Piston liners and rings sit inside the cylinders of your main engine block. They prevent harmful gases from escaping the combustion chamber. They also help control the temperature of your engine. Manufacturers use piston liners to form a space that allows each piston to move up and down. The liner is subject to constant force, as well as exposure to high temperatures (over 2000 °C). As a result, the liner will degrade over. It is possible to replace worn piston liners on some (but not all) engines. You should therefore check that this is possible on your vehicle before making a purchase. Symptoms of worn piston liners are overheating, stalling and issues starting your car. However, the last two symptoms could also mean that the pistons themselves are worn and may need replacing. Piston rings are usually sold in packets of three. These rings form an airtight seal between the engine piston and the liner. While small in size, each piston ring plays an essential part in keeping oil out of the engine cylinders, even at high pressure. Each piston ring also helps to contain fuel and allows the combustion process to work its magic. Due to these extreme conditions, piston rings will wear over time. Signs of worn piston rings include white or grey exhaust smoke, and excessive oil consumption.