On June 21, 1998, at 1145 eastern daylight time, a float equipped Cessna 180A, N9753B, operated by an airline transport rated pilot, nosed over on landing at a private airstrip in Bay City, Michigan. The pilot and passenger received minor injuries. The airplane was substantially damaged. The 14 CFR Part 91 personal flight was operating in visual meteorological conditions without a flight plan. The flight originated from the private airstrip at 1115 edt. The pilot reported that during the landing roll a "rumbling noise" was heard and the airplane pulled slightly to the left when he applied the brakes. He stated he pulled back on the elevator to no avail, and the airplane nosed down and flipped inverted. Post accident inspection of the airplane by an inspector from the Federal Aviation Administration (FAA) Grand Rapids, Michigan, Flight Standards District Office (FSDO), revealed the front wheel on the left float separated from the airplane. He also reported the wheel attachment bracket then dug into the sod airstrip which resulted in the airplane nosing over. The left and right forward wheel casters and associated hardware were forwarded to the NTSB North Central Regional Office from the Inspector from the Grand Rapids FSDO. It was noted that the axis bolt on the left wheel caster was separated from the caster. These parts were then sent to the NTSB Metallurgical Laboratory, Washington, D.C., for inspection. The metallurgist who inspected the parts reported, "Examination disclosed that about 90 percent of the bolt [left caster] fracture was flat, smooth, and contained crack arrest positions indicative of fatigue crack propagation." "The SEM examination of the separated bolt pieces revealed that both origins of fatigue cracking in the bolt from the loft bow gear were located at small steps, apparently created during machining of the transition radius between the thicker head and the thinner median portions of the bolt." See attached Materials Laboratory Factual Report for further details.