Due to the increased cylinder pressure in 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). To reduce wear, each VERTEX piston features an MOS2 coating on the piston skirt. VERTEX is an original equipment manufacturer for some of the largest motorcycle manufacturers, and many factory teams in MXGP rely on VERTEX's performance. The piston kit includes the piston, piston ring(s), piston pin, and piston pin clips! The 4-stroke piston design: Pro Replica = forged piston corresponds to the original shape, Pro HC = forged specialty piston (higher compression and performance-enhancing).