On January 19, 2000, at 1230 central standard time, a Mooney M20K, N1157G, was substantially damaged during a forced landing following a loss of engine power near Tyler, Texas. The private pilot, who was the registered owner and sole occupant of the airplane, was not injured. Visual meteorological conditions prevailed for the 14 Code of Federal Regulations Part 91 personal flight and a flight plan was not filed. The cross-country flight departed from the Addison Airport, Addison, Texas, approximately 1155, and was destined for the Tyler Pounds Field, Tyler, Texas. According to the pilot, he was flying between 1,500 and 2,000 feet agl and was 10 miles from the Tyler Pounds Airport when the "engine rpm gradually decreased," and the engine lost total power. He switched the fuel selector from the left tank to the right tank and attempted to re-start the engine, but was unsuccessful. He then initiated a forced landing to a field. During the landing roll, the left wing contacted a 6"X6" wood fence post, damaging the wing spar, and the airplane came to a stop upright 6.5 miles from the Tyler Pounds Airport. The 210-horsepower Continental TSIO-360-LB1 engine was examined by an FAA inspector and a representative from Teledyne Continental Motors. During the examination they removed the rear accessory case cover and observed that the crankshaft drive gear was fractured. Three consecutive teeth of the crankshaft drive gear separated. Two of the three teeth were recovered. The NTSB Investigator-In-Charge and an NTSB Metallurgist examined the fractured crankshaft drive gear and gear teeth at the NTSB Materials Laboratory in Washington D.C. Three areas of fatigue were observed. Each fatigue area originated at the relief radius at the root of one of the three consecutive teeth of the crankshaft drive gear that separated. No tooling marks or gouge marks were observed near the fracture surfaces or on the remainder of the crankshaft gear teeth. The outermost nitrided layer of the crankshaft drive gear appeared smooth and no cracks were noted. Additionally, machining marks through the entire crankshaft drive gear appeared consistent.