Due to the higher cylinder pressure in 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 features a MOS2 coating on the piston skirt to reduce wear. VERTEX is an OEM supplier for some of the largest motorcycle manufacturers, and many factory teams in MXGP rely on VERTEX for their performance. The kit includes pistons, piston ring(s), piston pin, and piston pin circlip! Piston types for 4-stroke engines: Pro Replica = Forged piston equivalent to the original form, Pro HC = Forged special piston (higher compression and performance enhancing).