oil filters and strainers must be maintained, and oil samples must periodically be drawn from the lowest point in the sump to determine the presence of abrasive materials or water. Strict adherence to recommended practices will reduce the failure of bearings and other parts because of the contaminated oil or insufficient supply of clean oil.


FRICTIONLESS BEARINGS: Maintenance of recommended oil pressures is essential to ensure an adequate supply of oil at all bearing surfaces. Refer to the oil pressure gage, as it is the best source of operational information to indicate satisfactory performance.

Since dirty bearings will have a very short service life, every possible precaution must be taken to prevent the entry of foreign matter into bearings. Dirt in a bearing which has been improperly or insufficiently cleaned may be detected by noise when the bearing is rotated, by difficulty in rotating, or by visual inspection. Do not discard an antifriction bearing until you have definitely established that something in addition to dirt has caused the trouble. You may determine this by properly cleaning the bearing.

Spalled or pitted rollers or races may be first recognized by the noisy operation of the bearing. Upon removal and after a very thorough cleaning, the bearing will still be noisy when rotated by hand. (Never spin a frictionless bearing with compressed air.) Roughness may indicate spalling at one point on the raceway.

Pay particular attention to the inner surface of the inner race, since it is here that most surface disintegration first occurs. Since pits may be covered with rust, any sign of rust on the rollers or contact surfaces of the races is a probable indication that the bearing is ruined.

Brinelled or dented races are most easily recognized by inspection after a thorough cleaning. Brinelling receives its name from its similarity to the Brinnell hardness test, in which a hardened ball is pressed into the material. The diameter of the indentation is used to indicate the hardness of the material. Bearing races may be brinelled by excessive and undue pressures during installation or removal, or by vibration from other machinery while  the bearing is inoperative.

If heavy shafts supported by frictionless bearings are allowed to stand motionless for a long time, and if the equipment is subject to considerable vibration, brinelling may occur. This is due to the peening action of the rollers or balls on the races. Brinelled bearings must not be placed back in service.

Steps can be taken to prevent brinelling. Proper maintenance will help a great deal, and the best insurance against brinelling caused by vibration is to rotate the shafts supported by the frictionless bearings at regular intervals (at least once a day) during periods of idleness. These actions will prevent the rollers from resting too long upon the same portion of the races.

Separator failure may become apparent bynoisy operation. Inspection of the bearings mayreveal loose rivets, failure of a spot weld, or cracking and distortion of the separator. Failure ofseparators can usually be avoided if properinstallation and removal procedure are followed,and steps are taken to exclude the entry of dirt.

Abrasion (scoring, wiping, burnishing) on the external surface of a race indicates that relative motion has occurred between the race and the bearing housing or shaft surface.

The race adjacent to the stationary member is usually made apush fit so that some creep will occur. Creep isa very gradual rotation of the race. This extremelyslow rotation is desirable as it prevents repeatedstressing of the same portion of the stationaryrace. Wear resulting from the proper creep isnegligible and no damaging abrasion occurs.

Cracked races will usually be recognized bya definite thump or clicking noise in the bearingduring operation. Cleaning and inspection is thebest means of determining if cracks exist. Cracksusually form parallel to the axis of the race. Thecracking of bearing races seldom occurs ifproper installation and removal procedures arefollowed.

Excessive looseness may occur on rare occasions  even though no surface disintegration is apparent. Since many frictionless bearings appear to be loose even when new, looseness is not always a sign of wear. The best check for excessive looseness is to compare the suspected bearing with a new one. Wear of bearings, which cause looseness without apparent surface disintegration, is generally caused by the presence of fine abrasives in the lubricant. Taking steps to exclude abrasives and keeping lubricating oil filters and strainers in good condition is the best way to prevent this type of trouble.

Some common problems in frictionless bearings and their causes are listed below


Dirty bearingImproper handling or storage.

Use of dirty or improper lubricant

Failure to clean housing.

Poor condition of seal

Spalled or pitted rollers or racesDirt in bearing.

Water in bearing.

Improper adjustment of tapered roller bearings.

Bearing misaligned or off square

Dented (brinelled) racesImproper installation or removal.

