On July 6, 2002, approximately 0845 mountain daylight time, a Grumman-Schweizer G-164B "Agcat," N8440K, registered to and operated by Weiser Air Service, Inc., and being flown by a commercial pilot, sustained substantial damage during a forced landing following a loss of power during a spray run near Weiser, Idaho. The commercial pilot was uninjured. Visual meteorological conditions existed and no flight plan had been filed. The flight, which was a crop dusting flight, was operated under 14 CFR 137, and originated from Weiser, about 0840 The pilot reported that as he departed the Weiser Municipal airport with 330 gallons of fluid and proceeded north to the destination field (about two miles north of the airport). During the first spray run over the field the engine began running rough ("popping" and "banging"). The pilot terminated the spray, pulled up and turned right away from obstacles and attempted to trouble shoot the problem. As he continued the right turn crossing the Snake River, he dumped the spray load and continued turning back toward the Weiser airport (refer to attached Diagram and NTSB Form 6120.1/2). During the right turn the engine abruptly stopped and the pilot executed a forced landing in a beet field directly ahead. During the landing roll, the aircraft crossed an elevated dirt/gravel roadway bounded on its north and south edges by a shallow ditch, and the aircraft's landing gear separated from the airframe. The aircraft then nosed over and came to a stop. The pilot reported that the aircraft's Pratt & Whitney R-1340, 9-cylinder engine had a total of 1591 hours and had been overhauled 81 hours previous to the accident. The last inspection was conducted May 22, 2002, 31 hours previous to the accident. The engine was shipped to Tulsa Aircraft Engines, Tulsa, Oklahoma, where an examination was conducted on August 16, 2002, under the oversight of an FAA Inspector from the Federal Aviation Administrations Oklahoma City Flight Standards District Office. The inspector reported that the examination revealed the number 3 cylinder exhaust valve steel pushrod was broken near the rocker arm end of the rod and the rod displayed very slight bending deformation (refer to Attachment PTRS-I). Removal or separation of the exhaust pushrod results in the exhaust valve's inability to open during the exhaust phase cycle for the associated cylinder, and can result in hot exhaust from the cylinder back-flowing out the intake valve when it opens during the intake cycle. The Inspector also reported that the separated ends of the push rod displayed extensive battering damage with the ends "mushroomed" over.