On July 5, 1996, about 1130 eastern daylight time, a Cessna 172N, N75814, collided with trees and was substantially damaged during a forced landing attempt near Hillsborough, North Carolina. The private pilot and his passenger received minor injuries. The airplane was being operated by the pilot under the provisions of Title 14 CFR Part 91, and visual flight rules. Visual meteorological conditions existed at the time, and no flight plan had been filed for the local personal flight. The flight departed Burlington-Alamance Regional airport about 1115. The pilot stated that after a loss of engine power and an unsuccessful engine restart, he attempted a forced landing in a pasture. The airplane impacted trees, separating the wings, the horizontal stabilizers, and the vertical stabilizer. The engine was examined following the accident. The single-drive dual magneto timing was found to be incorrectly timed by 90 degrees. After disassembly of the magneto, the impulse coupling spring was found broken. Records for the overhaul of the magneto indicated that the spring had been reused during the overhaul in March 1989. According to a Bendix magneto representative, the overhaul manual allowed reuse of the spring in March 1989. The overhaul manual was subsequently changed, in July 1989, to require replacement of the spring at each disassembly with new parts. According to the quality assurance representative for the overhaul facility, the facility has revised its procedures to comply with the Bendix overhaul manual for replacement of the spring at each disassembly of the magneto.