The pilot reported that about 30 minutes after takeoff, a partial loss of power was experienced and was accompanied by the sounds of metal-to-metal contact. He headed toward an airport about 12 miles away, but shortly thereafter, total engine failure occurred. A forced landing was made in a corn field. An examination of the engine revealed that the No 4 exhaust valve had failed. Excessive wear of the valve guide and valve stem had occurred. There was evidence of poor heat dissipation from the valve and uneven/high seating velocities on the valve head. The engine had accumulated 2156 hours of flight time and had never been overhauled. The manufacturer recommend overhauls at 2000 hour intervals and oil changes at 50 hour intervals. The engine log book indicated the oil was changed at about 200 hour intervals. Lycoming Service Bulletin No 388A recommended checking valve guide clearance at mid-point of service time or earlier, but the service bulletin had not been complied with.