On May 18, 1998, about 1420 eastern daylight time, a Grumman AA-5A, N9884U, registered to a private individual, was substantially damaged during a forced landing in a field near Sumter, South Carolina. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 instructional flight. The certified flight instructor (CFI) was not injured and the private-rated Biennial Flight Review (BFR) applicant sustained minor injuries. The flight originated about 30 minutes earlier from the Sumter Municipal Airport, Sumter, South Carolina. According to the CFI, the BFR applicant had completed the airwork portion of the flight and was returning to the airport when, while flying at 2,000 feet, he noted that the oil pressure was below the lower red arc. He asked the BFR applicant who was one of three owners of the airplane if that was normal, and the applicant advised no. The flight turned towards the departure airport and about 20 seconds later, the engine began to run rough. The CFI took the controls and executed the emergency procedures to restore engine power which was unsuccessful at that time. The CFI then maneuvered the airplane towards a field and while on final approach with the throttle partially applied, engine power was momentarily restored. The flight was then too fast and the CFI maneuvered the airplane towards another nearby field. The airplane was landed nearly perpendicular to the plowed direction and after touchdown, the nose landing gear collapsed and the engine nearly separated from the airframe. The engine was removed from the airplane and examined by an FAA inspector. Complete disassembly of the engine revealed no evidence or preimpact failure or malfunction; however, the solder at the oil pressure screen was melted, and resolidified. A copy of the FAA inspector statement is an attachment to this report. The oil cooler bypass valve was retained for further examination. Examination of the oil cooler by-pass valve revealed that by design, the valve is to expand a minimum of .160 inch between 150 and 185 degrees Fahrenheit. Testing of the valve at 170 degrees Fahrenheit, revealed it expanded only .105 inch, which is less than the specification. Review of the engine logbook revealed that the engine had accumulated a total time of 2,091 hours since manufacture, with no record of the oil cooler by-pass valve being replaced.