What was found
LT MAGNETO FAILED TO SUSTAIN ENG OPS DURING GROUND RUN-UP CHECK. UPON INTERNAL VISUAL INSP, FOUND BROKEN MAGNETO SHAFT BELOW DRIVE GEAR. ALUMINUM MAGNET CASTING FAILED AT STEEL SHAFT MATING JOINT. ROTATION OF STEEL SHAFT IN ALUMINUM CASTING YIELDED EXCESSIVELY RETARDED TIMING. MAGNETO WAS "FACTORY NEW" WITH 350.9 HOURS TT AND HAS NOT BEEN OPENED PRIOR TO THIS EVENT. A REPLACEMENT "FACTORY NEW" MAGNETO OF THE SAME TYPE WAS INSTALLED TO RESOLVE THE PROBLEM. FOR PRECAUTIONARY REASONS, THE RT MAGNETO WAS OPENED AND FOUND TO HAVE THE SAME CRACK BEGINNING. IT WAS ALSO REPLACED WITH A "FACTORY NEW" MAGNETO OF THE SAME TYPE.