On September 6, 2004, approximately 1745 mountain daylight time, a Cessna 182, N2725G, piloted by a private pilot, was substantially damaged during a forced landing 8 nautical miles southwest of Gillette, Wyoming. Visual meteorological conditions prevailed at the time of the accident. The personal flight was being conducted under the provisions of Title 14 CFR Part 91 without a flight plan. The pilot reported no injuries. The cross-country flight originated at Casper, Wyoming, at 1700. According to the accident report submitted by the pilot, he was in cruise flight when the engine began to run rough. In a telephone conversation with the pilot, he stated he heard sounds of grinding metal. Shortly thereafter, the "engine began to shake violently." The pilot shut the engine down and attempted a forced landing to a plowed field. During the forced landing, the nose wheel impacted a small rut, separating the nose landing gear tire from the nose landing gear assembly. The airplane nosed over wrinkling the fuselage, crushing the vertical stabilizer and bending both wings. The engine was examined at Mobile, Alabama, on February 22, 2005. An external examination showed no apparent damage. The top and rear seams of the crankcase and the accessory drive housing showed evidence of oil leakage. The crankcase seam at the front portion of the engine, just below the propeller flange gasket, also showed evidence of previous oil leakage. The gasket area between the front crankcase and the front of the oil sump showed oil leakage. Oil covered the bottom of the oil sump. An internal examination of the engine showed the number 3 piston, piston rings, and tension springs, and the number 3 exhaust valve head had fragmented into numerous pieces. These pieces were located in the oil sump. Approximately 2 tablespoons of oil was recovered from the sump. The inside of the crankcase halves showed impact damage in the areas aft of the number 4 piston and forward of the number 3 piston. The inside walls of the crankcase were significantly blackened. The top inside of the number 3 cylinder head showed numerous 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 which were consistent with a valve head edge impact. The number 3 exhaust valve seat was not present. The number 3 exhaust valve stem was removed and examined. Fractures observed on the valve stem were consistent with overload failure. The number 3 exhaust valve guide was elongated. An examination of the crankshaft, camshaft, other cylinders, pistons, valves, and valve springs showed no pre-existing failures. The rocker box housings, rockers, and springs, were oil stained. No evidence of the number 3 exhaust valve seat was found among the fractured pieces in the engine. The left and right magnetos were bench-tested. Both magnetos' leads consistently fired within the range of 300 to 3,000 rpm. According to maintenance records, the engine had accumulated approximately 200 hours since the annual inspection when the accident occurred. Previous to the annual inspection, the airplane had not been flown within the previous 10 years. An examination of the remaining airplane systems, conducted by the FAA, revealed no anomalies.