Due to the higher cylinder pressure of 4-stroke engines, the piston is generally manufactured 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, every VERTEX piston has a MOS2 coating on the piston skirt. VERTEX is an original equipment manufacturer for some of the largest motorcycle manufacturers, and many factory teams in the MXGP also rely on VERTEX's performance. The piston kit includes a piston, piston rings, piston pin, and piston pin circlip! The 4-stroke piston design: Pro Replica = forged pistons conform to the original shape, Pro HC = forged special pistons (higher compression and performance-enhancing).