50cc QMB139 4-stroke 50mm, Racing Big Bore Cylinder Set. Kit includes cylinder, piston, rings, piston pin, cir clips, head gasket, and base gasket. Performance items are not returnable. Please see our Warranty Returns Policy. We recommend that you use our QMB139 performance cylinder head (see Related Items) with this set as some stock cylinder heads will not fit. Use with an un-modified stock head can cause serious engine damage.*You may need to modify your engine case to accommodate the larger cylinder.