Sheared flywheel keys are caused by impact to the pto, loose tightened flywheel nuts, grease or dirt on flywheel tapers or damage to the tapers. What first needs understanding is the flywheel key is nothing more then an aligment aid, it has little strength in terms of retaining that alignment. The friction of the flywheel and crankshaft taper fit, being for forced together by proper torquing of the flywheel nut, is what holds the flywheel in alignment with the crank. If this alignment slips, the flywheel magnets will pass the ignition coil at the wrong time, and ignition timing will be off.

This taper force fit is compromised when oil or grease is applied by someone thinking, "I'll make it easier to remove the next time". The tapers must be clean to work right. The proper torque on the retaining nut must be applied or the proper force on those tapers will be not enough or if too tight it can alter, spread, the taper making it ineffective.

Using the improper method of flywheel removal can warp the taper of the flywheel, distorting the taper so it doesn't make good contact. Any damage to the tapers will cause the aligment to go off and shear the key, even normal engine operation can throw it off if damage or improper torquing is applied, or if lube is added. Minor taper damage can be overcome by lapping.

Apply fine grit valve lapping compound to the taper surfaces. Set the flywheel on the crankshaft without the aligment key. Then, much as you'd lap in a valve, lap in the flywheel to the crank. Clean the compound off and check for high and low spots. When you have good surface contact all around on both parts, clean the tapers with brake cleaner or solvent and install the key and flywheel with the proper retaining nut torque for your engine.

If you are unable to gain a good surface on both parts, leave the valve grinding compound (water based compound only) on the tapers to act as additional friction bearing ability. If sheared flywheel keys continue to be a problem due to warped or damaged tapers, either or both the crankshaft or flywheel will need to be replaced.

LMRM; Bob :<=