On October 15, 1997, at 1800 central daylight time, a Grumman-Schweizer G-164A agricultural airplane, N8819H, sustained substantial damage during a forced landing following a loss of engine power near Altus, Oklahoma. The commercial pilot, the sole occupant, was not injured. The airplane was registered to and operated by the pilot, who was doing business as Brown's Flying Service of Drumright, Oklahoma. No flight plan was filed and visual meteorological conditions prevailed for the Title 14 CFR Part 137 local aerial application flight. During a telephone interview conducted by the NTSB investigator-in-charge and in the attached NTSB Pilot/Operator Aircraft Accident Report, the pilot reported that the engine lost power as he was turning the airplane between spray passes. The pilot further reported that the ensuing forced landing was made to a soft field, and during the landing roll, the airplane nosed over and came to rest inverted. According to the FAA inspector who inspected the airplane at the accident site, the top wing and vertical stabilizer sustained structural damage. Examination of the radial engine, a Pratt & Whitney R-985-14B, by the FAA inspector revealed that a circumferential crack had completely separated the #4 cylinder head from the cylinder barrel.