Due to the higher cylinder pressure of 4-stroke engines, the piston is 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 the 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 piston, piston ring(s), piston pin, and piston pin circlips. For 4-stroke engines, Pro Replica equals a forged piston matching the original form, while Pro HC equals a forged special piston (higher compression and power-increasing).