Due to the higher cylinder pressure of 4-stroke engines, the 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 OEM supplier for some of the biggest motorcycle manufacturers, and many factory teams in the MXGP rely on VERTEX's performance. The piston kit includes piston rings, piston pin, and piston pin lock! Piston type for 4-stroke: Pro Replica = forged piston matches the original form, Pro HC = forged special piston (higher compression and performance-enhancing)