HISTORY OF FLIGHT On July 25, 2009, about 1100 Pacific daylight time, an experimental Kitfox IV, N6140S, nosed over after a forced landing near Escondido, California. The pilot/owner was operating the airplane under the provisions of 14 Code of Federal Regulations Part 91. The certificated sport pilot received minor injuries. The airplane sustained substantial damage to the aft fuselage and vertical stabilizer. The personal flight departed Hemet, California, about 1030, with a planned destination of Ramona Airport, Ramona, California. Visual meteorological conditions prevailed, and no flight plan had been filed. After departing Hemet, and as the airplane approached Pauma Valley Airpark (CL33), Pauma Valley, California, the pilot observed oil trailing along the left landing gear strut. He checked the engine oil pressure indicator, and confirmed it was displaying 45 to 50 pounds per square inch (PSI). He elected to continue the flight to Ramona; however, he altered his course so that he was able to fly over several areas, including a private ranch landing strip, where a landing could be made if it was necessary. He reported that as he approached the private landing strip the oil pressure did not change, and he decided to continue the flight to Ramona. After flying for an additional 2 miles, he observed the oil pressure level drop to zero, and shortly thereafter, the engine stopped. He then changed his heading to the north, and prepared for a forced landing. The airplane landed in a dirt field. During the landing roll the main landing gear struck a ditch, and the airplane nosed over. The pilot reported performing a preflight inspection on a prior flight earlier in the day. During the inspection he confirmed that the engine contained adequate oil. He stated that on the accident flight, he had not checked the engine oil level. AIRCRAFT INFORMATION The two-seat, experimental airplane, was issued a Special Airworthiness Certificate in December 2001. It was powered by a four-cylinder, liquid cooled, Rotax 912 ULS engine, and equipped with a Sensenich ground adjustable composite propeller. The engine incorporated a dry sump forced lubrication system with an oil tank capacity of 3.2 quarts. The engine was manufactured in 2001; 330.3 flight hours prior to the accident. Maintenance records revealed that an annual inspection was completed on June 1, 2009; 8.2 flight hours prior to the accident. TESTS AND RESEARCH Engine The NTSB investigator-in-charge and representatives from the Federal Aviation Administration and Rotax Aircraft Engines examined the engine. The engine could not be rotated by hand. Removal of the engine oil dipstick, and visual inspection of the oil tank revealed the absence of oil. Oil streaks were observed on the left main landing gear strut, with the left side of the belly mounted engine coolant radiator oil soaked. External inspection of the engine revealed no obvious sources of an oil leak. Investigators connected a 15 PSI regulated air source to the engine oil tank breather line, and observed air leaking from the base of the number four cylinder exhaust pushrod. Removal of the cylinder revealed the exhaust pushrod O-ring seal had spilt. The pilot/owner stated that about 2 years, and 100 flight hours prior to the accident, he had removed the number four cylinder head in order to perform a repair of the spark plug hole. He stated that he reused the original O-ring seal during reassembly of the cylinder head. In the 'Procedure Notes' section of the 912 Series Rotax Engine Maintenance Manual it states, "At reassembly of the engine, replace all sealing rings, gaskets, securing elements, O-ring and oil seals."