As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING ENGINE RUN-UP, BEFORE TAKEOFF WHEN THE RT ENGINE, RT MAGNETO WAS CHECKED IT WAS FOUND TO BE INOPERATIVE. ACFT RETURNED TO BLOCK. RT ENGINE, RT MAGNETO WAS REMOVED AND FOUND HARD TO ROTATE BY HAND. OPENED MAGNETO AND FOUND TO BE SCORCHED AND MELTED DUE TO ROTOR BEARING FAILURE. INSTALLED NEW MAGNETO AND ACFT WAS RETURNED TO SERVICE.