Small Engines (Lawn Mowers, etc.): Flywheel Key Shearing, briggs and stratton, hp briggs

Question
I have a 12.5 HP Briggs and Stratton engine that shears the flywheel key upon starting.  This started happening after I removed the flywheel to file out some rough places that were eating up the gear of the electric starter.  I am wondering if the shearing is being caused by the flywheel being out of balance.  I did not remove a lot of metal so I question whether this is the case.  Can an unbalanced flywheel cause the flywheel key to shear?

Answer
I suspect the problem is related to a loose drive pulley on the other side of the engine.  This pulley can be loose because of a worn key (or keyway), bent badly or a loose retaining bolt.  This can shear the key.  You would have to take a pile of material off the flywheel to put it that far out of balance....so I doubt that's the problem.  Hope this helps.