On June 3, 2006, at 1435 Pacific daylight time, an Aviat Aircraft Incorporated Pitts S-2B, N117PS, lost oil pressure in flight and the pilot performed a precautionary landing to a hay field in Antioch, California. The airplane nosed over and sustained substantial damage. The commercial pilot, the sole occupant, was not injured. The airplane was registered to a private company and the pilot was operating it under the provisions of 14 CFR Part 91. Visual meteorological conditions prevailed and no flight plan had been filed for the local, personal aerobatic flight. The pilot departed from Livermore Municipal Airport, Livermore, California, at 1400. The pilot reported that while performing an aerobatic sequence, the engine lost oil pressure and he elected to perform a precautionary landing in a field. The pilot shutdown the engine after 15 seconds when he had his landing spot secured. During the landing roll, the airplane nosed over in the muddy terrain. The Federal Aviation Administration accident coordinator examined the engine following the accident. The engine oil sump contained approximately 9 quarts of oil. The oil pressure gauge was tested and no operational anomalies were noted. The engine was test-run; however, the oil pressure gauge did not indicate positive pressure. The oil pump was removed from the accessory case and was turned by hand. Oil was present within the oil pump and coated the gears and shaft. No heat signatures were evident. The airplane was equipped with a Christen inverted oil system. The oil valve works through the means of gravity operated ball valves which includes two balls and a spring. During normal flight, the top ball valve is closed and the bottom ball valve is open. During inverted flight, the opposite is true. The oil valve was disassembled. Within the valve, a metal piece approximately 0.5-inch in length was identified between the two balls. Later disassembly of the engine showed that portions of the number 5 piston skirt had fractured from the piston. Pieces of the fractured skirt were identified in the oil sump and filter.