Cylinder (Cylinder Block)

A cylinder is the round bore in which the piston moves up and down, compressing the mixture and receiving the force of combustion. The casting that contains the cylinders is called the cylinder block, or on many motorcycles, especially singles and twins, the cylinder barrel. It is usually aluminium with cooling fins (air-cooled) or water jackets (liquid-cooled).

The number and layout of cylinders, such as single, parallel twin, V-twin, boxer, triple or inline four, strongly affect the engine's character, smoothness and width. In two-strokes the cylinder also contains the transfer and exhaust ports.

Bores wear over time, and most modern cylinders have a hard Nikasil or similar coating rather than a sleeve. Wear, scoring or out-of-round bores cause compression loss and oil burning; a worn cylinder may be re-coated, re-sleeved or bored oversize, depending on its construction.