Due to the higher cylinder pressure in 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 rely on VERTEX's performance. The piston kit includes a piston, piston rings, piston pin, and pin clips! The 4-stroke piston version: Pro Replica = forged piston matches original form, Pro HC = forged special piston (higher compression and performance-enhancing)