Vibration while bearing is inoperative

Failed separatorInitial damage during installation and removal.

Dirt in the bearing

Races abraded on external surfacesLocked bearing.

Improper fit of races

Cracked raceImproper installation or removal (cocking)
Excessive loosenessAbrasives in lubricant


Most of the problems listed above require the replacement of an antifriction bearing. The cause of damage must be determined and eliminated so that similar damage to the replacement bearing may be prevented. Dirty bearings may be made serviceable with proper cleaning, providing other damage does not  exist. In some cases, races abraded on the external surfaces can be made serviceable, but it is generally advisable to replace abraded bearings. Dirty frictionless bearings must be thoroughly cleaned before being rotated or inspected.

AUXILIARY DRIVE MECHANISMS Auxiliary drive mechanisms are used in internal combustion engines to maintain a fixed and definite relationship between the rotation of the crankshaft and the camshaft. This is necessary in order that the sequence of events necessary for the correct operation of the engine may be carried out in perfect unison. Timing and the rotation of various auxiliaries (blowers, governor, fuel and lubricating oil pumps, circulating water pumps, over-speed trips, etc.) are accomplished by a gear or chain drive mechanism from the crankshaft. (Some small engine auxiliaries may be belt-driven.)

GEAR MECHANISMS The principal type of power transmission for timing and accessory drives is a system of gears similar to those shown in figure 44.0. In some of the larger engines, there may be two separate gear trains, one for driving the camshaft and the other for driving other accessories. The type of gear employed for a particular drive depends upon the function it is to perform. Most gear trains use single helical or spur gears, while governor drives are usually of the bevel type.

Figure 44.0    Auxiliary Gear Drive

The causes of gear failure (improper lubrication, corrosion, misalignment of parts, torsionalvibration, excessive backlash, wiped gear bearingsand bushing, metal obstructions, and impropermanufacturing procedures) are basically the sameas the causes of similar troubles in other engineparts. The best method of prevention is to adhereto the prescribed maintenance procedures andfollow the instructions given in the manufacturer’stechnical manual.

Maintenance and repair of gear trains involvea thorough check (for scoring, wearing, pitting,etc.) of the gear shafts, bushings and bearings,and gear teeth during each periodic inspection.Be sure that the oil passages are clear, and thatthe woodruff keys, dowel pins, and other locking devices are secured to a correct fit in order toprevent axial gear movement. It is essential that all broken or chipped parts be removedfrom the lubrication system before new gears areinstalled.

Remember that an engine can not be barred over while the camshaft actuating gears are removed from the train. Should the engine be barred over, there is danger that the piston will strike valves that may be open and extending into the cylinder. Make certain that any gears removed are replaced in the original   position.

Special punch marks, or numbers (see figure 44.0) are usually found on gear teeth that should mate. If they are not present, make identifying marks to facilitate the correct mating of the gears later.

Bearing, bushing, and gear clearances must be properly maintained. If bushing clearances exceed the allowable value, the bushings must be renewed. The allowable values for backlash and bushing clearances should be obtained from the instruction manual.

Usually, a broken or chipped gear must be replaced. Care should be exercised in determining whether a pitted gear should be replaced.


CHAIN MECHANISMS In some engines, chains are used to drive camshafts and auxiliaries.

The principal causes of drive chain failure are improper chain tension, lack of lubrication, sheared cotter pins or improperly riveted joint pins and misalignment of parts, especially idlers. Chain drives should be checked regularly for any of the problems above  in accordance with the instructions in the appropriate engine manual. The tension should be adjusted as required during these inspections.

An idler sprocket and chaintightener are used on most engines to adjust chaintension. During operation, chains increase slightlyin length because of stretch and wear. Adjustments should be made for these increaseswhenever necessary.

When you are installing a new chain, peen the connecting link pins into place, but avoid excessive peening. After peening, make sure the links move freely without binding in position. Cotter pins must be secured or the joint pin ends riveted, whichever is applicable. Repair links should be carried in-stock at all times. Always check engine timing after installing a new timing and accessory drive mechanism.



Inspection and maintenance are vital to ensure smooth operation and to minimize engine failure. Through continuous and detailed inspection procedures, damaged parts or components can be discovered thus preventing early failure of the engine. Underlying conditions will generally include maladjustment, improper lubrication, corrosion, erosion, and other causes of machinery damage.

