Accident Overview
The pilot-in-command was initiating the descent to the destination airport when engine problems were reported. The engine, which was running roughly, eventually failed completely. The pilot performed a forced landing to an open field. During the subsequent landing, the aircraft encountered a ditch, resulting in the collapse of the nose gear.
Engine Examination
Examination of the engine revealed that the exhaust valve of one of the cylinders had failed due to fatigue. A metallurgical examination determined that the failure was a low-stress, high-cycle fatigue failure of the exhaust valve stem, occurring near the valve face.
Background
The engine had been completely overhauled 60 operational flight hours prior to the failure. The valves used during the overhaul met the manufacturer's specifications.