My shaft is 10mm. The #25 sprocket from on old scooter I had has the same shaft, so it must be pretty common in china.
http://www.hobbycity.com/hobbycity/s...idProduct=5144 - This is supposed to be my motor, but it looks longer and has some screws in the bell now. The shaft might be 12mm on the newer ones... But mine is 10mm.