Particular attention to be paid to the following abnormal condition in an engine:

  1. i) Unusual noises,
  2. ii) Vibrations,

iii) Abnormal temperatures,

  1. iv) Abnormal pressures,
  2. v) Abnormal operating speeds,
  3. vi) Any leaks in the engine,

Familiarization with the specific temperatures, pressures, and operating speeds of the engine required for normal operation will help detect any departure from normal operation.

If any gauge or other instrument for recording operating conditions of the engine gives an abnormal reading you must fully investigate the cause immediately and rectify the cause.

House keeping is important for a safe and healthy operation. Ensure the engine and the surrounding area is cleaned at regular intervals.

Promptly attend to any leaks observed. This can prevent major mechanical problems later.

Any changes in the operating speeds (those normal for the existing load) of pressure-governor-controlled equipment, variations from normal pressures, lubricating oil pressure and temperatures, system pressures, quality of exhaust gas and abnormal cooling system temperatures often indicate either improper operation or poor condition of the engine.

Always remember to promptly inspect all similar units to determine whether there is any danger that a similar failure might occur. Prompt inspection and remedial action may eliminate a wave of repeated failures in other equipment.

Pay strict attention to the proper lubrication of all equipment, including frequent inspection and sampling to determine that the correct quantity of the proper lubricant is in the unit. It is good practice to make a daily check of samples of lubricating oil in all auxiliaries. Allow samples to stand long enough for any water to settle. When auxiliaries have been idle for several hours (particularly overnight), you should drain a sufficient sample from the lowest part of the oil sump to remove all settled water. Replenish with fresh oil to the normal level.


18.0     OPERATION.

The following pointers on starting, operation and stopping of an engine are elementary and are useful reminder and guide. The manufacturers operating manual should be fully read, understood and followed; personnel properly trained in operations and understand the safety requirements associated with the particular engine prior to starting, operating and shutting down of an engine.

  1. Ensure all safety requirements associated with the engine and driven equipment are complied with and Occidental/ Ipedex HES safety guidelines and the Safah field work permit system are met.
  2. Carry out a general check of the engine and driven equipment. Complete the manufacturers and OXY pre-start check list. Do not start the engine or move any of the controls if there is a DO NOT OPERATE TAG or similar other warning tags are attached to the start switch or any system or control.
  • Perform required daily and periodic checks before starting the engine. Ensure all guards are in place. Repair or replace all damaged or missing guards.
  1. Positioning (Check mechanical interference by turning full cycle and put the cylinders in crank position if required)
  2. Cooling system: Inspect all hoses for cracks and loose clamps. Make sure water pump is healthy and cooling media tank is full, rid the system of air.
  3. Inspect the all belts (fan belts, accessory drive belts) for looseness, cracks, breaks and damage. Replace or tension as required.
  • Lubricating system (Check the oil level, operate the hand pump or turn engine over several times to ensure oil film at the bearings. Where the independent pumps are installed on the engine, run the pumps before starting the engine. Lubricate hand lubricated parts and also grease/oil from cups; operate mechanical lubricators a few times)
  • Starting system (ensure that the starting battery or the air starting system is fully charged). Remember that unburned gas left in the inlet manifold can ignite when the engine is restarted. Clear the system of unburned gas. Before starting a gas engine which was stopped by terminating the ignition system, crank the engine for approximately 15 seconds with the gas valve in off position in order to clear the exhaust system of unburned gas.
  1. Follow the manufacturers start up procedure and start the engine.
  2. If engine does not start promptly, stop cranking to avoid unnecessary loss of starting air or battery charge. Find and eliminate the cause of failure before cranking again.
  3. Immediately after starting check the lubricating oil pressure, cooling water flow and fuel supply. Listen for abnormal sounds and check for vibration and temperature rise. Watch the entire engine to see if all components are functioning properly. If possible run the engine at light load till it reaches running temperature. Recharge the starting system.


