What was found
(CAN) AFTER TAKEOFF, IN CRUISE FLIGHT, THE PILOT NOTICED THE LT ENGINE TACHOMETER NEEDLE FLUCTUATING. SHORTLY AFTER, THE ENGINE STARTED SURGING AND A NOTICEABLE VIBRATION WAS FELT. THE AIRCRAFT RETURNED, AN INVESTIGATION REVEALED THE LT ENGINE MAGNETO, UPPER BEARING, P/N 10-382971, HAD SEIZED IN THE DISTRIBUTOR BLOCK P/N 10 -682058. THIS IS THE FIRST INCIDENT OF THIS NATURE AND WITH OVER 15,000 HOURS ON THIS ENGINE TYPE, FEEL NOTHING COULD HAVE PREVENTED THIS OCCURRENCE.