What was found
DURING A PRE-TAKE OFF MAGNETO CHECK THE PILOT OBSERVED HIS #2 ENGINE RIGHT MAGNETO HAD A 1100 RPM DROP AND WAS FAILING HIS MAGNETO CHECK. THE PILOT THEN RETURNED THE BEECH BARON 58 TO MAINTENANCE. THE MAINTENANCE TEAM COMPLETED A RUN-UP OF THE AIRCRAFT CONFIRMING THAT THE MAGNETO HAD A PROBLEM. MAINTENANCE REMOVED THE EFFECTED MAGNETO AND OPENED IT FOR INSPECTION AND FOUND THAT THE MAGNETO TIMING GEAR HAD ENCOUNTERED THE COIL CASE GROUNDING WIRE AND RUBBED THROUGH IT OPENING THE CIRCUIT WHICH FAILED THE COIL. BECAUSE THE MAGNETOS WERE SO CLOSE TO THE 500-HOUR INSPECTION THE LEFT MAGNETO WAS ALSO OPENED FOR INSPECTION, IT WAS THEN FOUND THAT THE EXACT SAME PROBLEM EXISTED IN THAT MAGNETO. THE BACKGROUND ON THIS IO550-C71B WAS THAT IT WAS OVERHAULED BY TELEDYNE CONTINENTAL WITH A 478.7 TSO AND THE MAGNETOS WERE NEW AT OVERHAUL. THE LEFT MAGNETO P/N 10-500556-1 WITH A S/N OF D19GA114 AND THE FAILED MAGNETO WITH THE SAME P/N AND A S/N OF D19GA108.