Important note: This piston is for a stock or oversized bore. Please measure your cylinder to check clearance before ordering. Pro X 2-Stroke Piston. ProX pistons are precision machined on the latest, state of the art CNC equipment. The same low-expansion, low-friction piston alloy is used as is used for all major Japanese and European engine manufacturers. ProX Piston Kits are a first class solution to replace the original piston in your machine. Most ProX pistons come with a coating. The O.E.M. style tin-coated pistons assist the wear-in process as when you first started the bike. ProX has developed a new skirt coating that not only assists the wear-in process, but also helps reducing friction after the wear-in. The black coating is made from a delicate mix of materials and will increase performance. Cast piston puts engine back to standard specs without boring or replacing cylinder.