On June 26, 2007, about 1930 Alaska daylight time, a Piper PA-12 airplane, N78456, sustained substantial damage when it nosed over during an off-airport precautionary landing, about 6 miles northwest of Anchorage, Alaska. The airplane was being operated by the pilot as a visual flight rules (VFR) personal local flight under Title 14, CFR Part 91, when the accident occurred. The solo private certificated pilot was not injured. Visual meteorological conditions prevailed, and no flight plan was filed. The flight departed Merrill Field, Anchorage, about 1910. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on June 27, the pilot said that during cruise flight the engine started to run rough, and he was unable to get the engine to run smoothly, and elected to make a precautionary landing on a grass-covered field, where he had landed on previous occasions. He said the grass was knee to waist high, and the airplane nosed over during the landing roll. The pilot indicated that the wings and rudder were damaged when the airplane nosed over. After the airplane was recovered, a certificated aircraft mechanic who examined the airplane, told the IIC that during a test run of the engine, the engine appeared to have a malfunctioning magneto. He said the magneto was sent to a shop for examination. During a telephone conversation with the IIC on July 17, the technician who examined the magneto, said the upper housing had fractured, allowing the wire-lugs to float internally, and the engine to run rough. He said the damage to the magneto, in his experience, did not appear to be the result of the accident. He said the failure of the upper housing was not uncommon in older airplanes.