It is important to inspect the engine regularly. Look for leaks and loose fastenings, listen for mechanical noises and check temperatures by instruments or feel. Test emergency devices frequently. Prevent engine overloading as over loading causes combustion troubles and overheating. Ensure uniform temperatures at each cylinder for uniform cylinder loading.

Regulate cooling water temperature and flow within the range recommended by the manufacturer. Low cooling temperature causes condensation on lower cylinder walls and causes liner wear.  It also causes incomplete combustion. High outlet temperature promotes scale formation and may breakdown the oil film on the cylinder walls. Scale in jackets causes growth of cylinders resulting in warping, cracking and burning of liners.

Keep the lubricating system clean. Renew filter and purifier elements regularly. Check the condition of the oil by having a sample analyzed and renew on a regular schedule. Keep the lube oil at proper temperature. High temperature promotes oxidation and sludging; it also tends to increase leakage from crankcase. Investigate any increase in crankcase temperature; it may indicate a hot bearing. Investigate any oil pressure change. It signifies clogging of the system. Slow pressure fall indicates bearing or pump wear; a sudden pressure drop means a burnt out bearing.

Excessive lube oil reaching the combustion chamber will cause blue smoke in the exhaust. Keep a log of lube oil consumption and if the consumption is excessive check the piston rings or mechanical defects.

Check combustion conditions. Abnormally high exhaust temperature or rate of fuel consumption means that combustion is poor and is causing waste of fuel.



This chapter is concerned with problems that occurboth when an engine is starting and running. Theproblems are chiefly the kind that can be identified byerratic engine operation, warnings by instruments, orinspection of the engine parts and systems. Keep in mind that the troubles listed here are generaland may or may not apply to a particular engine.

When working with a specific engine, check themanufacturer’s technical manual and any instructionsissued within OXY.An engine may continue to operate even when aserious casualty is imminent. However, symptoms areusually present. Your success as a trouble-shooterdepends partially upon your ability to recognize thesesymptoms when they occur. You will use most of yoursenses to detect trouble symptoms. You may see, hear,smell, or feel the warning of trouble to come. Of course,common sense is also a requisite. Another factor in yoursuccess as a trouble-shooter is your ability to locate thetrouble once you decide something is wrong with theequipment. Then, you must be able to determine asrapidly as possible what corrective action to take. In learning to recognize and locate engine troubles,experience is the best teacher.

Instruments play an important part in detectingengine problems. You should read the instruments andrecord their indications regularly. If the recordedindications vary radically from those specified byengine operating instructions, the engine is notoperating properly and some type of corrective actionmust be  taken. You must be familiar with thespecifications in the engine operating and maintenance manuals,especially those pertaining to temperatures, pressures,and speeds. You should know the probable effect on theengine when instrument indications vary considerablyfrom the specified values. When variations occur ininstrument indications, before taking any correctiveaction be sure the instruments are not at fault before youtry corrective actions on the engine. Check theinstruments immediately if you suspect them of beinginaccurate.Periodic inspections are also important in detectingengine troubles. Such inspections reveal the failureof visible parts, presence of smoke, or leakage of oil,fuel, or water. Cleanliness is probably one of the greatestaids in detecting leakage.When you secure an engine because of trouble, yourprocedure for repairing the engine should follow anestablished pattern, if you have diagnosed the trouble.If you do not know the location of the trouble, find it by followinga systematic and logical method of inspection.

Once you have associated the problem with a particular system, the next step is to trace out the trouble until you find the root-cause of the problem. Problems generally originate in one system, but may cause damage to another system or to other basic engine parts. When a problem involves more than one system of the engine, trace problems in associated systems separately and make repairs as necessary.

To satisfactorily locate and remedy problems,you must know the construction,function, and operation of the various systems as wellas the parts of each system for a specific engine.


IMPORTANT NOTE: Always work within the guidelines of the Occidental/ Ipedex HES safety guidelines and the Safah field work permit system. If in doubt stop work and contact the HES department or the departmental supervisor for guidance.




