Due to the higher cylinder pressure of 4-stroke engines, pistons are generally made from forged pieces. To withstand the internal pressure of the engine, VERTEX pistons are forged from the highest quality aluminum alloy (2618). Each VERTEX piston features a MOS2 coating on the piston skirt to reduce wear. VERTEX is an original equipment manufacturer for some of the largest motorcycle manufacturers, and many factory teams in the MXGP trust the performance of VERTEX. The kit includes the piston along with piston rings, piston pin, and circlips! Piston design for 4-stroke: Pro Replica = forged piston corresponds to the original shape, Pro HC = forged special piston (higher compression and performance-enhancing).