This is a 48mm front caliper piston for Spitfire 1968 to 1980 and Jaguar XJ 1974 to 1987. Two are needed per caliper for the Spitfire and four are need for the Jaguar. The pistons are Plasma Nitrided which will give you the life of Stainless at half the price. The Nitriding becomes a part of the metal and will not flake off or seperate, you can't buy a harder caliper piston