A cylinder liner is a device that it pressed into an engine block and house the piston. it is much harder than the engine block and prevents the piston from earing through the cylinder bore. Typically used in aluminum engine blocks and diesel engines, the cylinder liner is either pressed into position or held in lace 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.