This kit includes everything you need to upgrade your 50cc 4stroke to 72cc. Many places commonly call this an 80cc kit. Bore diameter is 47mm. This is over a 60% increase in engine size. Expect overall engine performance to improve almost as much. Great deal. Fits most GY6 scooter ATV 139qmb ( QMB139 ) engines. Check your engine number on your case. Works great with your stock head, and bolts right up with no modifications. Super deal, no one is lower than this!