  1. Engine fails to start
1Not enough fuel or no fuelEnsure tank is full and the valves are open; Make sure the pumps and piping is primed and vented. Check the transfer pumps are working and filters are clean. In gas engines, check gas supply, check filters and check all valves are open.
2Water or dirt in gasoline fuelDrain fuel system and tanks; clean tanks; prime and vent pumps and lines properly. Replace filter.
3Weak or flat batteryRecharge and test battery. Replace if necessary.
4Damaged, corroded or loose battery terminalsClean, inspect and retighten battery terminal and clamps. If badly corroded replace the clamps and cables.
5Dirty or corroded distributor cap or rotorClean the distributor and inspect. If badly pitted or burned replace rotor or cap.
6Broken or loose ignition wiresReplace broken wires. Replace cables with damaged insulation. Ensure all cables are secure and retighten.
7Fouled spark plugsClean, adjust and re-install plugs. If badly damaged replace them.
8Faulty electrical starter circuit, solenoid or motor.Check and clear faults in electrical circuit(s), replace faulty motor or solenoid.
9Engine crank does not turn due to internal problemWhen the engine crankshaft does not turn after disconnecting the engine from driven equipment, remove the fuel nozzles and check for fluid inside cylinders. If inside fluid is not a problem, engine must be disassembled to check for other inside problems such as bearing seizure, piston seizure etc,.
10Dirty fuel filterReplace fuel filter
11Dirty or broken fuel linesClean or replace fuel lines
12Faulty fuel transfer pumpCheck the pump is pumping at the minimum fuel pressure. If not replace fuel filter element. Prime and vent pump and piping. If problem persists replace the pump.
13Bad quality fuelDrain out the fuel, replace the fuel filter and fill the tank with clean, good and recommended quality fuel.
14Dirt or water in gas line, fuel systemCheck upstream equipment on gas supply, clean filter, check functioning of auto drains system closed valves and rectify as required.



  1. Engine stalls
1 Idling speed lowReset throttle adjustment screw until engine idles at manufacturers specifications
2 Weak battery and charging systemRecharge and test battery and charging system. If necessary, replace any defective part with a new one of the same type and capacity.
3Spark plugs dirty, or gaps incorrectly set, worn tipsClean or replace spark plugs. Adjust plug gaps to manufacturers specifications
4Coil defective with low outputReplace defective coil
5Improper ignition timingSet ignition timing to manufacturers specifications
6Leaks in ignition wiringReplace broken ignition wires or those with cracked insulation. Tighten all connections at coil and distributor. Check spark plug wires are secure in distributor cap and coil cover
7Burned or pitted intake and exhaust valvesReplace or reface and grind valves
8Engine overheatingCheck the root cause of overheating and rectify as required


  1. Engine stalls or stops at low speed or RPM
1Fuel pressure low– Check fuel level in tank, fill

– Check for leak(s) in fuel lines, rectify as necessary

– Check for air in fuel system and bleed off.

– Ensure delivery pressure of feed pump is as specified in manufacturers operations manual and replace fuel filter.

– If problem persists replace pump

2Engine idle speed lowAdjust throttle until engine idles at manufacturers specifications.
3Leaks in ignition wiringReplace broken ignition wires or those with cracked insulation. Tighten all connections at coil and distributor.
Improper ignition timingSet ignition timing to manufacturer’s specifications.
Burned or pitted valvesReplace or regrind valves.
Spark plugs dirty or gaps incorrectly set or wornClean and adjust plug gaps to manufacturer’s specifications. Replace plugs if badly damaged .


  1. Engine has no power
1 Incorrect ignition timing Check and reset ignition timing. Replace defective parts to correct the condition
2 Coil has low output Replace defective coil
3Defective distributorReplace or rectify distributor
4Worn or misadjusted pointsInstall and adjust new points
Spark plugs are dirty or wornClean or replace spark plugs. Adjust plug gaps
5Dirt or water in gas lineDisconnect lines and clear with compressed air.
6Partially plugged fuel filterReplace filter
7Low compressionReplace or reface and grind valves. Replace piston rings. Check cylinder for scoring and rectify as required
8Plugged or restricted exhaust systemCheck for excessive carbon in combustion chamber. Check the exhaust system and rectify the blockage
9Engine overheatingCheck the root cause of engine overheating and rectify as required.

