Due to the higher cylinder pressure of 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). 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 MXGP trust the performance of VERTEX. The kit includes the piston, piston ring(s), piston pin, and piston pin retainer! 4-Stroke piston execution: Pro Replica = forged piston corresponds to original form, Pro HC = forged special piston (higher compression and performance enhancing).