Incident Summary
A pilot reported that the engine quit while he was en route to a job. He subsequently landed the aircraft in an alfalfa field. During the landing roll, the aircraft collided with a ditch that was obscured by the crops. A postcrash fire erupted and consumed the aircraft.
Aircraft Examination
An FAA airworthiness inspector examined the aircraft after the fire. The inspector reported that the engine crankshaft could be rotated by hand; however, no corresponding valve action was noted on any cylinder. This indicated a disconnection between the crankshaft and the valvetrain.
Engine Findings
The inspector observed a disassembly of the engine and reported that the eight screws securing the crankshaft hub to the crankshaft gear had sheared. The crankshaft gear is responsible for rotating all accessory drive gears, including the magnetos. The shearing of these screws likely resulted in the loss of valve actuation and magneto operation, consistent with the reported engine power loss.