On September 24, 1997, at 0930 central daylight time (cdt), a Beech C23, N1851Z, operated by a commercial flight instructor, sustained substantial damage when on approach for landing, the airplane's engine lost power. During the subsequent forced landing, the airplane impacted the terrain. Visual meteorological conditions prevailed at the time of the accident. The instructional flight was being conducted under 14 CFR Part 91. A flight plan was not on file. There were no injuries reported by the instructor pilot or the dual student. The flight originated at Champaign, Illinois, at 0830 cdt. In his written statement, the instructor pilot said that they were 4 miles from the airport, and in radio contact with the control tower, when the airplane's engine began to run rough. He noted "an immediate rpm loss from 2500 to 2200 rpm. The instructor pilot notified the control tower that they had engine trouble and requested a closer runway. "I tried to trouble shoot - switched [to the other] fuel tank, adjusted mixture, carburetor heat on, cycle the magnetos - but nothing worked, and the rpm dropped to 2000. At this point, we were about 1,300 to 1,400 agl (feet above ground level) and 2 1/2 miles out. Oil began to cover the windscreen and [oil] pressure dropped to zero. [The] rpm was decreasing and I believed seizure was eminent." The instructor set the mixture to lean and attempted to glide to the runway, but noted that he was low in altitude. "I sidestepped to aim for the grass and avoid [the runway] lighting and the localizer. I couldn't make the grass, so I landed in the corn." The National Transportation Safety Board examined the wreckage in a T-hangar at the University of Illinois - Willard Airport, on October 8, 1996, at 1030 cdt. The outboard 46 inches of the forward leading edge of the left wing were bent upward and aft. The left wing tip was broken off at the rivet line. The engine cowling showed minor damage. The nose gear was broken off. The lower engine mount and bottom of the firewall were bent down and aft. A 30-inch tear was observed along the bottom of the fuselage beginning at the firewall and running aft. The right wing showed dents and minor crushing along the leading edge. The right main landing gear was broken off. The left side of the windscreen was broken out. The right side of the wind screen was covered with a film of oil. The remainder of the airplane's structure was undamaged. Flight control continuity was confirmed. The engine was removed for further examination. The engine showed a 1/4-inch square hole in the top left forward seam of the case. There were two pushed out areas, approximately 1/4-inch square, one on each side of the top seam of the engine, midway down the case. Examination of the internal engine components revealed that all eight valve lifters were broken. Pieces from the broken lifters were found throughout the engine case and in the oil reservoir. Metal fragments were found in the oil screen and filter. The bases of all four cylinders and the bottoms of all four pistons showed nicks, and chips. Both halves of the inside engine case showed impact marks throughout. Engine controls, engine accessories and other airplane systems revealed no anomalies. The lifters, lifter pieces, and oil filter were retained for laboratory examination. The lifters, lifter pieces, and oil filter were examined at Engineering Systems, Incorporated, Aurora, Illinois, on November 6, 1997. Hardness testing on the surviving faces of three lifters showed the faces met specified hardness. The body hardness of one lifter was tested and found to meet the specified hardness. The examination of the lifters and lifter pieces showed brittle fractures due to overload. Some areas on the lifter faces showed evidence of heavy loading from the camshaft.