As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE THE ACFT WAS IN FOR MX, WHILE ATTEMPTING TO START THE ENGINE, A METAL GRINDING NOISE WAS HEARD BY THE MECHANIC. SUBSEQUENT REMOVAL OF THE STARTER ADAPTER REVEALED AT LEAST 2 TEETH MISSING ON THE CRANKSHAFT GEAR AND MARRED TEETH ON THE ADAPTER. THIS CONSTITUTES AND INTERNAL ENGINE FAILURE. THE THEORY IS THAT THE STARTER INDUCED A STRESS FATIGUE FRACTURE ON THE CRANKSHAFT GEAR.