HISTORY OF FLIGHT On March 7, 2003, about 0715 mountain standard time, a Stinson 108-1, N8344K, nosed over during a forced landing in a field near Eloy, Arizona. The pilot/owner was operating the airplane under the provisions of 14 CFR Part 91. The private pilot, the sole occupant, sustained minor injuries. The airplane sustained substantial damage. The personal cross-country flight departed Ryan Field (RYN), Tucson, Arizona, about 0650, en route to Casa Grande, Arizona. Day visual meteorological conditions prevailed, and no flight plan had been filed. The primary wreckage was located at 32 degrees 36 minutes north latitude and 111 degrees 31 minutes west longitude. The National Transportation Safety Board investigator-in-charge (IIC) interviewed the pilot at the scene. She was flying at 4,500 feet mean sea level (msl) when the engine started to make loud noises. She heard a bang, followed by a total loss of power. The pilot made a forced landing on a plowed field. During rollout, the landing gear caught in the soft dirt, and the airplane nosed over and came to rest in an inverted position. PERSONNEL INFORMATION A review of Federal Aviation Administration (FAA) airman records revealed that the pilot held a private pilot certificate with an airplane single engine land rating. The pilot held a third-class medical certificate that was issued on February 26, 2001, with the limitations that she must wear corrective lenses and must use hearing amplification. The pilot reported a total flight time of 706.8 hours. She logged 16.7 hours in the last 90 days, and 4.5 in the last 30 days. She had 209.5 hours in this make and model. AIRCRAFT INFORMATION The airplane was a Stinson 108-1, serial number 108-1344. A review of the airplane's logbooks revealed a total airframe time of 1,878 hours at the last annual inspection. An annual inspection was completed on May 31, 2002. The tachometer read 488.0 at the last inspection. The tachometer read 588.9 at the accident scene. The airplane had a Franklin 6A4-150-B4 engine, serial number 13608. Total time on the engine at the last annual inspection was 1,023.0 hours. The pilot reported that she departed RYN with 40 gallons of automotive fuel. A search of the Federal Aviation Administration (FAA) records revealed that a Supplemental Type Certificate had been issued for the use of unleaded automobile gasoline on April 2, 1994. Examination of the maintenance and flight department records revealed no unresolved maintenance discrepancies against the airplane prior to departure. Inspection of the engine logbook revealed that the number 4 cylinder assembly had been replaced due to a cracked head. The repair was accomplished on May 31, 2002, at the engine tachometer time of 488.03 hrs. WRECKAGE AND IMPACT INFORMATION Investigators from the Safety Board and the FAA examined the wreckage at the accident scene. The exterior of the airplane was covered with oil. Investigators observed that the number 3 and 4 cylinder assemblies were out of their normal position. When recovery personnel flipped the airplane over onto its landing gear, a large amount of oil and metal fragments fell out from the engine cowling onto the ground. Air Transport, Phoenix, Arizona, recovered the airplane and transported it to their facilities for further examination. TESTS AND RESEARCH Investigators examined the wreckage at Air Transport on March 8, 2003. Investigators examined the engine. The number 3 and 4 cylinders were badly damaged and dislodged from the engine. Both cylinder assemblies were fractured approximately mid-barrel. Both connecting rods were connected at the crankshaft and displayed torsional twisting. The wristpin was still attached to the number 3 cylinder crankshaft. Investigators removed the engine for disassembly. They removed the top and bottom spark plugs. Except for the number 3 and 4 cylinder spark plugs, the remaining spark plugs were clean with no mechanical deformations. The spark plug electrodes were gray in color, which corresponded to normal operation according to the Champion Aviation Check-A-Plug AV-27 Chart. The number 3 and 4 cylinder spark plugs sustained mechanical damage, and metal debris and oil fouled the electrodes. The number 4 cylinder exhaust valve had fractured between the valve head and stem. The valve head was not recovered. Except for cylinders number 3 and 4, an inspection of the remaining cylinder assemblies revealed no mechanical deformation on the valves, cylinder walls, or internal cylinder heads. Investigators were unable to manually rotate the engine due to the extensive damage to the number 3 and 4 cylinder assemblies. The IIC sent photographs of the fractured surface of the valve stem to the Safety Board Materials Laboratory, Washington, DC, for evaluation. According to the evaluation results, the fracture was consistent with fatigue cracking, with the primary initiation from one side of the stem from at least two origins. Propagation appeared to be almost all the way through before a small region of the final fracture was created. ADDITIONAL INFORMATION The IIC released the wreckage to the owner's representative.