Piston Assembly complete BSA C10/C11/C12 Piston Std. incl. rings, gudgeon pin, circlips STD= 63mm Piston Clearance 0.12mm or 45thou C11 1939-55 C12 1955-58 can also be used for C10, even if they usually had a flat topmeasurements see last pict. please and techn. data