On September 3, 2003, about 0900 mountain daylight time, a Piper PA-28-140, N15367, sustained substantial damage following a loss of engine power and subsequent hard landing at the Homedale Municipal Airport (S66), Homedale, Idaho. The airplane is owned by the pilot and was being operated as a visual flight rules (VFR) cross-country flight under the provisions of Title 14, CFR Part 91, when the accident occurred. The private pilot was seriously injured and his passenger received minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed. The flight originated from Ontario, Oregon, approximately one hour prior to the accident. The pilot reported that the airplane's engine developed an uncharacteristic vibration while in a cruise flight configuration. The pilot stated that he diverted to a nearby airport and shortly after entering a downwind for the intended runway, the engine "quit." The pilot stated he turned directly toward runway 31 after the engine failure. Shortly after completing the turn to final, the pilot landed on the intended runway. The pilot reported that the airplane "...hit very hard." It was later discovered that the pilot's seat collapsed as a result of the hard landing. On September 17, 2003, investigators from the National Transportation Safety Board, Piper Aircraft and Lycoming accessed the aircraft. Examination of the aircraft's engine, a Lycoming O-320 E2A, revealed a fracture in the number four cylinder head. The full thickness fracture emanated from the top sparkplug hole and traveled the circumference of the cylinder, terminating at the bottom sparkplug hole. Subsequent to the engine examination, the fractured cylinder was transported to Northwest Laboratories of Seattle, Inc, for further metallurgical examination. The metallurgical examination of the cylinder revealed that the cylinder head fractured due to fatigue. The exam report (attached) indicated that multiple fatigue failure initiation sites were observed, most notably a series of cracks that initiated at each of the two sparkplug holes. The report indicated the cracks propagated through the thickness of the cylinder wall and formed a single crack front. Maintenance records indicated that the failed cylinder (cylinder number four) was removed and repaired due to valve "pitting" in June of 1992. Following the repairs, the cylinder was returned to service. At the time of the repairs, the engines total time was 6,689 hours; 726 hours since major overhaul. The cylinder total time since manufacture could not be determined. According to maintenance records for the engine, the last annual inspection was completed on November 14, 2002. The maintenance logbook entry corresponding to the annual inspection indicated that the engine total time was 7,260 hours; 1,319 hours since major overhaul. The engine had accumulated approximately 36 hours from the time of the annual inspection to the time of the accident.