As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHEN CHECKING MAGNETOS DURING ENGINE RUN-UP PRIOR TO TAKEOFF, RIGHT MAGNETO GAVE A 350 FOOT RPM DROP ACCOMPANIED BY A VERY ROUGH RUNNING ENGINE. AFTER RIGHT MAGNETO WAS REMOVED FOR TROUBLESHOOTING, CRACKS WERE DISCOVERED ON THE TOP AND BOTTOM OF THE HOUSING EXTENDING FORWARD APPROXIMATELY 1.50 INCHES FROM THE FLANGE OF THE IMPULSE COUPLING.