What was found
(CAN) DURING THE SECOND GROUND RUN ON AN O/H ENGINE, THE ENGINEER SELECTED RT MAGNETO, THE ENGINE RPM DECAYED, AND WAS FOLLOWED BY A LOUD BACKFIRE. THE ENGINEER SELECTED BOTH MAGNETOS AND THEN SHUT ENGINE DOWN. AFTER AN INSPECTION, THE RT INTERNAL DRIVE GEAR HAD FOUND TO HAVE FAILED. TIME ON THE MAG WAS 0.0 TSO. THE RT MAGNETO, PN SB9RU-3, SN B90762 WAS REPLACED WITH AN O/H MAGNETO, PN SB9RU-3, SN BB28422 IAW MM. GROUND RUN PROCEDURES CARRIED OUT AND THE ACFT WAS RETURNED TO SERVICE.