What was found
ENGINE OPERATION NORMAL ON TAKEOFF, CLIMB, AND CRUISE SETTING. ROUGHLY 15 MINUTES INTO FLIGHT, THE ENGINE LOST MANIFOLD PRESSURE. FULL THROTTLE MANIFOLD PRESSURE WAS 25 INCHES. PILOT LANDED WITHOUT INCIDENT. INSPECTION FOUND THE NR 4 CYLINDER HAD CRACKED ON THE EXHAUST SIDE OF THE CYLINDER HEAD AND HAD STARTED MELTING THE NR 6 CYLINDER HEAD ASSY. THE TWO AFFECTED CYLINDER ASSEMBLIES WERE REPLACED. THE OIL FILTER INSPECTED AND REPLACED. NO OTHER DEFECTS NOTED. THE AIRCRAFT WAS RETURNED TO SERVICE.