On Monday April 5, 2004, approximately 0800 Pacific daylight time, a Grumman G-164, N48564, experienced a main gear collapse during a forced landing in an open field near Nezperce, Idaho. The commercial pilot, who was the sole occupant, was not injured, but the aircraft, which is owned and operated by the pilot, sustained substantial damage. The 14 CFR Part 137 aerial application flight, which departed Nez Perce Municipal Airport, Nezperce, Idaho, about 10 minutes earlier, was being operated in visual meteorological conditions. No flight plan was filed. There was no ELT activation. According to the pilot, while en route to apply dry fertilizer to a field, the aircraft's engine popped and then experienced a significant loss of power. Because there was not sufficient power to keep the aircraft airborne, the pilot elected to execute a forced landing in a nearby field. He therefore started dumping his load of fertilizer, and attempted to set the aircraft down on the soft terrain. Although the touchdown was successful, soon thereafter the main gear sank into the soft terrain, and the right main gear leg folded aft. As the gear leg collapsed, the nose and right wing of the aircraft impacted the terrain. According to the FAA Inspector who responded to the scene, a post-accident inspection of the engine revealed that the head of the number seven cylinder had fractured circumferentially just above the location where the head screws into the cylinder barrel. Although the head had not separated from the barrel, the fracture was far enough around the barrel that it created a misalignment of the cylinder geometry to an extent that the intake valve would not close. According to this same inspector, there was no indication of detonation or any other anomaly that would have been expected to apply greater than normal loads to the cylinder structure.