On July 28, 1999, about 0650 central daylight time, a Rockwell S-2R, N8482V, registered to and operated by Gary Flying Service, Inc., crashed while attempting a forced landing following loss of engine power near Isola, Mississippi, while on a Title 14 CFR Part 137 aerial application flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft was destroyed and the commercial-rated pilot was seriously injured. The flight originated from Belzoni, Mississippi, the same day, about 0630. The pilot stated that while performing aerial application, the engine backfired twice, lost oil pressure, backfired again, and then shut down. While making a forced landing, the aircraft collided with a ditch, which separated the landing gear. The aircraft then spun around 180 degrees and came to rest. A postcrash fire erupted in the left wing area of the aircraft and destroyed the aircraft. Postcrash examination of the engine was conducted at an engine overhaul facility under the supervision of an FAA inspector. The No. 4 cylinder compression was 35 over 80 psi., with leakage through the intake valve. Compression limits for the engine are 60 over 80 psi or above. Seven out of 18 valves were not adjusted within normal limits. All spark plug gaps were set above normal limits. See the FAA inspector's statements and mechanic's statement attached to this report.