On November 3, 1996, at 1330 central standard time, a Stinson 108-3 airplane, N798C, was substantially damaged during a forced landing near Tyler, Texas. The airplane, owned and operated by a private owner, was being operated under Tittle 14 CFR Part 91. The private pilot and his passenger were not injured. Visual meteorological conditions prevailed for the personal flight for which a flight plan was not filed. The flight originated from the Cherokee County Airport near Jacksonville, Texas, at approximately 1245. According to the pilot, he observed the "tachometer RPM changing drastically" while the airplane was on a 2 to 2.5 mile extended downwind leg for landing on runway 13 at the Tyler Pounds Field. The pilot reported the anomaly to the tower advising them that he needed to land. The tower immediately issued the pilot a clearance to execute a left turn and proceed for a straight-in approach to runway 17. The pilot added that he complied immediately and aligned the airplane for landing on runway 17. The pilot further stated that about 30 seconds after turning toward runway 17, the "engine started to make a loud popping and backfiring noise," followed by the total loss of power. The pilot notified the tower of his predicament and advised them that he would not be able "to make the runway." The pilot elected to land the airplane on a field to the right of his course, approximately 1.5 miles north of the threshold for runway 17. During the landing roll, "the mains hit a drainage ditch along side a road crossing the field" and the airplane nosed over, coming to a stop in the inverted position. Examination of the airplane by the FAA inspector revealed that both wing struts sustained structural damage. Further examination and teardown of the engine by the owner revealed that the loss of engine power was attributed to the failure of the crankshaft in the heavy cast (web) area between the #1 and #2 rod journals. See enclosed report for further details of the examination. The 1947 vintage airplane was rebuilt by the pilot in 1993. He stated that at that time he outfitted the airplane with shoulder harnesses. He credited the absence of injuries to either occupants to the use of the shoulder harnesses.