5 Engine “skips” or misses at idle or low speeds

1Spark plugs dirty, damp or worn Clean or replace spark plugs. Adjust plug gap to manufacturers specification
2Moisture on ignition wires, cap or plugsDry the ignition system with compressed air or a clean, dry cloth. Remove the individual spark plug wires from cap; dry cavity and wire ends thoroughly. Inspect inside of cap and remove all traces of moisture and dirt
3Leaks in ignition wiringReplace broken ignition wires or those with cracked insulation. Tighten all connections at coil and distributor. Be sure the spark plug wires are secure in distributor cap.
Incorrect ignition timingCheck and reset ignition timing
Burned, warped or pitted valvesReplace or reface and grind valves




  1. Engine misses on acceleration
1Distributor points worn or incorrectly spacedClean and inspect contact points; if badly burned or pitted, replace points and condenser. Adjust point gap to manufacturers specification and then check timing
2Coil defectiveReplace defective coil
3Incorrect ignition timingCheck and reset ignition timing
4Spark plugs dirty, damp or wornClean or replace spark plugs. Adjust plug gaps to manufacturers specifications.
5Poor ignition wiresReplace faulty ignition wires
6Fuel injection malfunctioningTest fuel injection according to manufacturers procedure and replace necessary components
7Defective electronic ignition componentTest electronic ignition according to manufacturer’s procedure and rectify as required


  1. Excessive engine vibration
1Loose bolts and nutsIdentify the loose bolts and nuts in the engine and tighten. If the bolts or nuts are damaged, replace them. Identify the underlying cause for the bolts getting loosened and rectify.
2Loose, out of alignment components in the engineIdentify the loose/out of alignment components, identify the base cause for looseness or out of alignment and rectify problem
3Unbalanced rotating elementsCheck fan blades and other rotating components for unbalance and re balance where necessary
4Engine misfiring or running roughCheck engine for faulty firing and rectify as required
5Broken or loose foundation boltsMajor maintenance required to replace the broken foundation bolts


  1. Engine Noise (Piston)
1Piston pin too tight or too looseRefit piston pin as required
2Excessive piston to bore clearance Replace piston as require. Check cylinder walls for excessive wear and if necessary replace cylinder liners and install new pistons
3Collapsed piston skirtReplace piston and check cylinder for scoring; recondition cylinder as required



4Insufficient clearance at cylinder headRemove cylinder head and clean carbon from chamber, piston and valves
5 Broken piston or skirt Replace piston as required. Check cylinder for scoring and recondition as required.


            Valve noise

1Excessive valve clearanceCheck and adjust valves with engine at normal operating temperature
2Worn or stuck valve liftersReplace valve lifters. Check camshaft for pitting and wear
3Gum formation on valves resulting in stickingRemove gum from valve stems, reinstall and adjust
4Weak valve springsCheck valve springs and replace if required
5Damaged valve springs or locksReplace the damaged parts. Defective locks can cause the valves to drop inside cylinders, which can damage the engine extensively.
6Lack of lubricantCheck lubricant pressure, lubricant flow and passages. Correct as necessary
7Worn camshaftReplace camshafts and install new lifters


  1. Connecting rod noise
1Low oil pressureCheck the pump, relief valve setting internal drainage etc and correct as required
2Insufficient oil supply, thin or diluted oilCheck oil level in crankcase; if necessary, add/drain as required. Test for possible loose or damaged bearings
3Misaligned rodsCheck rod alignment. Replace bent rods. Check bearings for damage and rectify as required
4Eccentric or out of round crank-pin journal


Replace or regrind crankshaft journals. Replace with new undersize bearings if shaft is ground
5Excessive bearing clearanceReplace worn bearings as necessary.





  1. Main bearing noise
1Low oil pressureCheck the pump, relief valve setting internal drainage etc and correct as required
2Insufficient oil supply, thin or diluted oilCheck oil level in crankcase; if necessary, add/drain as required. Test for possible loose or damaged bearings
3Excessive bearing clearanceReplace worn bearings as required. Fit main bearings to manufacturers clearance
4Excessive end play in thrust bearingReplace thrust bearing, measure and replace crankshaft if required
5Eccentric or out of round journalsReplace crankshaft or regrind journals as required. If regrinding is done fit undersize bearings to suit the new journal size
  1. High oil consumption
1Gasket failure and leakCheck for damaged gasket at the location of leak and replace with new gaskets.
2Oil seal leakage at shaftsCheck for damaged oil seals at main bearing, timing gear etc, and replace seals. Check for shaft wear, reposition seal, rebuild shaft at the seal position or replace shaft as required.
3.Oil loss at piston rings–  Replace worn rings after inspection of cylinder walls. Worn, wavy or scored cylinder walls can increase oil loss.

