On March 31, 2003, at 1440 eastern standard time, an Air Tractor AT-401, N45297, piloted by a commercial pilot, was substantially damaged during a forced landing following an engine failure near Knox, Indiana. Visual meteorological conditions prevailed at the time. The maintenance test flight was operating under the provisions of 14 CFR Part 91 and was not on a flight plan. The pilot sustained serious injuries. The aircraft departed the Starke County Airport (OXI), Knox, Indiana, approximately 1425. The intended destination was Arens Field Airport (RWN), Winamac, Indiana. In his written statement, the pilot reported that in the weeks prior to the accident flight a newly overhauled engine had been installed on the aircraft. Several engine test runs were conducted with no oil leaks or other anomalies noted. The pilot stated that immediately prior to the accident flight, the engine was warmed up for about 20 minutes prior to taxiing for departure. He noted the run-up was normal and the oil temperature was within the green arc prior to initiating the takeoff. The pilot reported that the takeoff was uneventful and he climbed to 400 feet above ground level before proceeding to the south. The pilot reported that the oil temperature rose to 180 - 200 degrees before dropping back to 135 degrees, indicating the oil was being circulated through the oil cooler. The oil pressure was "good" throughout the time. He stated that about two miles south of the airport the oil temperature and pressure indicated 135 degrees and 80 lbs., respectively. Approximately 5 miles from the airport the oil pressure had decreased to about 40 lbs. At this point, the pilot decided to divert toward Wheeler Airport (3C5), which was approximately 2 miles east of the aircraft's position. The pilot stated that when he was established on an easterly heading he noted the oil pressure had dropped to 25 lbs. The engine was losing power at this point and the aircraft was not able to maintain altitude. The pilot estimated that he was not going to be able to clear a line of trees along the west end of 3C5. He selected and made a forced landing in a field short of the airport. He stated that he touched down in the field "all right," but to avoid hitting trees at the end of the field, he applied the brakes causing the aircraft to flip over. The aircraft came to rest inverted at the edge of a harvested soybean field. Federal Aviation Administration inspectors conducted an on-scene inspection. Three cylinder heads had failed. The lower fuselage was covered in oil emanating in a fan-shaped pattern from the engine oil breather tube and extending to the tail of the aircraft. There were no other signs of oil leakage. No oil appeared to remain in the oil tank. The oil screen was free of debris and contamination. The engine was removed and subsequently torn down under NTSB supervision. The number 1 cylinder head had failed completely. The number 2 cylinder head was cracked. The number 8 cylinder was missing approximately 1-1/2 inches of the lower portion of the cylinder head. The threads of the cylinder body were completely exposed. The right magneto was cracked at the base, separating the unit from its mount. The magneto drive gear appeared intact. The ignition leads were secure and the points appeared undamaged. The left magneto was intact and securely attached to the engine. The oil pump would not rotate under hand pressure. The drive gear, housing and attachments appeared undamaged. The internal cover safety wire was intact. The oil check valve was free to move. Upon disassembly, the oil pump gears appeared undamaged. Scrape marks were observed on the interior of the pump housing. At the oil pump inlet a small piece of rubber hose was recovered. The piece measured 1-1/2 inches by 3/8 inch by 1/8 inch thick. The housing inlet cavity measured approximately 7/8 inch wide by 3/4 inch deep. A 15/16 inch diameter, 45-degree angled tube was installed outside the cavity inlet to provide an installation flange for the oil hose. The engine fittings were not capped when the NTSB arrived for the teardown. The master rod bearing was deformed and had extruded from the rod. It appeared to have a bluish tint consistent with overheating of the material. The newly overhauled Pratt & Whitney R-1340-59 radial engine, serial number 41-12095, was installed on the airframe on March 25, 2003. Total engine time was recorded as 5,283.9 hours. This was the first flight since the overhauled engine had been installed. The logs and the pilot's statement stated that the engine had undergone ground runs to verify the installation. They stated that no leaks were observed following those runs. The overhaul was conducted by Tulsa Aircraft Engines under work order 3926. According to documentation from the overhaul shop, the engine was test run for 4 hours on February 18, 2003, prior to shipment for installation.