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 TAKE OFF WHEN THE RIGHT ENGINE RIGHT MAGNETO WAS CHECKED THERE WAS EXCESSIVE RPM DROP. AIRCRAFT RETRUNED TO BLOCK. RIGHT ENGINE, RIGHT MAGNETO WAS REMOVED AND FOUND VERY HARD TO ROTATE BY HAND. OPENED MAGNETO AND FOUND NO VISUAL INDICATION OF CAUSE. IT IS BELIEVED THE ROTOR BEARINGS WERE FAILING. INSTALLED NEW MAGNETO AND RETURNED AIRCRAFT TO SERVICE.