Due to the higher cylinder pressure of 4-stroke engines, the pistons are generally forged. To withstand the internal pressure of the engine, VERTEX pistons are forged from the highest quality aluminum alloy (2618). To reduce wear and tear, each VERTEX piston is coated with a MOS2 layer on the piston skirt. VERTEX is an original equipment manufacturer for some of the largest motorcycle manufacturers, and many factory teams in the MXGP trust in the performance of VERTEX. The kit includes the piston, piston ring(s), piston pin, and piston pin clip! 4-stroke piston models: Pro Replica = forged piston replicates original shape, Pro HC = forged special piston (higher compression and performance-enhancing).