The higher cylinder pressure in 4-stroke engines generally requires pistons to be 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 an MOS2 coating on the piston skirt. VERTEX is an OEM for some of the largest motorcycle manufacturers, and many factory teams in the MXGP trust the performance of VERTEX. The piston is delivered including piston ring(s), piston pin, and piston pin lock! Piston type for 4-stroke: Pro Replica = forged piston conforms to original shape, Pro HC = forged special piston (higher compression and performance-enhancing).