Event Overview
While in cruise flight at approximately 3,700 feet mean sea level, the pilot heard a loud 'pop' followed by a complete loss of engine power. The pilot performed a forced landing to a field, where the airplane struck a rock and came to rest inverted.
Engine Examination
Post-accident examination of the engine revealed that the crankshaft had separated between the number two and number three connecting rods. Additionally, the main bearing from the number two cylinder had shifted rearward. Material transfer between the bearing and the area of the crankshaft near the fracture was consistent with the bearing rubbing against the crankshaft.
Maintenance Records Review
A review of maintenance records from the facility that previously performed work on the airplane indicated that all six cylinders were replaced approximately 100 hours of operation before the accident. One of the through-bolts was found to be at a torque setting lower than the specified value. However, the investigation could not determine whether the bolt was not at the proper torque before the accident or if the impact during the forced landing had loosened it. The owner of the maintenance facility stated that the bolts were set to the correct torque.
Conclusion
The investigation identified a fractured crankshaft and a shifted main bearing, but did not establish a definitive cause for the separation.