Due to the higher 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 a MOS2 coating on the piston skirt. VERTEX is an OEM supplier for some of the largest motorcycle manufacturers, and many factory teams in MXGP trust the performance of VERTEX. The kit includes a piston along with piston ring(s), piston pin, and piston pin clips! Piston Type 4-stroke: Pro Replica = forged piston matches original shape, Pro HC = specially forged piston (higher compression and performance-enhancing)