On May 14, 2005, about 1550 Pacific daylight time, a Republic RC-3, N6292K, experienced a loss of engine power and made a forced landing in a vineyard in Franz Valley near Geyserville, California. The pilot/owner operated the airplane under the provisions of 14 CFR Part 91. The airplane sustained substantial damage. The pilot and one passenger were not injured; a third passenger received minor injuries. Visual meteorological conditions prevailed for the local area flight that departed Charles M. Schulz/Sonoma County Airport (STS), Santa Rosa, California, about 1530. The flight was destined for Lake Berryessa Seaport Base (E20), Napa, California, and no flight plan had been filed. The National Transportation Safety Board investigator-in-charge (IIC) interviewed the pilot. The pilot reported that the first indication of an engine problem was the rise in oil temperature with a corresponding decrease in oil pressure. He decided to turn back for STS, made it over a ridge and had to make a forced landing. The pilot stated that the airplane had just come out of an annual inspection 2 weeks prior to the accident. There were no mechanical anomalies noted on the two flights prior to the accident flight. The pilot submitted a written report. The pilot reported that about 7 miles east of the airport he noticed the oil temperature had risen 10 degrees (180 degrees to 190 degrees), and the airplane was not climbing. He turned back for STS, but didn't think that he would be able to clear the ridgeline. He then made a slight turn to the north and headed for the Healdsburg airport, but the oil temperature kept rising and was now at 200 degrees and climbing, with a corresponding decrease in oil pressure (dropping down to 25 pounds). A couple of minutes later he heard a very loud noise from the engine compartment followed by a complete loss of engine power. The pilot identified a vineyard to make the forced emergency landing. He stated that he kept the landing gear retracted and lowered the flaps and made a belly landing in the vineyard. After touchdown the nose dug into the soft ground and the airplane came to rest inverted. A pilot/mechanic witness, in the airplane, stated that after start-up they taxied to the run-up area. The run-up was normal and they took off towards the east. About 7 miles from the airport, he noted that the pilot was paying close attention to the engine gages. Up to that point the witness had not noticed anything wrong with the engine. He noted that the pilot was concerned with the rising oil temperature and initiated a turn back for the airport. The witness also noted a rise in oil temperature with a corresponding drop in oil pressure. At that point they both knew the engine was failing. He heard a loud bang, and the engine quit. Another witness in the rear passenger seat indicated that she heard "strange sounds - banging and clattering, etc." As the pilot turned back for the airport she reported that the engine noise increased and then the engine quit. According to the aircraft logbooks an annual inspection was completed on May 6, 2005. The airframe had a total time of 291.7 hours, and the engine had a total time since new of 296.2 hours. TEST AND RESEARCH A visual examination of the airframe revealed streaks of oil around the outside of the engine cowling. The cowling was removed revealing oil in the engine compartment, and a hole in the top of the engine case. A piece of material identified as a portion of separated engine case lying in the engine compartment. A divet was noted on the interior portion of the separated engine case. The IIC noted that the number 1 connecting rod (adjacent to the front main bearing journal) had separated. Rod bearing material along with a bolt end and nut assembly was at the bottom of the engine case. The connecting rod cap was lying on top of the engine case. The connecting rod cap exhibited bluing and deformation. The rod journal exhibited bronze/orange coloration as well as some deformation. Oil was drained from the oil sump, which contained metal debris. The top spark plugs were removed. According to the Champion Aviation Check-A-Plug chart AV-27, with the exception of the spark plugs for the numbers 1 and 3 cylinders; coloration was consistent with normal operation. The two sets of spark plugs for the numbers 1 and 3 cylinders were oil sooted with foreign debris around the electrode.