The increased cylinder pressure of 4-stroke engines generally requires pistons to be 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 the original equipment manufacturer for some of the largest motorcycle manufacturers. Many factory teams in the MXGP also rely on the performance of VERTEX. The package includes piston ring(s), piston pin, and piston pin clips! Four-stroke piston design: Pro Replica = forged piston matching the original shape, Pro HC = forged special piston (higher compression and performance-enhancing).