With the increased 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 is equipped with an MOS2 coating on the piston skirt to reduce wear. VERTEX is an original equipment manufacturer (OEM) for some of the largest motorcycle manufacturers, and many factory teams in the MXGP rely on VERTEX performance. The piston kit includes the piston, piston ring(s), piston pin, and piston pin circlip! Piston Versions for 4-Stroke: Pro Replica = forged piston corresponding to the original shape, Pro HC = forged special piston (higher compression and performance-enhancing)