On December 5, 2012, at 1300 central standard time, a Stinson 108, N8775K, was substantially damaged during a forced landing to a field following a loss of engine power near Halfway, Missouri. The private pilot sustained minor injuries. The airplane was registered to and operated by the pilot. Visual meteorological conditions prevailed for the personal flight conducted under 14 Code of Federal Regulations Part 91. No flight plan was filed for the flight that departed Eads Ridge Airport (MO68), Fair Grove, Missouri, at 1240.A Federal Aviation Administration (FAA) aviation safety inspector interviewed the pilot and examined the airplane. According to the inspector, the pilot had been airborne for approximately 20 minutes when the engine began to make an unusual grinding noise followed by a loss of engine power. The pilot attempted to make a forced landing to a field and struck a powerline with the left main landing gear. The airplane landed hard, resulting in substantial damage to both wings, the fuselage and an engine mount. The left main landing gear and propeller were also damaged. Examination of the engine revealed the magnetos were not properly secured to the engine and were loose. When the magnetos were removed, the inspector could visually see that the camshaft was broken. A review of the engine maintenance logbooks revealed the airplane's last annual inspection was January 8, 2008. The pilot held a private pilot certificate for airplane single-engine land. His last FAA third class medical was issued on March 4, 2009, and his last biennial flight review was on August 31, 2008. The pilot reported a total of 761 hours; of which 437 hours were in the accident airplane. He stated that he had flown the airplane approximately 3 hours in the last 90 days and was trying to prepare for a biennial flight review when the accident occurred.