On August 19, 2014, about 0740 mountain daylight time, the pilot of an Air Tractor AT-401, N4542W, made a forced landing in a field near Wray, Colorado. The pilot, the sole occupant on board, sustained a minor injury. The airplane was substantially damaged. The airplane was being operated by Tri-County Ag of Wray, Colorado, under the provisions of 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed at the time of the accident, and no flight plan had been filed. The local flight originated from Wray Municipal Airport (2V5), Wray, Colorado, approximately 0700.According to the pilot's accident report, he had just finished spraying a field and was climbing out to return to his home base when he felt a subtle vibration from the engine and noticed manifold pressure dropping rapidly. When he attempted to increase manifold pressure by opening the throttle, the engine began to vibrate violently and lost all power. The pilot made a downwind forced landing. To avoid a drainage ditch and power lines ahead, the pilot applied heavy braking and the airplane nosed over. The vertical stabilizer and rudder, cockpit, and left wing sustained crushing damage. On October 14 and 15, the engine was partially disassembled and examined. Engine oil was drained from the rocker arm sump. In addition to carbon, a small amount of aluminum and steel filings were recovered. A compression check of all the cylinders was made with the following results: Cylinder 1 0/80 Cylinder 2 50/80 Cylinder 3 0/80 Cylinder 4 20/80 Cylinder 5 25/80 Cylinder 6 75/80 Cylinder 7 65/80 Cylinder 8 55/80 Cylinder 9 55/80 Cylinders 1, 3, and 4 were removed and examined. The intake valve in the number 3 cylinder had a ridge worn of its face, and the valve seat was worn excessively. The exhaust valve in the number 1 cylinder was stuck open and was found to be binding in the valve guide. It was removed with extreme difficulty. The exhaust valve clearances were then checked and found to be less than the Pratt and Whitney overhaul limits of .003" - .0055", with a maximum wear limit of .010", to wit: Cylinder 1 Clearance .002" Stem size .610" Guide size .613" Cylinder 3 Clearance .0022" Stem size .611" Guide size .6132" Cylinder 4 Clearance .0022" Stem size .611" Guide size .6132"