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 original equipment manufacturer for some of the largest motorcycle manufacturers, and many factory teams in the MXGP rely on VERTEX performance. The piston kit includes piston rings, a piston pin, and a piston pin retainer! Piston type 4-stroke: Pro Replica = forged piston conforms to original shape, Pro HC = forged specialty piston (higher compression and performance-enhancing).