This isn't something at all familiar. From the final appearance of the keyway it seems clear it was not a vibration problem. Briggs engines rotate clockwise viewed from the flywheel end, and as I read it, the side of the keyway that is damaged is the trailing side, not the leading side. It looks as if something stopped the flywheel and the crankshaft inertia caused the damage, not the much more usual situation where the crankshaft stops and the flywheel keeps going. Could something have jammed the flywheel?
