On December 19, 1996, at 1421 central standard time, a Piper PA-46, N446RD, sustained substantial damage during a forced landing due to loss of engine power near Red Oak, Iowa. The private pilot was not injured. The 14 CFR Part 91 flight departed Milford, Iowa, en route to Red Oak, Iowa. Visual meteorological conditions prevailed and an instrument flight plan had been filed. The pilot reported that the temperature at takeoff was about zero degrees Fahrenheit. He climbed to 7,500 feet and the temperature remained at zero degrees. After about 30 to 45 minutes of flight, he started his descent into Red Oak, Iowa. During the descent, the prop started surging. The pilot reported he adjusted the prop control to try remedy the prop surge. After about a minute, the engine started knocking and quit. The pilot picked out a field to land in and executed a forced landing. The aircraft went through a fence during landing rollout. The pilot reported that the aircraft had been kept in an unheated hangar. A Tannis oil heater was installed on the aircraft and the pilot reported that it was working properly. He reported that when he departed, the engine oil temperature was in the green, but the oil pressure was in the yellow. He expected the oil pressure to lower into the green range as the engine became warmer during the flight, but it stayed in the yellow arc. The pilot continued the flight to Red Oak, Iowa, although suitable airports were available for precautionary landings. He reported that when the prop started surging, the oil pressure went "way down." He checked the engine after the accident and oil was all over the top of the engine. He checked the oil cap and it was on tight. A check of the oil quantity by an aviation mechanic revealed that about 4 quarts of oil still showed on the oil dipstick. The engine was examined. The only visible external damage was a hole in the crankcase above the number 2 and 4 cylinders. The were no signs of a seal leak. The crankshaft was intact, although the number 2, 3, 4, and 5 rod journals were discolored from heat damage. The number 4 rod had separated and was black in color. The main bearings were normal in color, but the babbitt exhibited signs of heat damage. The pistons were intact and had normal coloring. The piston skirts exhibited recent scoring. The piston rings were free. The spark plugs were normal. The connecting rods were inspected. The number 2, 3, and 5 connecting rod bearings were burned with signs of meltdown. The number 4 connecting rod and bearing had separated from the crankshaft and exhibited heat damage and failure. The number 1 and 6 connecting rod bearings exhibited less severe heat damage. The oil passages were checked and no obstructions were found. The oil cooler was checked and fluids flowed through the cooler.