On April 1, 1997, at 1824 central standard time, a Piper PA-28-235, N9004W, sustained substantial damage during a forced landing due to loss of engine power near Des Moines, Iowa. The commercial pilot was not injured. The 14 CFR Part 91 flight departed Morning Star Airport, Des Moines, Iowa, on a local flight. Visual meteorological conditions prevailed and no flight plan had been filed. The pilot reported that he had preflighted the airplane thoroughly before departure. After takeoff he climbed to about 2,500 feet mean sea level (msl) and was at cruise power with the propeller set at 2,300 rpm. About 40 minutes after takeoff, the tachometer went from 2,300 rpm to 2,600 rpm, and then down to 2,200 rpm. The pilot reported that the engine sounded "okay," with no backfiring, mis-firing, or rough running of any kind. He pulled the carburetor heat, checked magnetos, fuel switches, and set the mixture to rich. He reported that he saw the oil pressure gauge read zero about 30 seconds before he made the decision to land in a field for a forced landing. The engine was knocking and it seized before the pilot landed the airplane in a soy bean field. During landing rollout the airplane hit some large ruts which collapsed the landing gear. The engine and engine logbooks were examined. It was determined that the engine was a remanufactured engine that had been installed on the airplane in 1980. The engine had about 600 hours total operating time since being installed on the airplane. The pilot reported that one of the reasons he had purchased the airplane was because the engine was a low time engine. The engine examination revealed that the number six connecting rod had separated and knocked a hole in the top of the crankcase. During the engine teardown, it was determined that the number three upper piston pin plug had been deteriorating and contaminating the engine with aluminum metal. The oil filter was opened and evidence of aluminum metal contamination was found. The oil pressure screen was inspected for contamination and it had some metal particles in the screen. The cam shaft and tappets indicated various stages of spalling, some advanced. Rust was noted inside all cylinder walls and all the connecting rods had surface rust on them. The number six connecting rod was black in color, and the number four and five connecting rods were blued and not free to move. The number five connecting rod bearing was burnished and heat distressed. The pilot reported that the airplane was flown about 75 hours a year. Oil was changed on the airplane about three times a year. The oil filter was not opened during the oil changes to inspect for metal contamination. The pilot reported that he was unaware that the engine oil filter indicated that the oil system was contaminated with metal particles.