With 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). To reduce wear, each VERTEX piston features a MOS2 coating on the piston skirt. VERTEX is an OEM for some of the largest motorcycle manufacturers, and many factory teams in MXGP rely on the performance of VERTEX. The kit includes a piston, piston ring(s), piston pin, and piston pin retainer! 4-stroke piston execution: Pro Replica = forged piston matches the original form, Pro HC = forged special piston (higher compression and performance-enhancing).