The cylinder, being part of the combustion chamber, is subjected to extremely high temperatures. Since excessive heat can seriously distort the shape of the cylinder or cause piston seizure, the outside of the cylinder is finned to increase the heat radiating surface for better cooling efficiency. To minimize distortion from heat and to maximize durability, the cylinder is made of special cast iron.

The piston is made from an aluminum alloy, which expands and distorts slightly from heat during engine operation. So that the piston will become cylindrical after heat expansion, it is designed such that, when cold, it is tapered in towards the head and is elliptical rather than perfectly round. The piston diameter is made so that there is enough clearance between the piston and cylinder to allow for expansion. Two rings are fitted into grooves near the top of the piston so that gas does not escape between the piston and the cylinder wall into the crank chamber.

The full floating type of piston pin is used to connect the piston to the con-rod. The middle part of the piston pin passes through a caged needle bearing fitted into the small end of the con-rod, and a snap ring is fitted at each end of the piston pin in a groove to prevent the pin from coming out. Since the pin is the full floating type, a small amount of clearance exists between the piston pin and the piston when the engine is at normal operating temperatures.

Proper inspection and maintenance of the cylinder and piston include checking the compression; removing carbon from the piston head, piston ring grooves, and cylinder exhaust port; and checking for wear and proper clearance during top end overhaul. Heavy carbon deposits in the combustion chamber raise compression, which results in overheating, detonation, and preignition. A worn cylinder, worn piston, or worn or stuck piston rings cause a loss of compression from gas blowby past the rings since the rings will not form a satisfactory seal between the piston and cylinder wall during compression. This gas blowby will result in difficult starting, power loss, excessive fuel consumption, and possibly engine destruction. A worn piston pin causes piston slap, which will result in accelerated piston and cylinder wear.

Engine problems may be caused not only by carbon deposits and wear or damage to the engine itself, but also by poor quality fuel or oil, improper oil, improper fuel/air mixture, improper supply of oil, or incorrect ignition timing. Whenever knocking, pinging, piston slap, or other abnormal engine noise is heard, the cause should be determined as soon as possible. Neglect of proper maintenance will result in reduced engine power and may lead to accelerated wear, overheating, detonation, piston seizure, and engine destruction.

Compression Measurement

A compression test is very useful as an aid in determining the condition of the engine. Low compression may be due to cylinder wear; worn piston ring grooves; worn, broken, or sticking piston rings; cylinder head leaks; or damage to the engine such as piston seizure. Too high a compression may be due to carbon build-up on the piston head and cylinder head.

Before measuring compression, check that the cylinder head is tightened down to 0.6 – 0.9 kg-m (4.5 – 6.5 lb-ft) of torque, and then thoroughly warm up the engine so that engine oil between the piston and cylinder wall will help seal compression as it does during normal running. While the engine is running, check that there is no gas leakage from around the spark plug or the cylinder head gasket.

Stop the engine, remove the spark plug and screw the compression gauge hose securely into the spark plug hole so that there will be no leakage. With the throttle fully open so that air can flow freely to the engine, turn the engine over by the kick starter until the compression gauge stops rising. The compression is the highest reading obtainable.

Cylinder Compression

Cylinder, Piston Decarbonization

Carbon readily accumulates around the cylinder exhaust port, which reduces exhaust efficiency. To remove the carbon, take off the cylinder, and scrape the carbon out of the exhaust port carefully. At this time, the muffler should also be inspected, and cleaned if necessary.

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