On August 22, 1995, approximately 1700 mountain daylight time, a Mooney M20K, N1154L, received substantial damage in a forced landing 9 miles north of Roundup, Montana, following an inflight engine failure. Neither the private pilot, who owned the airplane, nor his passenger were injured. The 14 CFR 91 flight was en route from Aurora State Airport, Oregon, to Bismarck, North Dakota. Visual meteorological conditions existed and no flight plan had been filed. The pilot reported that the flight had been proceeding uneventfully. He stated that he was cruising at 11,500 feet at approximately 65 percent power during the flight. He stated that approximately three minutes before the event, he noticed oil temperature in the upper third of the green arc and opened 1/2 cowl flaps; and that about one minute before the event, he noted oil temperature "dropping normally." Approximately 28 miles north of Roundup, he noticed a slight propeller surge "like speeding up for 1/2 second, then [the engine] sound[ed] normal." He scanned the engine instruments and noted that the engine oil pressure had dropped to zero. He contacted the automated flight service station (AFSS) at Great Falls, Montana, informed them of the problem and told them he would attempt a landing at the Roundup airport. He stated that the engine "[made] grinding noises and start[ed] to smell hot" approximately 3 minutes after the oil pressure drop. At this point, the pilot stated, he advised Great Falls AFSS that "we will not make [the] airport." Deciding that there was too much vehicle traffic to attempt a forced landing on a highway in the vicinity, he landed the aircraft in a cow pasture. During the forced landing, the airplane went through a barbed wire fence and into an area of rough terrain. The passenger submitted a written statement which was in agreement with the report submitted by the pilot. A FAA on-scene investigator was able to rotate the airplane's engine, a Continental TSIO-360-GB, with difficulty. He noted one propeller blade bent slightly back. He reported that there was no visible oil on the external surfaces of the aircraft and noted no oil leaks from the engine. He reported that the engine contained 7 quarts of oil. Investigators from the NTSB and Teledyne Continental Motors (TCM) examined the engine at Discount Aircraft Salvage, Deer Park, Washington, on October 8, 1995. The examination revealed a ferrous metallic chip on the outlet side of the oil inlet screen, between the screen and the oil pump inlet. The oil inlet screen was ruptured open along its solder seam. The metallic chip was small enough to pass through this gash. The oil pump inlet was gouged; the metallic chip matched the gouge at the pump inlet. The TSIO-360-GB oil pump design is such that the pump rotor is mated to the pump drive shaft via nut torque and resultant friction between the mating surfaces. Investigators discovered the mating nut on the accident engine's oil pump to be only slightly more than finger tight, with rotational scoring on the mating surfaces. The oil inlet screen and the ferrous metallic chip were shipped to the TCM factory in Mobile, Alabama, for metallurgical analysis. A scanning electron microscope examination of the chip revealed the chip material to be copper-plated steel; TCM reported that the material was characteristic of that used in the engine gears and camshaft. The TCM product analysis manager reported that the piece appeared to be a small chip from an engine gear. He also reported that the oil inlet screen appeared to have been subjected to an axial compressive load which split the screen open along its solder seam. A FAA manufacturing inspector who coordinated the testing at TCM reported that the time at which the screen was subjected to this compressive load could not be determined. A work order filed with the aircraft logbooks, dated July 3, 1995, stated the following: "Vacuum pump [seized], drive [assembly] has frozen bearing. This [assembly] also drives the standby generator....Removed generator and adaptor [assembly]. Installed used starter drive [assembly]. Standard without generator pad....Installed new Sigmatek vacuum pump...."