On May 23, 1997, at 1930 central daylight time (CDT), a Ercoupe 415-G, N94415, sustained substantial damage during landing rollout when it nosed over in a field after a forced engine-out landing near Colusa, Illinois. The Airline Transport Pilot (ATP) reported engine knocking during initial climb. Both pilots sustained minor injuries. The personal 14 Part 91 flight was operating in visual meteorological conditions. No flight plan was on file. The flight departed Douglas Airport (IL84), Illinois at 1845, for a local instruction flight. In a written statement by the Pilot in Command, an ATP and Certified Flight Instructor (CFI), he stated that the during the initial climb after a series of touch and go's, "at an altitude of 250 feet, the engine began running rough and made a clattering noise." The CFI said he then took control of the aircraft. The pilot reported that the "engine quite completely with the propeller stopping in the horizontal position." The CFI next executed a forced landing in a "freshly planted" field. The CFI said, [the] "Aircraft touched down firmly and we rolled approximately 50 feet when the nose wheel dug in and the aircraft went over onto its back, nose first." Post crash investigation of the airplane showed that the exhaust valve rocker arm on the number two cylinder was broken in two pieces. Further inspection of the number two cylinder showed that the exhaust valve rocker arm had been interchanged with the intake valve rocker arm of the same cylinder. The exhaust valve rocker arm differs from the intake valve rocker arm by having a drilled oil port. This oil port aids in cooling of the exhaust valve. In a telephone conference with the engine manufacturer, it was confirmed that the switched rocker arms would produce conditions which could fracture the switched rocker arm. No other anomalies were found with the airplane.