On October 6, 1996, at 0800 central daylight time (cdt), a Bell 47G-5, N7925J, registered to Wayne Hursey Aerial Blades, of Flandreau, South Dakota, piloted by a commercial helicopter pilot, was substantially damaged following a loss of engine power and subsequent autorotion near Nunda, South Dakota. The pilot reported no injuries. The 14 CFR Part 137 flight was operating in visual meteorological conditions. No flight plan was on file. The flight departed Flandreau Airport, Flandreau, South Dakota, at 0640 cdt. According to the pilot's written statement he was flying his second agricultural chemical load that morning when the engine loss total power. The pilot successfully autorotated the helicopter. Post accident examination of the helicopter by a Federal Aviation Administration Principal Maintenance Inspector (PMI), stated that a teardown of the engine revealed six sheared accessory drive gear bolts. The six attachment bolts, three lock plates, three dowel pins, shaft assembly and the accessory drive gear were sent to the NTSB's Materials Laboratory for further examination. The attachment bolts for this assembly separated at the thread area located between approximately 0.1 and 0.3 inches below the lower surface of the bolt head. Fragments of these bolts were found attached to the accessory drive gear. The surface of the shaft facing the lock plates contained fretting damage all around the bolt through holes. The dowel pin through holes on the shaft contained elongation damage and the diameter surface of each dowel pins was severely burnished. No cracking was found on the shaft, accessory drive gear, and dowel pins. This engine assembly separated 400 hours since the last overhaul. The metallurgist's factual report is enclosed with this report.