Small Engines (Lawn Mowers, etc.): Flywheel key shear, valve lapping compound, briggs stratton

Question
I have a 12.5 HP Briggs & Stratton I/C engine that shears the flywheel key upon starting.  The starter teeth on the flywheel were damaged and were chewing up the teeth on the starter.  I filed out the rough spots on the flywheel and am wondering whether the flywheel is now out of balance.  Will an unbalanced flywheel cause the key to shear?  

Answer
It could shear keys but not very likely.  Are you torquing the flywheel with a torque wrench?  Is there a flat washer that sits below the flywheel nut?

One trick that may help is to use some valve lapping compound and lap the flywheel taper to the crankshaft taper.  Just put a small about of lapping compound on the crankshaft, a little in the flywheel hole then place the flywheel on the crankshaft and rotate by hand.

Make sure you clean all the lapping compound off the crank and flywheel when you're finished.

How old is the engine and what is is on?

Let me know.
Eric