Due to the higher cylinder pressure in 4-stroke engines, the piston is generally 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 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 MXGP rely on the performance of VERTEX. The piston kit includes piston ring(s), piston pin, and piston pin circlip! Piston Type for 4-Stroke: Pro Replica = forged piston corresponds to the original form, Pro HC = special forged piston (higher compression and performance boosting).