As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
RT MAGNETO FAILED TO SUSTAIN ENG OPS DURING GROUND RUN-UP CHECK. UPON INTERNAL VISUAL INSP, FOUND BROKEN MAGNETO SHAFT BELOW DRIVE GEAR. ALUMINUM CASTING SPLIT AT STEEL SHAFT MATING JOINT. ROTATION OF STEEL SHAFT IN ALUMINUM CASTING YIELDED EXCESSIVELY RETARDED TIMING. MAGNETO WAS "FACTORY NEW" WITH 286.7 HOURS TOTAL TIME AND HAS NOT BEEN OPENED PRIOR TO THIS EVENT. A REPLACEMENT "FACTORY NEW" MAGNETO OF THE SAME TYPE WAS INSTALLED TO RESOLVE THE PROBLEM.