Pilot Report
The pilot stated that a preflight inspection showed the oil capacity was full (8 quarts). Approximately two hours into the flight, while cruising at 10,500 feet, the engine began running rough. He notified air traffic control (ATC) and attempted corrective actions, which were unsuccessful. Shortly afterward, the engine developed severe roughness, and he heard a bang accompanied by smoke entering the cockpit. He declared a mayday with ATC and requested vectors to the nearest airport. Unable to land there, he performed a forced landing on a nearby highway. During the landing roll-out, the airplane collided with a vehicle.
Engine Examination
Examination of the engine revealed that the No. 3 cylinder had separated from the crankcase. All cylinder studs and/or thru-bolts exhibited various forms of fatigue. The No. 3 cylinder connecting rod bolts were fractured, showing evidence of either bending or shear overstress. The No. 2 cylinder connecting rod bearing was found welded to the crankpin, and a piece of aluminum-lined bearing material was present inside the oil passage of the No. 2 cylinder crankpin journal. Fretting was noted at the No. 3 cylinder contact area and at the split-line surface of the right crankcase half at the main bearing locations. The most significant fretting occurred at the cylinder contact area between studs Nos. 1 and 2, and at the split-line surface on the right crankcase half at the No. 3 main bearing location. The engine had accumulated 1,390 hours since major overhaul and approximately 873 hours since the No. 3 cylinder was removed and reinstalled.