What happened
During flight, the pilot observed a gradual reduction in engine power. Deciding to land immediately, he selected a gravel bar for an emergency touchdown. As the aircraft touched down, it rolled into a shallow slough. The left main landing gear struck a submerged log, causing the nose gear to impact the bank at the edge of the water. This sequence forced the plane to roll onto its back. The engine stopped completely when the airframe inverted.
The investigation
Post-accident examination of the engine revealed that the No. 3 rocker arm exhaust valve adjustment bolt had failed and was loose within the valve cover. Evidence suggested the bolt may have been overtorqued during previous maintenance. The correct torque specification for this bolt is 450 inch-pounds. Inspection of the remaining bolts showed inconsistent values ranging from 150 to 450 inch-pounds. The engine had accumulated only 3.7 hours of operation since its last top overhaul.
Findings
The primary factor was the failure of the No. 3 rocker arm exhaust valve adjustment bolt. This mechanical failure led to a loss of power, necessitating an emergency landing in unsuitable terrain. The subsequent impact with a log and bank caused the aircraft to overturn.