Event
During cruise flight at 2,500 feet, the pilot reported feeling increasing vibration in the airplane. The engine then 'froze' and could not be restarted. The pilot conducted a forced landing in a freshly plowed field, which was soft. During the landing roll, the airplane nosed over.
Postaccident Findings
A postaccident engine teardown revealed that the exhaust valve on the #4 cylinder had loosened and fallen into the cylinder. The airplane had accumulated 6 hours of flight time since its most recent annual inspection and 77 hours since the engine underwent a major overhaul.
Additional Information
No other factors were identified in the source material. The aircraft, its registration, operator, and location were not specified. No injuries or damage beyond the nose-over were reported.