On March 23, 2001, at 1845 central standard time, a Cessna 172RG single-engine airplane, N4823V, was substantially damaged during a forced landing following a loss of engine power near Waco, Texas. The airplane was registered to and operated by the pilot. The commercial pilot and passenger were not injured. Visual meteorological conditions prevailed, and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 personal flight. The cross-country flight originated from the Fort Worth Meacham International Airport, Fort Worth, Texas, at 1800, with a destination of Georgetown, Texas. The pilot reported that while in cruise flight, at 3,500 feet msl, the engine "began to vibrate excessively and would not produce enough power to maintain altitude." The pilot contacted Gray approach control and requested a heading to the nearest airport. Gray approach control vectored the pilot toward the McGregor Executive Airport near Waco with instructions to turn to a heading of 100 degrees. Approximately 5 miles west of the airport, the engine "quit completely and locked up," and the pilot initiated a forced landing. During the ensuing landing, the airplane touched down in a field, struck a fence and a ditch, nosed over, and came to rest inverted. Examination of the aircraft by the FAA inspector, who responded to the accident site, revealed that the vertical stabilizer was crushed, the was empennage twisted, and the left wing strut was bent. In addition, the inspector found that the engine's number four cylinder lower spark plug housing was separated from the cylinder. On April 26, 2001, NTSB investigators and a representative of the engine manufacturer examined the Lycoming O-360-F1A6 (serial number RL-31901-36A) engine at Air Salvage of Dallas, near Lancaster, Texas. Examination of the engine revealed that the number four exhaust valve separated from its stem and was found "wedged" in the spark plug hole. The number four exhaust valve push-rod and push-rod tube were bent aft at the cam follower end. According to the engine maintenance records, the engine was field overhauled on June 22, 1999, and had accumulated approximately 450 flight hours since overhaul at the time of the accident.