Sleeve

A cylinder liner (sleeve) is the replaceable cylindrical surface inside the engine block in which the piston runs.

It gives the piston and rings a hard, wear-resistant wall to slide against and helps transfer combustion heat to the coolant; worn liners can be replaced without scrapping the block.

Liners sit in the engine block, one per cylinder. The correct liner matches the engine model, bore size, and type (wet or dry).