Incident Description
Approximately 30 minutes after takeoff, the pilot reported a partial loss of power accompanied by sounds of metal-to-metal contact. The pilot initiated a heading toward an airport about 12 miles away, but shortly thereafter, total engine failure occurred. A forced landing was subsequently made in a corn field.
Engine Examination
An examination of the engine revealed that the No 4 exhaust valve had failed. Investigators found excessive wear on both the valve guide and valve stem. There was evidence of poor heat dissipation from the valve, as well as uneven and high seating velocities on the valve head.
Maintenance History
The engine had accumulated 2,156 hours of flight time and had never been overhauled. The manufacturer recommended overhaul intervals of 2,000 hours and oil changes every 50 hours. However, the engine log book indicated that oil was changed at approximately 200-hour intervals. Additionally, Lycoming Service Bulletin No. 388A recommended checking valve guide clearance at the midpoint of service time or earlier, but the service bulletin had not been complied with.