The Incident
The pilot reported that while in cruise flight, the engine began to run rough. Shortly thereafter, he heard sounds of grinding metal and the engine started shaking violently. He shut the engine down and initiated a forced landing to a plowed field.
Forced Landing and Damage
During the forced landing, the nose wheel impacted a small rut, which caused the nose landing gear tire to separate from the assembly. The airplane nosed over, resulting in substantial damage to the aircraft.
Engine Examination
An internal examination of the engine revealed that the number 3 piston, piston rings, tension springs, and the number 3 exhaust valve head had fragmented into numerous pieces. These pieces were found in the oil sump. The crankcase halves showed impact damage in areas aft of the number 4 piston and forward of the number 3 piston. The top inside of the number 3 cylinder head exhibited multiple impact marks, including a 7/8-inch long, 1/8-inch wide, and 1/8-inch deep indentation at one of the spark plug openings, and a 1/2-inch long, 1/8-inch wide, and 1/8-inch deep indentation at the area of the exhaust valve seat. These marks were consistent with a valve head edge impact. The number 3 exhaust valve seat was missing.
Additional Findings
The number 3 exhaust valve stem was removed and examined. Fractures observed on the valve stem were consistent with an overload failure. The number 3 exhaust valve guide showed elongating distortion. Examination of the crankshaft, camshaft, other cylinders, pistons, valves, and valve springs revealed no pre-existing failures. No evidence of the number 3 exhaust valve seat was found among the fractured pieces in the engine. Examination of the remaining airplane systems revealed no anomalies.