What was found
(CAN) THE ENGINE DEVELOPED A SLIGHT VIBRATION INFLIGHT. THE ENGINE OIL PRESSURE & VACUUM SUCTION DROPPED TO ZERO. THE STANDBY VACUUM RESTORED SUCTION. THE ENGINE OPERATING TEMPERATURE INCREASED BUT STAYED IN THE GREEN. THE MANIFOLD PRESSURE WAS NORMAL. POST FLIGHT INSPECTION REVEALED THAT A CRANKSHAFT COUNTERWEIGHT PIN RETAINER PLATE HAD FAILED ALLOWING THE PIN TO FALL INTO THE CRANKCASE, THE LOOSE PIN FRACTURED AN INTAKE VALVE PUSH ROD CREATING A REDUCTION IN OIL PRESSURE. THE VACUUM PUMP SHAFT SHEAR BUSHING ALSO FAILED. NO OBVIOUS CAUSE FOR THE RETAINER PLATE WAS DETERMINED.