On August 11, 2010, about 0715 Pacific daylight time, a Grumman Aircraft Engineering Corporation Schweizer G-164B, N6781K, experienced a loss of engine power while maneuvering to spray a field near Tracy, California. Trinkle AG Flying Inc., operated the airplane under the provisions of 14 Code of Federal Regulations Part 137 as an agricultural crop dusting flight. The commercial pilot/owner was not injured; the airplane sustained substantial damage to the wings, fuselage, and tail section. Visual meteorological conditions prevailed for the local area flight and no flight plan had been filed. The National Transportation Safety Board (NTSB) investigator-in-charge (IIC) interviewed the pilot. According to the pilot, he had started flying about 0630 the morning of the accident. The accident flight was the second flight of the day. He reported that the airplane was loaded with 240 gallons of pesticide and about 35 gallons of fuel. While spraying a cornfield, he heard a loud bang, followed by a loss of engine power. The pilot also reported that the engine began shaking. He was able to land the airplane in an alfalfa field, but the field was slick and as he activated the left brake to intentionally ground loop the airplane, it did not respond. The airplane continued its forward movement until it struck a fence at the end of the field. The airplane came to rest inverted in a nearby ditch adjacent to the alfalfa field. The engine was examined under the supervision of a Federal Aviation Administration inspector. During the examination, it was discovered that the head to the number five cylinder was cracked. In addition, the connecting links for all but the number 7 cylinder were broken and the master rod had sheared. They were unable to determine if the master rod or the connecting link failed first. No other anomalies were noted with the airframe or the power plant.