–  Replace incorrect rings with new piston rings of correct size

–  Rebore out of round cylinders or replace liners.

–  Remove rings and clean ring slots on piston with suitable cleaning tool.

–  Replace rings stuck in piston grove and check clearance between ring and grove.

High oil consumption due to lubricating oil–  Check oil level and maintain correct level

–  Replace filter cartridge and filter, fill with correct grade of oil

–  Check oil relief valve and repair as required

–  Check all return drain holes, check for sludge in crankcase. Clean crankcase if sludge build-up and replace oil

External oil line leaksCheck for oil leaks at filter tubes, oil gauge lines. Replace tubing or fittings to correct the condition. Ensure filter is mounted properly.


  1. Broken piston rings
1Wrong type or sizeReplace rings. Check cylinder for scoring. Use manufacturers original rings.
2Excessive clearance due to worn pistonFit new piston and rings. Rebore cylinder or replace cylinder liner
3Ring striking top edgeRemove ridge and recondition cylinders or replace liner. Replace rings
4Worn ring groovesReplace pistons and rings, check cylinder wall for scoring and grooving and rectify as required.
5Broken ring glandsReplace pistons and rings, check cylinder wall for scoring and grooving and rectify as required.
6Insufficient end gap clearanceReplace rings with correct end gap as specified by the manufacturer. Check cylinder wall for damage and recondition as required.
7Excessive side clearance in grooveReplace piston. Check cylinder wall for damage and rectify as necessary.
8Uneven cylinder walls due to wearFit new piston and rings


  1. Broken piston
1Undersize pistonRecondition cylinder or replace liner and fit correct size pistons
2Eccentric or tapered cylindersRecondition cylinder walls and fit new piston and rings
3Misaligned connecting rodsFit new piston and rings, realign connecting rod. Rectify cylinder if damaged
4Engine overheatingIdentify the cause of overheating and rectify. Replace piston and rings. Rectify cylinder if damaged
5Excessive engine speed or loading at low RPMAvoid racing or lugging the engine, replace pistons and rings. Rectify cylinder if damaged
6Water or fuel leakage into combustion chamberCheck cylinder head, gasket and cylinder block for leaks and repair as necessary. Replace piston and rings. Rectify cylinder if damaged
7Detonation or pre-ignitionCheck for excessive spark advance and incorrect octane fuel. Fit new piston and rings. Rectify cylinder if damaged




  1. Engine has early wear
1Lube oil contaminationRemove dirty oil, flush the entire system and fill correct grade of fresh oil. Replace the oil filter
2Air inlet leakCheck the inlet system. Clean or replace filter. Repair any leaks in the inlet system.
3.Fuel or water leak into the lubricating oilIdentify the source of leak and carryout necessary repairs.


  1. Fuel consumption too high
1Fuel system leaksLarge changes in fuel consumption may be the result. Inside leaks probably will cause low engine oil pressure and an increase in oil level. Identify the leaks and rectify
2Incorrect timingMake an adjustment to timing
3Damaged combustion system internalsIdentify the damaged internals (valves, spark plug, piston rings etc) and repair or replace as required.


  1. Knock (mechanical noise in engine)
1Defect in connecting rod bearingsIdentify the damaged bearing and replace (engine overhaul)
2Damaged timing gearsReplace damaged gears and reset timing
3Damaged crankshaftDisassemble and repair engine


  1. Low oil pressure
1Defective oil pressure gaugeRecalibrate or replace oil gauge.
2Dirty oil filter or oil coolerReplace oil filter element. Check the filter bypass valve and repair if required. Clean the cooler tube bundle. If the cooling oil is dirty, replace the oil.
3Thin or diluted oilDrain, flush and refill with correct grade of oil.
4Excessive bearing clearance at the crank-shaft, camshaft and connecting rodInspect bearings and replace if necessary.
5Oil pump relief valve does not operate properly, spring brokenClean, replace damaged parts and recalibrate the valve to correct pressure


  1. Low oil pressure (continued)
6Worn oil pumpCheck and rebuild pump as required.
7Lube oil piping leakIdentify the leak and repair.


