Cylinder liners are the interior metal components within the piston that protect it from the wear and tear of the operation of the motor. Three basic types of liners are used: hot, dry and finned. The purpose of each type is to protect the piston from heat and impurities using slightly different methods. Cylinder liners are expensive, precisely manufactured products and are primarily purchased from speciality shops.
Dry Cylinder Liners
Dry cylinder liners are among the basic piston protectors. They must withstand extremely high temperatures and guard against impurities, so they are constructed of high-grade materials, such as cast iron and ceramic-nickel plating. Dry liners are much thinner than their counterpart, wet liners. They do not interact with the engine coolant but instead provide a very close fit with the jacket in the cylinder block to protect the piston from heat and impurities.
Wet Cylinder Liners
Wet cylinder liners protect the pistons in a different way than dry ones, but they are made from the same hardy material. They 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 but are simply another type of wet cylinder liner. If the liner doesn't have a cooling jacket, one is created by the liner by interacting with the jacket present in the cylinder block.
Finned Cylinder Liners
Finned cylinder liners are constructed of the same type of heat and impurity-resistant metal. This type of liner is designed for the air-cooled engine, and in operation works 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.