On June 15, 1995, about 1030 hours mountain daylight time, N6669Q, a Grumman G-164B airplane, operated by the owner/pilot, impacted terrain while maneuvering near Helena, Montana. The airplane was destroyed. There was a post crash fire. The commercial pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed and no flight plan had been filed. The flight was conducted under 14 CFR 137. According to an FAA aviation safety inspector from Helena, the pilot stated that he was in the process of maneuvering close to the ground during an agricultural aerial application when the engine "... lost partial power." The pilot stated that he was not able to sustain enough engine power to prevent the airplane from descending into terrain during the maneuver. The airplane impacted terrain and burned. The pilot stated in writing to the Safety Board that the engine "... backfired 2 [or] 3 times...then quit." He also stated that he attempted to level the wings and perform a forced landing, but the airplane was too close to the ground to do so. According to airframe and engine maintenance logs, the airframe and engine received an annual inspection on March 30, 1995. No unresolved discrepancies were found. The recorded tachometer time of the engine at the time of the inspection was 3,074 hours. The engine had received a major overhaul about 400 operating hours prior to the accident. The engine was removed from the wreckage, partially disassembled, and inspected by FAA aviation safety inspectors from Helena. According to their report (attached): ... the number 5 cylinder had a gaping crack running from the forward spark plug port, across the upper cylinder head, and engine at the rear spark plug port. This crack appeared to have existed for a period of time prior to the accident and was not a result of the accident. The report also stated: ... examination of the interior area of the engine disclosed the crankshaft at the forward bearing area showed bluing discoloration. Binding was noted when the crankshaft was rotated. This discoloration was not a result of the fire in the rear of the engine.