On August 10, 1998, at 0830 hours Pacific daylight time, a Cessna 150M, N704UV, made a forced landing in a riverbed due to a rough running engine, struck a boulder, and nosed over near Santa Paula, California. The airplane, operated by C.P. Aviation of Santa Paula as an instructional flight under 14 CFR Part 91, sustained substantial damage. The student pilot, the sole occupant, was not injured. Visual meteorological conditions existed for the local instructional flight and no flight plan was filed. The flight had originated from the Santa Paula airport at 0800 on the day of the accident. In an interview, the pilot stated that he was practicing a series of climbs, descents, and ground reference maneuvers, when the airplane began experiencing a rough running, vibrating engine that was producing smoke. The airplane was examined by a Safety Board investigator and it was noted that the number one cylinder had a circumferential crack approximately mid-barrel, and the corresponding top spark plug was oil fouled. According to the Champion Spark Plugs Check-A-Plug chart AV-27, the remaining spark plugs exhibited coloration consistent with normal operation. The oil screen was removed and visually inspected. It contained residual oil and metallic flakes. Examination of the number one cylinder by a metallurgist from the Safety Board, disclosed that the fins of the barrel were covered with iron oxide. Closer examination revealed a fatigue crack, which emanated from corrosion pits at the root radius between the 8th and 9th cooling fins. The corrosion pits measured approximately 0.002 inch in depth, and the fatigue crack propagated 325 degrees circumferentially round the cylinder. The metallurgist reported that the remaining portion of the fracture exhibited features typical of overstress separation. Examination of the airplane records revealed that the total time on the engine and the No.'s 1, 2, and 3 cylinders was approximately 1912.4 hours, 112 hours over the engine manufacturer's recommended time between overhaul (TBO). The number 4 cylinder had been replaced on December 20, 1996, due to a crack around the barrel between the cylinder head and base of the cylinder. The crack encompassed approximately 50 percent of the circumference of the cylinder barrel.