Modern Vespa

Scooter was humming along fine about 300 miles after the engine rebuild and suddenly things went south fast...

Long story short, the woodruff key on the flywheel had sheared in half, the flywheel spun and correct timing was a distant memory!

So, a couple of questions to the more experienced folk on here:

1) How common is this? The broken key was new when the engine was rebuilt.
2) Could there be another reason for the key to break that, if left unattended, will simply lead to another break?

Background info: PX125 with BGM177 kit, new crankshaft, new flywheel, new ignition, bearings, etc. Not set up as a performance scooter, so no crazy HPs...

I have found in my last two engine builds that new woodruff keys have been on the small side. This has been when fitting to a new crank too. Usually a woodruff key is a snug fit. A used one should take some tapping to get it out of the crank.

The taper locks the FW to the shaft. The key just indexes the two for timing. It is designed to be a sacrificial element.

You should be able to run a motor without the key if it's indexed properly.
A loose ( thinner) key will work but can throw the accuracy of the timing off.

The only way a key can shear is if the tapers are not mated correctly and/or the nut holding the FW loosens up. Never run a motor without torquing the nut.

( How many have sheared a key just setting and checking the timing with the FW on and the nut 'tight' but not torqued ?)

*Raises hand*

If you don't have a torque wrench so you know how much torque 45 ft-lbs is, you're most likely not getting the flywheel torqued down enough.

* Raises both hands *

Me too and we need to bring up LAPPING in the taper w/ fine valve grinding compound, especially after a sheared key! If the taper is not a perfect fit it won't do the job and you'll likely shear another key.

I like to use a fine flat file going lightly over the crankshaft first to remove any obvious burrs and high spots. You need to use a small amount keeping it away from the seal. Then gently rotate the flywheel until the pattern on the crank is uniform and ~95% lapped in visually. Be sure to clean well & remove all the grinding compound after.

Also make sure the nut is in good condition and bottoms all the way on the crank threads. I had one once that I finally discovered got tight BEFORE getting tight on the flywheel because of buggered threads on the crank. That ate another key before I figured it out and dressed the threads w/ a metric thread file.

no comment

So you didn't lap the flywheel onto the crank?
It happens.... but when you lap it in, then it WILL hold!


Ohh, I've got to use this one twice in a week!

Good/simple video on taper lapping. Absolutely essential knowledge.

&ab_channel=GrindingGearsGarage

As Voodoo said, it is important to level out any high spots / keyway damage beforehand so the tapers mate as well as possible. Otherwise, the FW can be cocked and you can be removing material where you don't want to.