On October 11, 1995, at 1345 central daylight time, a Cessna 172F, N8740U, piloted by a private pilot, sustained substantial damage when during a precautionary landing to a dirt road the airplane's right wing struck a sign and the airplane overturned. Visual meteorological conditions prevailed at the time of the accident. The flight was operated under 14 CFR Part 91. A flight plan was not on file. The pilot sustained no injuries. The airplane was on a local flight originating at Cumberland, Wisconsin at 1100 central daylight time. The pilot reported that he was cruising at 500 feet above ground level when the "engine started to run roughly." He decided to land the airplane on "what appeared to be a level road" and "investigate the problem." The pilot said, "it was the only place to land for many miles." On landing roll "at a ground speed of approximately 20 to 30 miles per hour," the pilot saw a sign which had been "obscured by brush." "The right wing tip struck the sign and spun the airplane around to the right." The airplane's nose gear struck a sand embankment on the road's edge and the airplane nosed over. The Federal Aviation Administration (FAA) inspector who examined the wreckage found the left magneto rear cover had separated from the magneto's main case exposing the gears and points. Three of the four screws which held the magneto cover in place were missing. The fourth screw had sheared off in the case. Both magnetos were tested for spark. No abnormalities were found. Examination of fuel in the tanks showed evidence of water and sediment.