On October 7, 2002, approximately 1830 eastern daylight time, an amateur-built Wolf Rotorway Exec-162F helicopter, N1955Z, owned and piloted by a student pilot, was substantially damaged when it lost power to the main rotor and executed an auto-rotation into an open field near Gregory, Michigan. The flight was being conducted under 14 CFR Part 91 and was not on a flight plan. Visual meteorological conditions prevailed at the time of the accident. The pilot reported no injuries. The flight departed the Livingston County Airport (OZW), Howell, Michigan, at 1800 edt for a local flight and was returning to the airport when the accident occurred. The pilot stated that the "secondary shaft bearing temperature started to rise." He reported that approximately one minute "elapsed from the time the [bearing] temperature increase was noted, to the time the temperature reached 150 degrees Fahrenheit," and the "drive let go", causing the engine RPM to surge. At this point, the pilot noted that all power to the main rotor was lost and he initiated an auto-rotation. He stated that, although the touchdown point was as planned, the forward speed was faster than intended and the helicopter flipped forward and rolled to the left. Following a post-accident examination, the owner/pilot reported that the secondary drive shaft had failed completely at a point inside the upper bearing race. He noted that the shaft had been in service for approximately 155 hours at the time it failed. Due to a history of secondary drive shaft failures, Rotorway had released a "re-designed configuration" in April 2001. This configuration increased the shaft diameter from 30 mm to 35 mm. The 30 mm shaft was installed in the accident aircraft. According to Rotorway, owners were notified of the availability of the new shaft design by an Advisory Bulletin in May 2002. In part, this bulletin states: "The larger [35mm] secondary shaft was supplied with new aircraft and as an upgrade to existing aircraft over a year ago. ... It is the suggestion of Rotorway International that all Rotorway owners consider upgrading to the 35mm secondary shaft." The owner/builder stated that he had planned to upgrade to the 35mm shaft over the coming winter. In addition, the accident aircraft included an after-market main rotor drive system which incorporated a cog belt in place of the chain drive.