Cylinder walls can become very worn or damaged from use. If the engine is not equipped with replaceable sleeves, there is a limit to how far the cylinder walls can be bored or worn before the block must be sleeved or replaced. In such cases, the use of a sleeve or liner can restore proper clearances to an engine. Sleeves are made out of iron alloys and are very reliable. A sleeve is installed by a machinist at a machine shop. The engine block is mounted on a precision boring machine, where the cylinder is then bored to a size much larger than normal and a new cast-iron sleeve can be inserted with an interference fit. The sleeves can be pressed into place, or they can be held in by a shrink fit. This is done by boring the cylinder (between 3 and 6 thousandths of an inch) smaller than the sleeve being installed, then heating the engine block and while hot, the cold sleeve can be inserted easily. When the engine block cools down it shrink fits around the sleeve holding it into place. Cylinder wall thickness is important to efficient thermal conductivity in the engine. When choosing sleeves, engines have specifications to how thick the cylinder walls should be to prevent overworking the coolant system. Each engine’s needs are different, dependent on designed work load duty cycle and energy produced. After selecting and installing the sleeve, the cylinder needs to be finish bored and honed to match the piston. Care needs to be given to the finish of the cylinder walls to prevent improper ring seating at break in.