On April 25, 2002, about 1257 Pacific daylight time, a Grumman-Schweizer G-164C, N6702K, experienced a total loss of engine power while maneuvering during an aerial application flight about 2 miles east of Olivehurst, California. The pilot made a forced landing in a rough field. During the landing rollout, the airplane nosed over and was substantially damaged. The commercial pilot was not injured. The airplane was operated by Moe's Crop Dusting, Marysville, California. Visual meteorological conditions prevailed. The flight was performed under the provisions of 14 CFR Part 137, and it originated from a field near the crash site about 1254. The pilot holds a Federal Aviation Administration airframe and power plant mechanic certificate. He reported to the National Transportation Safety Board investigator that at the time of the mishap, the airplane's fuel tanks were about 1/2 full. The pilot stated that the airplane had been operating normally during the flight while he was applying fertilizer to the underlying field. Without warning, the Garrett turbine engine lost all power. Subsequently, the engine was removed from the airframe and examined. The pilot reported that the examination revealed the engine's torque sensor-bearing shaft was broken. This broken gearbox component inhibited proper operation of the fuel control, and the fuel supply to the engine was thereby shut off.