On September 21, 2002, about 1450 Pacific daylight time, a Cessna 172RG, N6303V, made an off airport forced landing following a loss of engine power near Del Mar, California. Air Charter Express, Inc., was operating the airplane under the provisions of 14 CFR Part 91. The airline transport pilot, the sole occupant, was not injured; the airplane sustained substantial damage. The personal cross-country flight departed McClellan-Palomar Field (CRQ), Carlsbad, California, about 1430, en route to Montgomery Field (MYF) San Diego, California. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported that approximately 20 minutes after takeoff, during cruise flight, the engine stopped, and he made a forced landing in a dirt field. During the landing roll the nose wheel fell into a culvert, which resulted in substantial damage to airframe structural members. A Federal Aviation Administration (FAA) inspector interviewed the pilot and personnel at Air Charter Express. The previous night, another pilot reported an "rpm bump" to maintenance personnel. The maintenance personnel discovered a leak in the oil cooler return line and it was repaired. No other inspections were preformed. Subsequent investigation determined that the overspeed was not the 50 rpm that was reported by the previous pilot, but rather the engine speed topped out at over 3,000 rpm, which requires engine disassembly and inspection. Textron Lycoming Service Bulletin (SB) 3691,states that overspeed is defined as an increase of no more than 10 percent of rated engine rpm for a period not exceeding 3 seconds. In Chart 11 of SB 3691, the engine overspeed described was greater than 10 percent, which requires engine disassembly and inspection in accordance with the applicable overhaul manual. The maximum rated speed on the engine was 2,970 rpm. An FAA inspector supervised disassembly of the engine. The rod cap separated from the rod on the number 3 cylinder. Pieces of metal were throughout the interior of the engine.