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 comes with a MOS2 coating on the piston skirt to reduce wear. VERTEX is an OEM supplier for some of the largest motorcycle manufacturers and is trusted by many factory teams in the MXGP for its performance. The piston kit includes piston ring(s), piston pin, and circlip! Piston version 4-stroke: Pro Replica = forged piston that matches the original form, Pro HC = forged special piston (higher compression and performance enhancing)