On December 12, 1994, approximately 1330 mountain standard time, N93290, a Commonwealth Rearwin 185, was substantially damaged during a forced landing in Longmont, Colorado. The commercial pilot was not injured. Visual meteorological conditions prevailed. The following is based on the pilot report. The pilot planned to practice touch and go landings. After takeoff and while on the downwind leg, the engine "sputtered" and he immediately turned towards the runway. The engine then seized and the propeller stopped turning. Unable to glide to the airport, the pilot made a forced landing in an open field. During the landing roll, the airplane struck a depression and the landing gear was torn off. After examining the airplane, a Federal Aviation Administration inspector reported the oil drain plug was missing and there was no evidence of oil in the crankcase. The engine had accumulated five hours time-in-service since undergoing a major overhaul.