A cylinder liner is a replaceable cylindrical insert used in the cylinder block of a liquid-cooled internal combustion engine. The liner is a tube-shaped component with relatively thin walls, made from cast iron or steel. Cylinder liners come in two types:

• “Wet” liners, where the walls are directly cooled by the liquid coolant (antifreeze). These liners are primarily used in heavy-duty diesel engines of trucks. Typically, the outer surface of such liners has special grooves for precise installation in the cylinder block and for fitting sealing rings.
• “Dry” liners, which have thin walls and are used for repairing (sleeving) the cylinders of many truck or car engines. The outer surface of these liners is smooth, without grooves or channels, except for a possible outer flange at the top.

Regardless of the liner type, the internal surface is almost always subjected to special treatment—honing.

Liners typically have a long service life, but they may need to be replaced during a major engine overhaul. Additionally, a repair liner may be necessary if a cylinder suffers severe damage due to an accident.

Liners are usually designed for specific engines. However, in some cases, liners can be selected based on the required dimensions to restore the cylinder block.