On April 25, 1998, approximately 1330 mountain daylight time, a Cessna 172N, N6356D, operated by Aspen Air Corporation, was substantially damaged during a forced landing near Parker, Colorado. The flight instructor and his student were not injured. Visual meteorological conditions prevailed, and no flight plan was filed. The instructional flight was operated under Title 14 CFR Part 91 and originated at Englewood, Colorado, approximately one hour before the mishap. According to the flight instructor, he and his student were returning to the airport after an instructional flight. The instructor reported that while on an extended right base for runway 35R approximately 5 miles southeast of the airport, he "suddenly heard the engine lose a major portion of power." While performing an emergency check of the instruments, the instructor noted that the engine power had dropped to 1,800 rpm. Realizing they would be unable to glide to the airport due to the loss of engine power, the instructor executed an emergency landing in a field near a golf course. The ground was muddy due to an accumulation of rain and snow from the previous night. Upon touchdown, the airplane traveled a short distance before the nose wheel struck a rut left by a tractor, and the aircraft nosed over. This investigator examined the engine at the facilities of Beegles Aircraft Service, Greeley, Colorado, on May 18, 1998, and again on June 17, 1998. The no. 1 (right front) cylinder intake pushrod was bent. After removing the valve cover, it was discovered that the stud that anchors the exhaust valve rocker arm had fractured. The exhaust valve was in the closed position. There was no damage to the valve lifters and camshaft.