On July 26, 1998, approximately 0915 mountain daylight time, a Piper PA-18, N1252C, sustained substantial damage when it nosed over in a field during a precautionary landing south of Columbus, Montana, after the pilot noted that the engine was running roughly. The private pilot, the sole occupant, sustained minor injuries. Visual meteorological conditions prevailed for the flight, which had originated at Laurel, Montana about 0830. There was no report of an ELT actuating. The pilot stated that the engine was running rough, and he elected to make an off-field precautionary landing. He stated that he had checked the carburetor heat and magnetos, and had pumped the throttle; however, his measures were insufficient to improve the situation. He elected to make a precautionary landing in a hayfield; braking for the short field and obstructions resulted in the airplane nosing over. FAA inspectors inspected the airplane and engine after it was removed from the accident scene and found no evidence of a mechanical problem that could have resulted in a rough-running engine.