The increased cylinder pressure in 4-stroke engines generally necessitates that pistons are 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 biggest motorcycle companies, and many factory teams in MXGP rely on the performance of VERTEX. The piston kit includes piston ring(s), piston pin, and piston pin circlip! The Pro Replica 4-stroke piston version refers to a forged piston that matches the original shape, while the Pro HC version refers to a forged special piston which is designed for higher compression and performance enhancement.