On July 3, 2002, approximately 2030 mountain daylight time, a Grumman G-164B "Agcat," N297AL, registered to and operated by Nef Flying Service, Inc., and being flown by a commercial pilot, was destroyed during a forced landing following a loss of power during a spray run approximately six nautical miles northwest of Shelley, Idaho. The pilot sustained minor injuries. Visual meteorological conditions existed and no flight plan had been filed. The flight, which was a crop dusting flight, was operated under 14 CFR 137, and originated from a private landing site just west of Shelley, approximately 2000 on the evening of the accident. The pilot reported that he had just rolled out of a turn to begin a spray run when there was an abrupt loss of power accompanied by engine roughness and vibration. The pilot, unable to maintain altitude, executed a forced landing into the field he was spraying. The aircraft touched down roughly perpendicular to the furrows and the aircraft tumbled end over end several times shedding both the engine and the empennage before coming to rest inverted (refer to photographs 1 and 2). Post-crash examination revealed the top of the number five cylinder had separated from the lower half of the cylinder and departed the engine (refer to photographs 3 and 4). The cylinder had separated circumferentially about an inch inside the barrel. The pilot reported that the engine had undergone a zero-time overhaul within the previous 120 hours. The number five cylinder and associated piston was sent to the Safety Board's metallurgical laboratory for subsequent examination and testing. The examination revealed that the cylinder had split circumferentially between the fifth and sixth inboard cooling fins and that the fracture surfaces displayed characteristic crack arrest lines (fatigue). Multiple fatigue crack points of origin were noted within the first and second thread roots of the cylinder threads on the inside of the cylinder head, and no mechanical damage was observed within the threads. Dimensional examination revealed that the threads between the cylinder base and head had been improperly machined resulting in the cylinder's abutting surfaces not coming into complete contact during assembly of the two components (refer to attached metallurgical report).