On June 26, 1997, at 0915 mountain daylight time, N6844X, a Cessna 172A, operated by the owner/pilot, nosed over during a forced landing and was substantially damaged near Ogden, Utah. The forced landing was precipitated by a loss of engine power during cruise flight. One private pilot (first pilot) received minor injuries, and the other private pilot (second pilot) was not injured. Visual meteorological conditions prevailed and no flight plan had been filed. The personal flight departed from the Ogden-Hinkley Airport on a local flight about 0845, and was en route back to the airport when the accident occurred. The flight was conducted under 14 CFR 91. In a written statement to the Safety Board, the first pilot (pilot-in-command seated in the left front seat) stated that he was flying at 4,400 feet about 15 miles north of Ogden when the engine "...started missing." Engine power began to decrease despite the pilot's efforts to regain full engine power. The airplane then began to descend under partial power. The pilot declared an emergency with the Air Traffic Control (ATC) tower at Ogden and attempted to return for an emergency landing there. The pilot further stated: Called Ogden Tower [and] told them I was having power problem and did not know if I would make it back to Ogden. Called Ogden [again and] told them I was going to land in a hay field in Slaterville. Attempted to land in hay field, but [the landing area] was obstructed by wires. Attempted to go under wires, but...ground was to close. According to a Federal Aviation Administration (FAA) aviation safety inspector from Salt Lake City, Utah, the second pilot stated that both pilots knew that they would not be able to make it to the Ogden-Hinkley Airport, so they decided to attempt a landing on a private grass airstrip located about four miles north of the airport. During the forced landing, which was performed during a prevailing tailwind, the airplane was forced downward by the first pilot onto a hayfield. It then nosed over during the landing roll. The inspector examined the accident site and reported that fuel was found on board the airplane. The wreckage was then moved to facilitate a detailed examination of the engine. During the examination, a fractured exhaust valve was found in the no. 4 cylinder assembly. The valve pieces and the entire cylinder assembly was removed from the engine and sent to the Safety Board's Materials Analysis Division in Washington, D.C. for further analysis. According to the Safety Board's Materials Laboratory Factual Report (attached), the fracture features of both valves were completely obliterated. One of the valves was missing its stem. One of the push rods "... revealed circumferential scratches below one of the ballends." The report also noted that "dissimilar wear patterns... were observed on the rocker to valve stem contact faces." Corrosion pits were found in several locations on the exterior of one of the rocker shafts. Damage was also found on the valve seat and valve spring seats. According to FAA records, the aircraft N6844X, was manufactured in 1960. It was acquired by the registered owner in July 1995. The airplane was powered by a single 145-horsepower Continental model O-300-C engine. The Safety Board recovered and examined the maintenance records (excerpts attached) of the airplane. The examination revealed that the engine and airframe had accumulated 2,566 hours of operating time at the time of the accident. The engine had accumulated 866 hours since its last major overhaul (SMOH), and 20 hours since its last 100-hour inspection on April 12, 1997. The records revealed the following recent entries: August 5, 1994 Engine Time SMOH = 711.3 hours No. 5 cylinder "impaired" / Compression test of cylinders indicate "all within manufacturers recommended limits." November 1, 1994 Engine Time SMOH = 758.09 hours No. 2 and no. 4 cylinders "removed due to low compression" and "reworked" February 6, 1995 Engine Time SMOH = 807.96 No. 2 cylinder removed "due to venting exhaust valve" December 3, 1995 Engine Time SMOH = 833.81 Engine receives annual inspection / All cylinders exhibit acceptable compression values during compression check. April 12, 1997 Engine Time SHOM = 846.33 Engine receives 100-hour inspection / All cylinders exhibit acceptable compression values during compression check except for no. 5 cylinder. No. 5 cylinder removed for "venting exhaust valve" / No. 5 cylinder reinstalled after rework / No metal found in oil screen. An examination of the maintenance records also revealed that all of the aforementioned cylinder repair work occurred at an FAA repair station in Salt Lake City, Utah, known as Precision Air-Power. According to FAA records, both pilots held Private Pilot certificates with a rating in airplane single engine land airplanes. Both pilots also held expired FAA Class III medical certificates and had not received any flight reviews during the two years prior to the accident.