On June 23, 1995, at 1510 central daylight time, a Piper PA-32R, N39620, sustained substantial damage from an emergency landing in a field. The pilot had shut down the engine after an engine fire and smoke in the cockpit. The commercial pilot reported no injury. The 14 CFR Part 91 flight departed Iowa City, Iowa, en route to Point Lookout, Missouri. Visual meteorological conditions prevailed and an instrument flight plan was filed. The pilot reported that he was cruising at 10,000 feet mean sea level and that all instrument indications were normal. At about 18 to 19 miles north of the Columbia Regional Airport, Columbia, Missouri, he heard a muffled explosion from the engine compartment. The engine quit, and flames started coming out of the top left louvre of the engine cowling. The cabin began filling with smoke, and oil began spewing onto the windscreen. He shut off the fuel flow and began an emergency descent. He reported he successfully executed the in-flight fire, and smoke in the cockpit procedures. He then began looking for a field to make an emergency landing. He picked out a hay field and put the gear down after clearing a powerline. However, the gear did not have enough time to fully extend, and the gear collapsed at the time of impact. The airplane came to a stop after about a 200 foot skid with no subsequent fire. The airplane was inspected by an Airworthiness Inspector from the Federal Aviation Administration. He reported that the Bendix dual magneto had separated from the engine. Futher investigation revealed that the lower magneto mounting nut and holding clip were missing. The upper magneto mounting nut and clip were still attached to the stud bolt. Attached to the top clip was the magneto gasket and a portion of the housing flange that had separated from the magneto housing. He reported that from the conditions viewed, it appeared that the lower nut and clip came off at some time, and that the upper nut and clip held the magneto in place until it failed from overstress. An examination of the Bendix dual magneto, model number D6LN-2031, serial number 19875K, was conducted by the Materials Laboratory Division of the National Transportation Safety Board on October 31, 1995. An inspection was made of the bottom surface of the magneto housing in the area where the bottom clip normally fastens to the magneto. The examination revealed that two longitudinal markings (chipped paint) were apparent away from the flange. The bottom clip had a spacing which matched the chipped paint on the magneto housing, as if the clip had moved longitudinally relative to the housing which produced longitudinal marks in the housing. The Materials Laboratory examination of the top magneto housing flange, which had separated from the magneto, revealed fracture surface markings indicative in fatigue cracking before the onset of overstress separations. (See attachments)