This is a 54mm front caliper piston for a Triumph TR6 from CC29930 to 1976 with 16PB type calipers. The 16PB caliper piston can be checked by looking for the 1/8 Dust Boot Slot. Two are needed per caliper. 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 separate, you can't buy a harder caliper piston. Sold as a set of 4