A cylinder liner is a device that is pressed into an engine block and houses the piston. It is much harder than the engine block and prevents the piston from wearing through the cylinder bore. Typically used in aluminum engine blocks and diesel engines, the cylinder liner is either pressed into position or held in place by the cylinder head. In large engines such as the engines found in diesel locomotives, the liner is part of an assembly containing a new piston, piston rings and a connecting rod and is changed as a complete unit during scheduled maintenance or repair.
In aluminum engine blocks, the block material is much too soft to house a piston. The friction of a piston moving up and down inside the alloy block would soon wear out, resulting in a loss of compression and severe oil consumption. The steel cylinder liner is pressed into the engine block and then the engine block is machined to assure that the cylinder head mating surface is smooth and flat. By machining the engine block to receive a steel liner, the engine is able to operate for many years without engine failure.
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The flat surface resulting from the machining of the engine block assures a proper seal of the head gasket between the cylinder head and the engine block. An improperly sealed head gasket will result in over-heating of the engine, loss of power, and the potential to ruin the block and cylinder head. Great care must be taken when installing a cylinder liner into an aluminum engine block since the aluminum block will react to heat at a different rate than the steel cylinder liner. Improper fit at installation could result in a broken liner or a cracked engine block.
There are also instances when a cylinder liner can be used to repair a cast iron engine block that has suffered a catastrophic failure to a cylinder wall. Often, when an engine fails or "blows up," the cylinder wall takes the brunt of the trauma rendering it unrepairable by over-boring. In this instance, the block might be machined to receive a liner and then all of the cylinders could be over-bored to the same size, rendering the engine useful once again. For street driven vehicles, this is a viable option which typically offers the vehicle owner a significant savings over the cost of a replacement engine.