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). To reduce wear, each VERTEX piston features a MOS2 (molybdenum disulfide) coating on the piston skirt. VERTEX is an OEM supplier for some of the largest motorcycle manufacturers, and many factory teams in the MXGP also trust the performance of VERTEX. The piston kit includes the piston ring(s), piston pin, and piston pin lock! For 4-stroke engines: Pro Replica = forged piston matching the original form, Pro HC = forged specialty piston (higher compression and performance enhancing).