On June 9, 2000, about 0850 Eastern Daylight Time, a homebuilt S-51, N51FY, was substantially damaged a force landing to a field at Forty Fort, Pennsylvania. The airplane departed from Wilkes-Barre Wyoming Valley Airport (WBW), Wilkes-Barre, Pennsylvania. The certificated commercial pilot sustained minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed for the local personal flight conducted under 14 CFR Part 91. The pilot stated that during the initial climb, about 150 feet above the ground, the engine lost all power. The pilot performed a forced landing to a field. During the landing, the airplane struck a fence and sustained damage to the right wing. Examination of the wreckage by a Federal Aviation Administration inspector revealed that the propeller reduction gear box input shaft had sheared. When asked about the shaft, the pilot stated that it failed "at the weakest point." He added that the failed area was too thin, and needed to be re-machined. He believed that over an approximate 150 hours of operation, stress caused the component to fail. According to the kit manufacturers, the failure was limited to the pilot's airplane. A machine shop had provided the reduction gearbox input shaft for the production models. The pilot decided to machine his own parts for his particular airplane. The pilot obtained some rough castings from the machine shop, but that was the only similarity between the failed shaft and the production shafts. The manufacturer added that the shafts in the production models are larger, and use aviation splines, whereas the pilot used automotive splines.