Cylinder & piston

How it works

The cylinder is the bore in which the piston moves up and down. The piston takes the force of combustion and passes it to the crankshaft through the connecting rod. To keep everything sealed, piston rings sit in grooves in the piston: they hold compression above and stop oil from climbing into the combustion chamber. The cylinder is either cast iron or aluminium with a very hard coating (such as Nikasil). As it wears, piston-to-bore clearance grows: compression drops, the engine loses power, burns oil and may smoke blue. Overheating, lack of oil or a lean mixture can cause a seizure: the piston grabs in the bore, leaving deep scoring. On small 2-strokes, the cylinder-and-piston kit is a routine wear part; on a 4-stroke, the job is far bigger. Diagnosis starts with a compression test, done on a warm engine with a compression gauge.

Maintenance tips

• ✓Do a compression test if the engine loses power, burns oil or starts poorly: it is the basic measure of top-end condition.
• ✓Follow a gentle running-in after a new kit or rebuild: no full load or long constant revs for the first kilometres.
• ✓On a 2-stroke, use suitable oil at the right ratio: lack of oil seizes the engine.
• ✓A clattering noise when cold that fades when warm can come from excessive piston clearance: have it diagnosed.
• ✓Opening the top end needs tools and precise measurements: on a 4-stroke, leave it to a professional.