On May 30, 1995, at 1410 central daylight time, a Bell 47G-5, N1502L, sustained substantial damaged while maneuvering near Falfurrias, Texas. The commercial pilot was not injured. The helicopter was being operated as a business flight by Tiller Helicopter Service under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the local cattle herding flight and a flight plan was not filed. During interviews and on the Pilot/Operator Report, the operator reported that the pilot had been working cattle on the Felix Tapp ranch for approximately 5.5 hours when he stopped for lunch and refueling of the helicopter. Upon departure, he had one pasture where he needed to finish the roundup of the cattle. Approximately 45 minutes into the flight, at 50 to 60 feet above the ground, the "helicopter didn't feel right." The pilot landed the helicopter in the mesquite thicket area and checked the instruments, magnetos, tailboom, drive shaft, and tailrotor. Following a subsequent 45 seconds to 1 minute hover, the helicopter checked "ok no vibration" and the pilot continued the flight over the mesquite thicket to the edge of the pasture. Upon maneuvering at the outer perimeter of the pasture, as the pilot leveled the helicopter from a right turn, the helicopter started spinning to the right and "application of the left pedal had no effect." The pilot initiated an autorotation from 75 feet above the ground into a confined area with a left turn to clear the mesquite trees. As the pilot pulled in collective, the helicopter started spinning, the main rotor blade struck a tree, the skids touched the ground, and the helicopter rolled to the right impacting the ground. The operator recovered the helicopter from the ranch. Following his examination the separated tailrotor drive shaft was reported to the Federal Aviation Administration. The Federal Aviation Administration inspector examined the helicopter and secured the gear and bearing for metallurgical examination by the NTSB. According to the operator, the bearing P/N 47-620-929-1 was ordered (invoice copy enclosed) on December 23, 1994, from Helicopter Spares, Inc., of Bridgeport, Pennsylvania. Following installation of the bearing, the helicopter was returned to service by the operator's maintenance personnel on January 29, 1995. During a telephone conversation, conducted by the investigator-in-charge, with Mr. Jack Carson, President of Helicopter Spares, the bearing shipped was P/N 47-620-929-1 as shown on the invoice. Mr. Carson further stated that their parts are received and placed in a bin prior to filling the customer's order. Helicopter Spares does not run a quality control check on the incoming parts as the parts are received in Bell packaging with accompanying documentation from Horshan Valley Aviation of Horshan, Pennsylvania. During a telephone conversation, conducted by the investigator- in-charge, Mr. Hoyt Bangs of Horshan Valley Aviation, stated that they receive the parts directly from Bell Helicopter Textron, Inc., of Fort Worth, Texas. Mr. Bangs further reported (invoice copies enclosed) that the bearing P/N 47-629-929-1 was invoiced from Bell Helicopter Textron on November 9, 1995, and shipped to Helicopter Spares, Inc., on November 21, 1994. On August 10, 1995, the bearing and gear components from Model 47G-5, S/N 7945 were examined by the investigator-in-charge and a representative of Bell Helicopter at Hurst, Texas. The bearing, received by the operator from Helicopter Spares, as P/N 47-620-929-1 showed identity "NDH1303T17." It was determined that the bearing was P/N 1303TS17 with the marking of "1303T17" on the bearing being the part number for the outer race only. The manufacturer representative stated that the Hyatt Roller Bearing (General Motors Corporation at Sandusky, Ohio) assembly P/N 1303TS17 is the "only qualified bearing and manufacturer source for Bell P/N 47-620-605-1." This P/N was replaced by P/N 47-620-929-1 by Alert Service Bulletin 47-80-6 issued in May 15, 1980, by Bell Helicopter. Bearing P/N 1303TS17 is not part of the type design for the Model 47G-5 civilian helicopter and is considered an unapproved part. On the enclosed statement, dated December 8, 1995, the Bell Helicopter representative reported that the last sale of a bearing P/N 47-620-605-1 was in 1972. During a telephone interview, conducted by the investigator-in- charge, Mr. Chuck Kromer of Del Phi Chassis (formerly Hyatt Roller Bearing) at Sandusky, Ohio, stated that the company stopped manufacturing bearing P/N NDH1303T17 in 1987. The gear from the fan drive assembly was identified with an etched P/N 47-620-568-3-A. Parts produced by or for Bell are "not marked by this method." Bell helicopter's design drawing for the gear indicates P/N 47-620-568-3 with a black oxide finish per BPS 4084. Visual examination of the gear did not indicate a black oxide finish and the gear was forwarded to the NTSB metallurgists. The gear could not be identified as conforming to the type design data and is considered an unapproved part. The source and maintenance history for the gear P/N 47-620-568-3-A could not be determined. The Materials Laboratory NTSB Metallurgist (enclosed report) visual and metallographic examination of the gear revealed that a portion of the gear's shaft that constitutes the inner race for the cylindrical bearing was not carburized. The inner race area of the shaft contained an "approximately 0.31 inch long groove where material appeared to be worn away." The microstructure at the surface of the gear's shaft was "heavily deformed with the most severe grain flow in the grooved area." The rollers from the bearing were "heavily deformed and discolored by heat tinting." Pieces of the bearing retainer revealed features "consistent with overstress separations." The gear and bearing components were released to the owner.