On May 13, 1998, at 1415 central daylight time (cdt), a Cessna A185F, N7140H, piloted by an airline transport rated pilot, was substantially damaged when it nosed over during a forced landing following a loss of engine power while in cruise flight. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 personal flight was not operating on a flight plan. The pilot reported no injuries. The flight departed a restricted landing area near Webster, Minnesota, at 1315 cdt. According to the pilot's written statement, "...the propeller RPM started to increase..." while the airplane was at cruise altitude. She said she adjusted the propeller control to reduce its RPM and the "...engine seemed to settle down." While attempting to adjust the mixture, RPM and manifold pressure the propeller RPM increased again. The pilot said the propeller RPM went to its limit and "...the engine clunked a couple of times and lost all power." The airplane was equipped with amphibious float landing gear. The pilot said she chose a grassy field for the forced landing. As the airplane touched down the main landing gear tires "...dug into the soft ground [followed by] the nosewheels... causing the airplane to flip onto its back." According to the Cessna 185 Skywagon Pilot's Operating Handbook Supplement, "Emergency Landing On Land Without Engine Power, 1) Landing gear-- UP on soft or rough ground. DOWN on firm and smooth ground." The handbook continues, "The landing approach attitude and flare is the same for an airplane equipped with a tricycle gear." According to the engine logbook, the engine was a factory overhauled on March 15, 1990. The engine was installed on a Piper PA-31 Navajo, N3590Z, on April 27, 1990. The engine was removed from N3590Z on November 25, 1991. N3590Z was destroyed when a large hangar fell on it during July 1993. N3590Z's left engine was installed on N7140H on April 9, 1992. At that time, the engine had 536.1-hours since its factory overhaul. The engine had 1,056-hours since factory overhaul at the time of the accident. It had received a top overhaul on May 1, 1995, when it had 789.3-hours since its factory overhaul. N7140H's engine crankcase halves, fractured crankshaft, piston and connecting rod assembly for the number 3 and 5 cylinders were sent to the NTSB's Materials Laboratory in Washington, D.C., for examination. The examination revealed that the crankshaft had separated through the number 4 crank cheek and the number 6 crank cheek. The report states, "The fracture through the number 4 crank cheek intersected the forward radius of the number 3 rod journal and the aft radius of the number 2 main [bearing] journal. The fracture through the number 6 crank cheek intersected the forward radius of the number 3 main bearing journal and the aft radius of the number 4 rod journal." The report continues, "Binocular microscopic examination of this crankshaft fragment revealed the surface of number 3 main bearing journal and its forward and aft radius contained moderate circumferential scoring marks. The fracture through number 6 crank cheek contained crack arrest positions typical of fatigue cracking that emanated from multiple origins at the forward radius of number 3 main bearing journal... . The fracture surface in the vicinity of the fatigue origin contained ratchet marks that were coincident with ladder crack... . These ladder cracks were noted all around the forward radius of the number 3 main bearing journal. Fatigue propagation was in the forward direction, through the thickness of the crank cheek... ." The report said "The cap for the number 5 connecting rod... fracture contained fatigue cracks that emanated from multiple origins... [and] were nearly as wide as the width of the cap and propagated through as much as 60-percent of the wall thickness... ." The metallurgists factual report is appended to this report. A review of the Federal Aviation Administration's Service difficulty Report Data for the Lycoming IO and O-540 series engine revealed 22 reports of a broken crankshaft at either one or more of the journals or flange. The reports covered the period between 1986 through June 1998. According to a Lycoming representative there are no material or hardness differences between the crankshafts for the O-540, the IO-540, or TIO-540 series engines. Regarding the dimensional and configuration aspects of the crankshafts, the representative stated, "The difference between the three crankshafts could be bearing journal dimensions, stroke, or counterweight applications." His letter is appended to this report.