On July 9, 2004, about 1300 central daylight time, a Beech BE-N35, N80HL, sustained substantial damage during a forced landing following a loss of engine power in cruise flight near Austin, Minnesota. The private pilot and three passengers were not injured. The 14 CFR Part 91 flight was operating in visual meteorological conditions on an instrument flight rules flight plan. The flight departed from the Ankeny Regional Airport, Ankeny, Iowa, at 1215, and was en route to the Chetek Municipal-Southworth Airport, Chetek, Wisconsin. The pilot reported the airplane had been operating properly at 5,000 feet mean seal level (msl) until the propeller started to overspeed between 2,950 - 3,025 rpm and the overspeed annunciator light illuminated. He attempted to slow the propeller with the propeller pitch control but without effect. The oil pressure gauge read zero. He reduced the throttle to about 15 inches of manifold pressure to slow the engine to 2,500 rpm. He declared an emergency with Rochester Approach Control and received radar vectors to Austin, Minnesota. About 30 seconds to 1 minute later, the airplane began to shake and vibrate violently, white smoke filled the cabin, and oil covered the windshield. The pilot reported that he decided to execute a forced landing to one of the flat farm fields below. He lowered the gear and established about a 90 knot descent. He shut down the engine by pulling the mixture to idle cutoff. He slipped the airplane to lose altitude prior to landing. The airplane skidded in the mud for about 80 yards. The pilot and passengers exited the airplane normally after it came to a stop. The landing gear was found partially extended and the landing gear circuit breaker was popped. The on-site inspection of the engine revealed a hole in the right side of the engine case. The engine, a 260 horsepower Continental IO-470N, was shipped to an engine overhaul facility for an engine teardown. The teardown revealed that number 1 cylinder piston pin plug was fractured in two pieces. The smaller of the two pieces was deformed with substantial material loss with the fracture surface being obliterated. The number 4 cylinder piston pin plug rotated within the piston pin. Metal contamination was found in the oil pressure relief valve and valve seat. There was a hole in the engine case next to the number 5 cylinder. The number 5 connecting rod was broken and the end of the rod was "hammered" and blackened. The number 5 rod bearing was scored and blackened. The number 1, 4, and 6 rod bearings exhibited melting and protruding. All main bearings were scored. The oil pump gears turned freely, and the sides of the pump walls were scored. The number 1 and number 4 cylinder piston pins and piston pin plugs were sent to the National Transportation Safety Board's Materials Laboratory for examination. The examination revealed that the number 1 cylinder piston pin plug had a fracture surface with visible crack arrest lines, features consistent with fatigue. Black deposits were observed on the head of the larger piece of the number 1 piston pin plug and on the ends of the number 4 cylinder piston pin plugs, although the deposits were less thick than on the number 1 piston pin plug. An impression was observed on the larger piston pin plug piece in the bevel between the head and the shank. The impression was located on the same side of the piston pin plug as the fatigue origin. The surface of the impression was covered with black deposits. The number 4 cylinder piston pin plug rotated within the number 4 piston pin. The plug could be rotated by hand about 90 degrees relative to the piston pin. The inspection revealed corrosion pits around the circumference of both piston pins. The pilot reported the engine had 1,000 hours of operation since the last overhaul, and 47.7 hours of operation since the last inspection. The engine did not have an oil filter installed on it. The number 1 and number 4 cylinder piston pins and piston pin plugs were released to Myers Aviation, Inc., Oshkosh, Wisconsin.