On May 9, 2002, about 1955 central daylight time, a Grumman-Schweizer G-164B, N48485, piloted by a commercial pilot, sustained substantial damage when it nosed over during a forced landing following a loss of engine power while maneuvering near Hordville, Nebraska. Visual meteorological conditions prevailed at the time of the accident. The aerial application flight was operated under the provisions of 14 CFR Part 137 and was not on a flight plan. The pilot was not injured. The local flight originated from the Knox Landing Airport, York, Nebraska at 1900. The airplane suffered a total loss of engine power while climbing from a low altitude application of herbicide. The pilot stated that he, "...pushed mixture rich and added power. I checked the mags and started looking for a landing site. I landed in a disked field 1/2 mile southeast of the field I was spraying. The aircraft rolled in a 3 point attitude for 300 feet. The wheels sank in the soft dirt and flipped the aircraft over." Upon inspection of the engine, a rocker arm shaft on one of the exhaust valves was found to have split. No other anomalies were found with respect to the airframe, engine, or systems that could be identified as existing prior to impact.