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). Each VERTEX piston features 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 the MXGP trust the performance of VERTEX. The piston comes with piston ring(s), piston pin, and piston pin clips! Piston Design 4-Stroke: Pro Replica = Forged piston corresponds to the original shape, Pro HC = Forged special piston (higher compression and performance-boosting).