O.S Engines cylinder and piston set have advanced Bi-Metallic Liner (ABL) cylinder liner construction. This process uses a double layer of plating material rather than one thick layer. This helps the metal bond to the cylinder and is more durable. Also, the plating process is more consistent, resulting in a more precise fit between piston and sleeve for better compression and longer life. Machined aluminum piston and 3-port cylinder liner.