Here are two pictures of the 11-tooth and 12-tooth sprockets to get an idea of the size difference.

11-tooth sprocket on top.

11-tooth

12-tooth

Arrow shows where the chain comes closest to the transmission case. In the second picture, you can see that the chain can not rotate freely. It will only fit when the sprocket is rotated so that the chain is “flat” against the transmission case. I had to remove the “lip” between the top and bottom parts of the engine.

11-tooth rotates freely (bad picture angle).

12-tooth stuck on lip

This is the grinding wheel that I used. As you can see, it attaches directly to a normal dill. It has worked extremely well for me in all of my applications so far. On the right, you can see that I removed the “lip” by using the edge of the grinding wheel to gently remove a little at a time. The thickness of the transmission case is plenty thick enough to withstand this little bit of metal removed. (see www.mopedjunkyard.com for a picture of motor case thickness.)

Lip of the motor case removed.

After my first round of grinding, I put the chain back on and tried to pedal the moped through one complete revolution of the chain. As you can see from this picture, the chain still touched the case. The left side of the chain touched more than the right side indicating that my grinding was not even. I evened out the surface and repeated with the revolution of the chain several times until I did not see any more oil marks on the transmission case. I made sure to remove a little at a time. You do not want to remove more than is necessary! Once I did not see anymore oil stains, I know that I did not need to grind any more. To be sure, I ground off just the slightest bit more and smoothed out the whole area for aesthetics. The 12-tooth sprocket fit like it was meant to be there! As you can see, I forgot to cover opening to the air intake (stupid me!!!). Oh well, a good carb cleaning never hurts . Also be sure to clean all surfaces of filings! You do not want to get ant of them into your engine!!!!

Left oil spot

Right oil spot.