  1. Oil at the exhaust
1Too much oil in the exhaust compartmentOil seeping into the exhaust from valve compartment. Identify the source of leak and rectify.
2Worn valve guidesReplace valve guides, recondition the cylinder head.
3Worn piston ringsInspect the ‘O’ rings and replace with new rings


  1. Engine coolant too hot
1Not enough coolant in the systemAdd coolant to the system.
2Clogged radiator or heat exchangerClean the radiator or the heat exchanger. Check the cooling fan and fan belt. Check if the piping and hoses are clogged and clean or replace them.
3Defective temperature gauge, thermostat or temperature controllerIdentify the defective instrument and repair or replace as appropriate.
4Scale build-up in the cooling systemDescale the cooling system. Source and replace the existing cooling water with demineralised water.
5Defective cooling water pumpRepair the water circulation pump
6Too much load on the systemIdentify the cause of the load and correct the system as appropriate.


  1. Miscellaneous
1Damaged valve spring(s) or locksReplace spring(s). Check the clearances and set correct clearance. Check the locks in their slots and if the groves are enlarged, may need replacement of valves. Check other springs in the engine and replace if there are nicks, cracks or surface corrosion.
2Damaged or worn camshaftReplace camshaft (or cams if the cam alone is damaged), check the rocker clearance and reset with correct clearance. Check the lubrication to camshaft. Check the drive gear train
  1. Miscellaneous (continued)
3Little movement due to damaged rocker arms and push rods–   Make adjustments according to specifications

–   Check lube oil passages and flow in valve lifter assembly, rectify as appropriate

–   Check for wear on valve stem end, rocker arm face and push rods and replace worn item(s)

–   Check valve clearances and reset with correct clearance.

–   Check for wear and tightness in other associated parts.

4Damaged valve(s)Replace damaged valve(s). Check the cylinder combustion temperature, Check the valve setting and entire valve lifting assembly. Check the cooling around valves. Check inside the cylinder, piston head and inside of cylinder head for damage if the valve is fallen inside the cylinder. Check other valves if the damage is repetitive.
5Damaged piston and piston ringsReplace worn and broken rings. Free the jammed rings in piston, check and reinstall if found good. Check grove clearance in piston. Ensure ring clearances (both gap and clearances are correct)

Check the clearance between piston and liner for wear. If wear is excessive check piston lubrication, (both pressure and passages). Check coolant temperature

6Damaged piston pins and sleeve bearingPull out the pin. Check the bushing and pin contact area for wear with a micrometer. Replace if the wear is excessive. Check lubrication pressure and passages for clogging and clean as necessary.
7Connecting rod damageIf the rod is cracked, replace the rod. Check the connecting rod bore for out of roundness. Replace if found excessive out of roundness. Clear the oil passages
8Crankshaft damageIf broken or bent, replace crankshaft. Check web deflection using a deflection gauge. Check for scoring and wear. If mild scoring, dress the scoring using a oil stone. Protect the oil passages while dressing to prevent abrasives entering the oil passage. If heating or burning observed, replace crankshaft. If deflection is outside the acceptable range, recheck alignment if still outside the range, replace crankshaft.
9Faulty air starting system–  Check the starting air/gas charging system and service

–  Check the starting air tank RV’s and service them.

–  Check for clogged air passages and clean.

–  Check if the packing nut is over tightened on starting valve and ease off.

–  Check for stuck open valve and release.

–  Check for damaged or broken valve and springs, replace.

–  Completely disassemble the valve and service if problem persist.

–  Check the gas supply system. Make sure that the strainer is clean.

12Governor problems–   Check the control linkages and free them.

–   Check oil in the hydraulic reservoir and replenish.

–   Check oil leakage and repair leaks, replace leaky oil seals

–   Always refer to governor manufacturer’s service manual for servicing the governor.