On July 1, 1995, about 1600 hours mountain daylight time, N4GY, a Cessna 210B, operated by the owner/pilot, was substantially damaged during a forced landing near Lewiston, Montana. The forced landing was precipitated by a loss of engine power while maneuvering. The private pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed and no flight plan had been filed. The personal flight departed Butte, Montana, on July 1, 1995, at 1445, and was conducted under 14 CFR 91. In a written statement, the pilot reported that he was maneuvering in mountainous terrain about 500 feet above ground level. The airplane passed over a ridge line and then dropped down into a canyon. The pilot stated that a "downdraft jolted the airplane severely enough to spill cherries from an open flat." The pilot then stated that the engine lost power. The electric boost pump was turned on and the fuel selector was switched to another fuel tank; however, the engine did not regain power. The pilot tried to lower the landing gear for the forced landing; however, the gear did not indicate down and locked even after utilizing the auxiliary gear pump. During the forced landing to an open rough field, the nose gear was sheared off, the belly was crushed inward, and the right wing was damaged. During an examination of the wreckage by FAA inspectors from Helena, Montana, fuel was found on board the airplane. The engine rotated, and compression developed in each cylinder. Rocker arm, valve train and accessory gear continuity was established. All spark plugs appeared unremarkable. The right magneto produced an intermittent spark due to internal contaminants. The left magneto produced a spark with hand rotation. Small amounts of contaminants (water) were found in one of the fuel header tanks, the fuel input filter, and the fuel sump.