On August 4, 1995, at 0945 central daylight time (cdt), an Air Tractor AT-401, N4543F, registered to Aurora Aircraft, Incorporated, of Aurora, Nebraska, and piloted by an airline transport rated pilot, was substantially damaged during a forced landing following a pilot reported total loss of power. The pilot said the power loss occurred during the beginning of a crop spray run. The 14 CFR Part 137 aerial application flight was not operating on a flight plan. Visual meteorological conditions prevailed at the time of the accident. The pilot reported no injuries. The flight departed Aurora, Nebraska, at 0920 cdt. The pilot said he had begun a swath run when he noticed white smoke coming from the right side of the engine. He said he stopped the swath run and turned back toward the airport. After turning toward the airport the pilot said the engine began to run "...very rough... ." Shortly after the roughness began he said the engine lost most of its power. Upon experiencing the power loss, the pilot said he headed toward a pasture. He said he was unable to glide to the pasture and had to land in a corn field. The airplane nosed over when the tires contacted the corn. A Federal Aviation Administration Principal Maintenance Inspector (PMI) represented the NTSB on-scene. The PMI said the engine examination revealed the number four cylinder's exhaust tower was cracked where the rocker arm shaft went through the head. The owner/mechanic of the airplane stated the cracked exhaust tower causes the cylinder's exhaust valve not to open. This in turn causes a "blow back" condition to exist for the other cylinders. The "blow back" causes' the engine to loose most of its power, according to the airplane owner. The PMI confirmed this information.