On July 6, 1997, at 2000 central daylight time (cdt), a Grumman- Schweizer, G-164B, N6633K, operated by a commercial pilot, sustained substantial damage following a loss of engine power during the aerial application flight. The pilot subsequently force landed the airplane in a field, 3 miles north of Donnelly, Minnesota. During the landing, the airplane nosed over. Visual meteorological conditions prevailed at the time of the accident. The aerial application flight was being conducted under 14 CFR Part 137. A flight plan was not on file. The pilot reported no injuries. The local flight originated at Morris, Minnesota, at 1930 cdt. In his written statement, the pilot said that he was in a turn to line up on the field he was about to spray when the "number 9 cylinder blew." The pilot turned towards a road which he intended to use for landing the airplane on. On approaching the road he "noticed a person on the road." The pilot abandoned the attempt to land on that road and began to maneuver for a landing on a perpendicularly intersecting road. As he was maneuvering, the "engine failed completely." The pilot put the airplane down in the field. During the landing roll, one main landing gear leg broke. The airplane nosed over coming to rest inverted in the field. The Federal Aviation Administration (FAA) inspector who examined the airplane said that the right wing had broken off forward of the attach point. The lower wing spar was broken. The vertical stabilizer and rudder were crushed inward and bent over to one side. The engine had broken off of the engine mounts. A large amount of oil was observed on the exterior of the airplane. Examination of the engine revealed the number nine cylinder head broken into three pieces. The number nine cylinder connecting rod was also found broken. Flight control continuity was confirmed. No other anomalies were found with the airplane.