On September 13, 2004, approximately 1245 mountain daylight time, a Cessna P206A, N4661F, nosed over during a forced landing in an open field near Clearwater, Idaho. The airline transport pilot and his three passengers were not injured, but the aircraft, which is owned and operated by the pilot, sustained substantial damage. The 14 CFR Part 91 personal pleasure flight, which departed Weiser, Idaho, about 75 minutes prior to the accident, was being operated in visual meteorological conditions. No flight plan had been filed for the flight to Moose Creek, Idaho. The ELT, which was activated by the accident sequence, was picked up by a Forest Service aircraft, whose crew notified the local Sheriff's Department. According to the pilot, while en route, the aircraft's engine began to run rough, vibrate, and make a lot of unusual noises. He therefore checked the oil pressure, which was indicating zero psi, so he reduced the power and turned toward lower terrain. About 30 seconds after the onset of the symptoms, the engine seized, and the pilot decided to make an emergency power-off landing in an open field. Due to the fact that the pilot was landing without power on a "fairly steep upslope," the nose gear contacted the surface with sufficient force to result in its collapse, and as the aircraft began to slide across the soft terrain, the nose gear strut dug into the dirt, and the aircraft nosed over onto its back. A preliminary inspection of the engine revealed that the case was cracked in an area above the number three piston, and the small end of the number three connecting rod was protruding slightly from the top of the case. A further teardown inspection of the engine revealed that the number three piston was no longer connected to its associated connecting rod, but that both the piston pin boss in the piston and the piston pin boss in the small end of the connection rod were intact. A significant portion of the internal components of the engine had been destroyed, and eleven separate pieces of the piston pin were able to be recovered from areas inside the case and in the oil sump. Although the damage to the pin had obliterated any manufacturer's identification markings, all the other piston pins in the engine had the Superior Air Parts "S" stamped into their aluminum caps. The recovered piston pin pieces were submitted to the NTSB's Materials Laboratory Division, where close examination revealed that four pieces of the pin contained areas of crack propagation along the longitudinal axis of the pin in a plane normal to the surface, with arrest marks consistent with fatigue. Further inspection of the pin revealed that the fatigue regions were all part of a single fatigue crack that initiated from the inner diameter surface of the pin. None of the recovered pieces contained the origin of the crack, but examination of the inner diameter surface of the recovered pieces did not reveal any clear signs of corrosion or other preexisting surface damage. One small additional fatigue crack was observed in one of the fragments, but the location of that fragment within the pin could not be determined. The macroscopic superficial hardness measurement of the outer diameter surface of the failed pin (HRC 44) was within the required hardness range of HRC 42-45. Due to the lack of an origin area, a clear determination of the initiating cause could not be made.