There are engine blocks in which the cylinders are made of the material of the block, and there are also blocks in which separate cylinder sleeves are used. The cylinder liners must be sealed with rubber rings when fitting into the block, because the engine oil in the crankcase at the bottom must not mix with the coolant at the cylinder walls. There may be cases where the factory-cast coated cylinder wall wears out or is damaged, in which case it is customary to fit a retrofit steel bushing into the block that is more durable. Special cylinder bushings are also used for motor tuning, which replaces the factory cast bushings.