On February 2, 2007, approximately 1130 Pacific standard time, a Republic RC-3 single engine airplane, N6248K, made a forced landing following the separation of the propeller assembly from the crankshaft flange near Julian, California. The commercial pilot and one passenger were not injured. The airplane sustained substantial damage. The owner/pilot was operating the airplane under the provisions of 14 CFR Part 91. Visual meteorological conditions prevailed, and no flight plan had been filed. The cross-country personal flight departed Gillespie Field Airport (SEE), El Cajon, California, about 1100, with a planned destination of Blythe, California. The pilot reported that during cruise flight, the Hartzell HC-A3VF-3L propeller assembly departed from the crankshaft flange and fell from the airplane. The pilot then executed a forced landing to an open field about 2 miles west of Julian. An examination of the airframe revealed the underside of the fuselage was crushed upward, the left and right horizontal stabilizers were bent, and the six propeller to crankshaft attachment bolts were fractured. The attachment bolts were retained for further examination. Examination of the six attachment bolts by the National Transportation Safety Board Materials Laboratory revealed similar fracture features. The fractures were generally flat and relatively smooth with curved crack arrest marks typical of fatigue progression. Multiple ratchet marks indicated fatigue initiation on opposites side of each bolt indicative of reverse bending loading. However, the majority of the fatigue progression was from one side penetrating an estimated 75 to 85 percent of the cross section. The individual fatigue regions initiated at multiple sites in the root radii of the first fully formed threads. No mechanical damage or corrosion was noted in the thread roots at the origin locations. Measured hardness was consistent with 180,000 pounds per square inch tensile strength steel. A review of the maintenance logbooks revealed the propeller was overhauled and installed on February 5, 2003, at a total airframe time of 802 hours. On January 10, 2007, at a total airframe time of 810 hours, the airframe underwent its most recent annual inspection with no anomalies noted. The pilot reported that at the time of the accident, the propeller assembly had accumulated 8 hours since installation.