Incident Overview
The pilot of a single-engine aircraft reported that while cruising at an altitude of 7,000 feet, the engine began vibrating, lost power, and eventually stopped. The pilot elected to perform a forced landing in an open field, which resulted in substantial damage to the aircraft.
Engine Examination
Post-accident examination of the engine revealed that the intake valve stem on the number three cylinder had failed. Further metallurgical testing indicated that the valve failed due to excessive hardness in a localized area. This condition created a micro-crack, which served as the stress riser for a fatigue failure.
Conclusion
The engine failure was caused by a fatigue failure of the intake valve stem, initiated by a micro-crack resulting from localized excessive hardness in the valve material.