On August 9, 1997, at 1100 Alaska daylight time, a Cessna 185 tundra tire equipped airplane, N5246E, sustained substantial damage when it contacted terrain during landing roll on a Chickaloon River off airport landing site, 30 miles north of Chickaloon, Alaska. The commercial certificated pilot and two passengers were uninjured. The airplane was operated by Talkeetna Air Taxi, Inc., of Talkeetna, Alaska. The flight was conducted under 14 CFR Part 135 as an on demand air taxi from Palmer to the Chickaloon River. Visual meteorological conditions prevailed at the time of the accident, and a company VFR flight plan was filed. The pilot stated that during landing roll, while decelerating through 25 knots, the left main wheel separated at the axle mounting bolts. The airplane ground looped, and the propeller and wing contacted the ground. Metallurgical inspection conducted at the NTSB laboratory revealed multiple fatigue crack origin sites on the attaching bolt hole walls of the left hand main landing gear leg. Scanning electron microscope inspection revealed a fatigue crack originating from corrosion pits. Metallurgical composition was consistent with 6150 steel specified by Cessna. Hardness was tested to be 50.8 to 52.7 HRC. The specified HRC range is 48.3 to 50.0 HRC. The operator reported that this landing gear leg had been damaged during a lake landing in February 1996. It was removed, arched, heat treated, and reinstalled on April 12, 1996. At this time, landing gear leg serial number 814048 underwent a magnaflux inspection for cracks at 5437 airframe hours. This was 806 hours prior to the accident. Previous magnaflux inspections had been performed at 5086 airframe hours, and 5415 airframe hours. Cessna maintenance procedures do not have required inspections, or life limits, on landing gear legs. The FAA does not mandate any inspection interval for landing gear in off airport use. Landing gear legs are replaced "on condition." The air taxi company maintains the airplane on an FAA approved continuing airworthiness inspection schedule, and voluntarily performs non-destructive magnaflux inspections of landing gear legs for cracks. These inspections are performed due to the use of the airplane for off-airport wheel and ski landings.