On July 24, 1993, at approximately 2220 central daylight time, a Cessna 172N, N4884G, was substantially damaged when it collided with rough terrain during a forced landing near Purcell, Oklahoma. The forced landing was executed following a partial loss of engine power. The airplane was on a night cross country dual instructional flight. A VFR flight plan was filed and visual meteorological conditions prevailed throughout the area. Neither the instructor pilot nor the rated student pilot were injured. According to the instructor, the engine lost power down to 1,500 RPM while in cruise flight, about twenty minutes after takeoff from the Westheimer Airport in Norman, Oklahoma. The instructor stated that his efforts to regain power were unsuccessful and the airplane would not sustain level flight at the reduced power. He subsequently executed a forced landing to what appeared to be an open field. The airplane impacted rough terrain during the landing roll. The number 3 cylinder exhaust rocker arm attachment stud had separated at the face of the cylinder head. The separated stud was found lodged under the number 3 intake valve mechanism. The number 3 intake valve push rod was bent. The operator stated that the stud was examined by a metallurgy lab at the University of Oklahoma and the failure mechanism was determined to be fatigue. The engine and subject parts had accumulated a total of 2,903.5 hours. Neither the instructor nor the student submitted a pilot/operator accident report. Both subsequently left the country.