This is a 39 mm outer diameter piston for 50cc GY6 139QMB scooter engines The 39 mm piston is a stock size for many Chinese built 50cc GY6 139QMB gas scooter engines, however you should carefully check the dimensions on the Specs tab to see if this is the correct 39 mm outer diameter piston for your scooter Description This is a 39 mm outer diameter piston for 50cc GY6 139QMB scooter engines. The 39 mm piston is a stock size for many Chinese built 50cc GY6 139QMB gas scooter engines, however you should carefully check the dimensions on the Specs tab to see if this is the correct 39 mm outer diameter piston for your scooter. The Chinese built 50cc GY6 139QMB engine has been in production by many brands and manufacturers over the years, and it is safe to say that these little pistons are powering millions of scooters around the world.,Specifications Outer Diameter: 39 mm Overall Height: 33 mm Piston Pin: 13 mm Compression Height: 17.25 mm Circlips included