Flight History
The pilot reported that after takeoff, the flight climbed to 7,500 feet mean sea level (msl). Approximately 40-45 minutes into the flight, the engine began to shake. The pilot repositioned the fuel selector, which slightly decreased the shaking. The shaking continued, and the pilot spiraled down to 3,500 feet msl, where the shaking worsened. The pilot glanced at the cylinder head temperature gauge, oil pressure gauge, and oil temperature gauge, noting that all were within the green range at that time. The pilot searched for a landing area and, due to cars on a nearby roadway, landed in a ditch adjacent to the road.
Engine Examination
A representative of the engine manufacturer, under FAA oversight, examined the engine. The examination revealed that the No. 2 cylinder connecting rod and one of the connecting rod bolts were fractured. The other connecting rod bolt was not fractured and remained secured to the connecting rod cap and a section of the connecting rod. Additionally, the No. 2 crankpin was dry but not heat discolored, and the oil port for the No. 2 crankpin was free of obstructions.
Metallurgical examination of the fractured components by the NTSB Materials Laboratory revealed that the fracture surface of the connecting rod bolt exhibited features consistent with tensile overstress. The bolt was determined to meet specification related to hardness. Mechanical damage to the fracture surface of the connecting rod precluded determination of the failure mode.
Maintenance History
Review of maintenance records showed that the engine was last overhauled on March 29, 1977, and installed in the accident airplane on April 15, 1977. The engine was disassembled, inspected, and reassembled on July 15, 1977, or approximately 50.0 hours since the overhaul in March of that year. The engine was then installed in the airplane, test run, and found to be airworthy. Cylinders 1, 4, and 6 were removed and replaced on April 4, 1981. At the time of the accident, the engine had accumulated approximately 1,652 hours since the last overhaul.