On May 31, 2001, about 0700 Pacific daylight time, a Schweizer G-164B, N6902K, collided with terrain following a loss of engine power near Butte City, California. Chuck Jones Flying Service was operating the airplane under the provisions of 14 CFR Part 137. The commercial pilot sustained serious injuries; the airplane sustained substantial damage. The local aerial application flight departed a private strip about 0615. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported that a heavy vibration developed in the engine and he shut it down. He tried to return to a nearby dirt strip, but had to land in a hay field. On the landing roll, the airplane encountered tall vegetation, which damaged the wings and landing gear attachment points. A Federal Aviation Administration (FAA) inspector examined the airplane. He found a fractured bolt on the propeller. A specialist from the National Transportation Safety Board Materials Laboratory examined the fractured bolt. He determined that it fractured at the base of the transition radius between the shank and the minor diameter of the threads. The fracture was on a flat plane normal to the surface, though the fracture was at a slight angle from the circumferential direction. The fracture surface contained arrest marks, which propagated through approximately 90 percent of the cross-sectional area. The origin area was at the surface where the specialist observed two ratchet marks. However, he observed no obvious material defects at the origin, and no defects such as nicks or scratches at the surface. The bolt contained the markings "FAA-PMA-MWA 50001."