The cylinder liner also known as cylinder sleeve, is fitted into the engine block as the inner wall of a cylinder, forms a sliding surface for the piston rings while retaining the lubricant within. Cast iron is the material used for cylinder liners. There are three types of liners, dry, wet and finned cylinder liners.
Dry cylinder liners are thin. They do not interact with the engine coolant. Instead they provide a very close fit with the jacket in the cylinder block to protect the piston from heat and impurities.
Wet cylinder liners come in direct contact with the engine coolant. Sometimes the wet cylinder liners are fitted with tiny openings to help disperse the heat and impurities. These types of liners are called water-jacket liners which are simply a type of wet cylinder liner.
Finned cylinder liners are designed for air-cooled engines. They operate much like the dry cylinder liner in that the cooling medium for the motor is air. However, these liners are fitted with tiny fins which allow the inflowing air to draw with great force around the cylinder to provide cooling.