On May 25, 1999, at 1830 central daylight time, a Grumman-Schweizer G-164B, N6524K, piloted by a commercial pilot, received substantial damage on impact with terrain during a forced landing attempt on runway 09 (2,000 feet by 20 feet, asphalt) at Central Valley Aviation Airport, near Buxton, North Dakota. Visual meteorological conditions prevailed at the time of the accident. The aerial application flight was conducted under 14 CFR Part 137. The pilot reported no injuries. No flight plan was on file. The local flight originated from Central Valley Aviation Airport at 1815. In his written statement, the pilot said that the engine began to run rough during a cruise segment between the fields he was spraying. He stated that the engine blew smoke and oil and that the windshield became covered with oil. He stated he turned the airplane towards Central Valley Aviation Airport. He said that on his approach to runway 09, he had no visibility through the oil during his forced landing. He stated that the airplane touched down prior to the runway on a road leading to and aligned with the centerline of runway 09. He said that the airplane had run off that road and a bridge embankment in the safety area before the approach end of runway 09. The airplane came to rest inverted next to the embankment. The pilot reported that the airplane's last inspection was an annual inspection on May 1, 1999. He said that the time since last overhaul on the engine was 1,103 hours and the time since last inspection was 50 hours. The number one cylinder head was found separated from its cylinder and was inspected by the National Transportation Safety Board's materials laboratory. The inspection revealed no indications of preexisting cracks in the cylinder head. The separation occurred in the combustion dome between the cylinder head and the screw on cylinder barrel. The cylinder head revealed marks and dents consistent with piston ring or piston crown contact.