On October 24, 1995, approximately 1125 central daylight time, a Piper PA-28R-180, N7471J, operated by Airman Flight School, was substantially damaged when it collided with terrain during a forced landing near Dibble, Oklahoma. The commercial flight instructor received minor injuries, and the commercial pilot receiving flight instruction and another commercial pilot-rated passenger were not injured. Visual meteorological conditions prevailed and no flight plan was filed for this instructional flight being conducted under Title 14 CFR Part 91. The local flight originated at Norman, Oklahoma, on October 24, 1995, approximately 1 hour, 25 minutes before the accident. The following is based on the Pilot/Operator Report and a conversation the flight instructor had with a Federal Aviation Administration inspector. The pilots were practicing maneuvers at 3,500 feet msl (above mean sea level) when they heard a loud noise. The airplane began to shake and oil covered the windshield. The instructor took control of the airplane, secured the engine, and made a forced landing in a field. The airplane went through a fence, across a road, and collided with a ditch. The engine was disassembled and examined at Textron Lycoming and observed by a Federal Aviation Administration inspector. The #4 connecting rod was fractured in the bearing bore area. One connecting rod bolt was broken with the head end remaining with the end cap. The threaded end of this bolt and the nut were not recovered. The other connecting rod bolt was not recovered. A connecting rod nut was found embedded near the #1 cylinder bore. An imprint of the threaded end of a connecting rod bolt with a nut installed was observed near the #4 tappet bosses. One of the #1 connecting rod nuts was loose and the other was "somewhat tight." Examination of the undamaged fracture surfaces of the #4 connecting rod bolt disclosed a failure mode consistent with overload. Metallurgical examination revealed all failed components met design material specifications. It was the opinion of Textron Lycoming that the #2 and #4 top connecting rod nuts had loosened and had separated from their respective bolts. This caused the no. 4 bottom bolt to fail from overload. The no. 4 connecting rod then failed from high stress, low cycle fatigue. Examination of the maintenance records disclosed that at the time of the accident, the engine had accrued 3,083 total hours. The engine was overhauled on June 19, 1978, and received a top overhaul on April 18, 1991. At the time of the accident, the engine had accrued 1,894 hours and 1,096 hours since major overhaul and top overhaul, respectively.