On July 2, 2000, about 1200 mountain daylight time, a Cessna U206B, N3488L, registered to and operated by the pilot as a 14 CFR Part 91 personal flight, nosed over during the landing roll at Johnson Creek, Yellow Pine, Idaho. Visual meteorological conditions prevailed at the time and no flight plan was filed. The airplane was substantially damaged and the commercial pilot and his three passengers were not injured. The flight had departed from McCall, Idaho, about 20 minutes prior to the accident. During a telephone interview and subsequent written statement, the pilot reported that he flew the normal traffic pattern for the soft field landing technique on runway 17. The airplane touched down on the main landing gear, and immediately after the nose wheel touched down, grass and dirt started to come over the windshield and the airplane rapidly decelerated, subsequently nosing over. A deputy from the Valley County Sheriff's Department responded to the accident site and diagramed the location of where parts were found on the airstrip that separated from the aircraft. The deputy also noted ground disturbances leading up to the inverted aircraft. (see attached Sheriff's report) The deputy noted finding a 3/8-inch nylon nut, an axle bolt with end cups, and the nose wheel fork on the airstrip north of the main wreckage. The nose wheel also separated from the nose wheel yoke, however, the deputy did not make note of its location. Maintenance records indicate that on February 27, 1991, a PA32 nose fork, s/n 594, was installed in accordance with Cessna Service and Parts Manual. A receipt indicated that the "PA32-206L Nose Fork, serial number 594" was purchased from Airglas Engineering Company, Anchorage, AK, by AeroWest Inc., of Troutdale, Oregon, on September 18, 1990. This installation was accomplished prior to the current owner's purchase of the aircraft. The most recent annual inspection accomplished under the current pilot's ownership was completed on July 8, 1999. On September 15, 1999, a logbook entry indicated that a new nose wheel tire and tube were installed approximately 98 hours prior to the accident by the same mechanic who signed off the July 1999, annual inspection. (see Maintenance Information attachment) The Cessna Accessory Kit nose wheel and main wheel conversion applicable to this aircraft, (see Cessna Gear and Tire Installation attachment) indicated that an AN5-55 axel bolt (5 5/8 inches long, and 5/16 inch in diameter) be used with an AN310-5 castle nut and cotter pin. The parts that were recovered at the accident site indicate that the aircraft was equipped with an axel bolt that was not drilled for a castle nut and cotter key configuration. A representative for the Cessna Aircraft Company reported that there is no Cessna approved axle bolt installation without the cotter key configuration. The NTSB Investigator-in-charge contacted a representative from Piper Aircraft to request parts information for the applicable nose gear strut assembly on a Piper PA-32. The parts catalog (see Piper Nose Gear Strut Assembly attachment) indicated use of an AN6-76A axle bolt and secured with an MS20365-624C Nut. Measurements taken of the axle bolt recovered at the accident site indicated that the bolt was 7 3/4 inches long, and 3/8 inch in diameter. These dimensions match the specifications for an AN6-76A bolt. The bolt and nut were sent to the National Transportation Safety Board Materials Laboratory in Washington DC, for further inspection of the locking tightness of the nut. The specialist reported that, "The thread on both the bolt and nut showed no evidence of excessive loading or stripping. Detailed examination of the threads from the bolt revealed the presence of minor wear on the pressure flanks of all the threads, with the exception of one or two threads adjacent to the unthreaded bolt shank. Wear was also noted on the non-pressure flanks of about half of the threads (those closer to the end of the bolt.)" The specialist further stated that, "Detailed examination of the threads of the lock nut revealed the presence of minor wear on the pressure flanks along the entire length of the nut, indicating that the nut had been threaded completely onto a bolt at some time." "Wear was also noted on the non-pressure flanks in the deformed area associated with the locking cuts."