As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING GROUND RUN, OPERATOR NOTED A LOUD BANG AND SUDDEN OIL LEAKAGE. AFTER SHUT DOWN FOUND THAT LT MAGNETO CAME OFF THE ACCESSORY HOUSING, CAUSING OIL LEAK. BANG WAS CAUSED BY MAG HOUSING WHICH CAME APART AT UPPER ATTACHMENT AREA (ATACHMENT LUG).