On July 17, 1995, at 0815 mountain daylight time, an Air Tractor 301, N4326S, owned by Lewis Flying Service, Inc., a 14 CFR 137 agricultural aircraft operator certificate holder, received substantial damage in a forced landing following a loss of engine power on takeoff from the pilot's private airstrip near Roberts, Idaho. The commercial pilot, a co-owner of the single-seat agricultural aircraft, was not injured. The flight was a 14 CFR 91 positioning flight to Rigby, Idaho. Visual meteorological conditions existed and no flight plan had been filed. The pilot stated in his report on the accident: "During climb- out at approx[imately] 40 foot altitude engine lost power and [began] backfiring. All terrain (360 deg) was cultivated fields with near mature crops...Established glide into barley field - 3 foot tall crop...attempted soft-field landing. Wheels contacted barley; aircraft immediately pitched over and cartwheeled on left wing...." A post-accident inspection of the Pratt & Whitney R-1340-AN1 radial engine revealed that the boss securing the exhaust valve rocker arm shaft to the number 3 cylinder head had separated from the cylinder head, allowing the entire rocker arm to move freely rather than pivot. This failure prevented the number 3 cylinder's exhaust valve from opening during engine operation. No other evidence of engine damage or malfunction was noted. According to the accident report and copies of the aircraft and engine logs submitted by the airplane's owner, the engine had been operated 201.3 hours since the airplane's last inspection (an annual inspection conducted on April 7, 1995), and 233.8 hours since its last overhaul. The engine was installed on the airplane at zero time since overhaul on August 7, 1994 according to the engine log. The FAA publication, "General Aviation Airworthiness Alerts" (AC 43-16), December 1995 issue, which the FAA distributes to FAA- certificated Inspection Authorization (IA) airframe and powerplant mechanics, contained an advisory for cylinder cracks on Pratt & Whitney model R-1340 engines. The advisory reported: "After receiving reports from three engine overhaul shops concerning cracks in the cylinder exhaust rocker shaft housing area, an investigation and interviews suggest this may be a systematic problem. Two repair stations stated they reject approximately 3 percent of the cylinders passing through their shops for this type defect. The defects in the exhaust rocker shaft housings range from slight cracking to complete separation of the housing above the rocker shaft....The Service Difficulty Reporting (SDR) data base revealed 42 similar reports, and 7 others which may have been related to this topic....the exact cause for cracking in this area has not been determined...Since the rocker shaft uses an 'O-ring' seal under each of the nuts, it is possible that when oil seepage is found, the nuts are tightened beyond the torque limits established by the manufacturer's maintenance manual....It is also possible that thermal expansion and contraction...may result in this type defect....It would be wise to pay close attention to this area during scheduled inspections."