Engine Failure
About 30 minutes after departure, the engine began to run rough and lose power. The pilot-in-command attempted to regain full power and return to the departure airport. As the airplane continued to descend, the pilot decided to land in a hayfield located approximately four miles from the airport.
Forced Landing
During the forced landing, which occurred with a tailwind, the pilot observed power lines ahead. He maneuvered the airplane onto rough terrain beneath the power lines. Upon touchdown, the airplane nosed over and sustained substantial damage.
Post-Accident Examination
An examination of the engine revealed that the exhaust valve stem on the No. 4 cylinder had separated from the valve head. The fracture surface was obliterated, preventing determination of the failure mode. Further damage to other cylinder components occurred as the engine continued to operate in a degraded condition after the valve failure. The No. 4 cylinder had been reworked approximately 108 flight hours prior to the accident.