Puch ZA50 21 Tooth Sprocket Fit Up

I wanted to share my experiences on fitting up a 21 tooth front sprocket onto a ZA50 motor.  I know a lot of Hawaii tuners like GotRigid? been using them for years but I never got a chance to run them because they weren't as readily available back then as they are now.  I did some research on the normal forums and asked our local Puch community on methods used for fit up.  Here's my lessons learned:




Measure Twice, Grind Once!!

1. Install your 21T front sprocket onto your ZA50 motor.
2. Run the chain on the top of the sprocket until it hits up against the case.  (Note:  Some chains are different in width.  I'm running a standard sized chain from Treats.)
3. Mark the width of the chain where it starts hitting the case.  This will be the approximate width you'll need to grind.  Repeat for the bottom side of the sprocket.
4. Remove the 21T and clearly mark the areas you need to grind on the motor block.

Grind That Thing!!

1. With a Dremel, carefully grind the areas you marked.  Use a sandpaper drum grinding bit.  Start from 60 grit and work down to 120 grit.  REMEMBER TO CAREFULLY GRIND!!
2. Continuously check your progress by installing the sprocket and running the chain.  After your satisfied with the fit, clean up the ground areas by hand-sanding.

I took off more than I expected.  I also tried to grind off as little as possible to provide the tightest fit for my chain.  What remained is a extremely thin magneto wall.

UPDATE:  I ruined my grind job when I was disassembling my transmission.  The 10mm socket punctured through the wall when I was removing that pesky counter-threaded screw without the use of any factory tools.  Learned the hard way how thin that wall became after my grind work.  The thinnest part was towards the center of the grind.  It punctured through into the magneto compartment.  The good thing was the area it punctured through was into a nice part of the magneto compartment where it would be easy to fix.  If your gonna grind through, try to make your mistake in this area.  Patching it was easy.  I did my repairs while the motor was on the bike.  That wasn't ideal but still doable.  I'll probably redo the patch when I rebuild my motor.

1. Prep hole for epoxy repair (or similar; I used JB Weld).
2. Put a masking tape cover on magneto side compartment for epoxy fill.
3. Fill hole with epoxy.  Use a generous amount.  Fill until epoxy kind of oozes out of hole.  Check magneto side to make sure epoxy is not oozing out through the taped cover.
4. Quickly place masking tape to cover hole.  Tape entire ground area.  Its OK for epoxy to spill over the exterior ground area.
5. Allow epoxy to cure per manufacture specifications.
6. After dry, remove tape on the magneto side and exterior.
7. Clean any excess epoxy on the interior or exterior with sand paper.  Don't over do it especially the exterior side.  Let the chain remove the rest.

If I had to do it all over again, I would epoxy fill the magneto compartment before grinding the exterior for sprocket fit up.  Ideally I would do this while the motor was off the bike.  By filling the magneto compartment it would   (1)  Give support to the thin magneto wall after grinding for fit up and  (2)  If you do go through the wall, the epoxy repair would already be there.

That's my lessons learned.

So get modifying and happy tuning!!