Cylinder Liner (Sleeve)

A cylinder liner, or sleeve, is a tube, usually cast iron, fitted inside the cylinder block or barrel to form the surface the piston and rings slide on. It allows a light aluminium cylinder to have a hard-wearing bore.

Dry liners are pressed or cast into the block and don't touch the coolant; wet liners are directly surrounded by coolant and sealed with O-rings. Many modern motorcycle engines don't have a separate liner at all, but use a hard Nikasil or similar coating applied directly to the aluminium bore, which is lighter and dissipates heat better.

A worn sleeved cylinder can usually be bored and honed to an oversize piston, and when it reaches the limit a new liner can be fitted. Coated cylinders are usually re-plated by a specialist. Measure the bore with a bore gauge to decide which repair is needed.