On August 27, 2017, about 1620 Pacific daylight time, a North American Navion A airplane, N8695H, experienced a total loss of engine power during the initial climb from Rohnerville Airport, Fortuna, California. The private pilot was not injured; the airplane sustained substantial damage to the firewall during the forced landing. The airplane was registered to and operated by the pilot as a Title 14 Code of Federal Regulations Part 91 personal flight. Visual meteorological conditions prevailed and no flight plan had been filed for the local flight which departed from Fortuna about 1615. The pilot stated that he departed with about 10-12 gallons of fuel on board. After the airplane reached about 1,000 ft above ground level (agl), the engine experienced a total loss of power. He attempted to return back to the airport, but was unable to maintain altitude and he made an off-airport landing in a pasture about .5 miles from the runway at Fortuna. A Federal Aviation Administration (FAA) certificated airframe and powerplant mechanic completed a post accident examination under the auspice of an FAA inspector. The inspector noted that the original engine for this airplane was replaced by a Teledyne Continental Motors IO-470H which was installed in June 2003. The spark plugs were removed from all cylinders revealing no signs of deterioration, wear or oil/carbon fouling. Continuity was established for each magneto, which produced spark at each cylinder. A borescope inspection of each cylinder revealed no evidence of excessive wear, deterioration, broken rings, or broken valves. The FAA inspector further stated that he observed fuel in the left, right, and main tanks. He sumped samples from each tank disclosing that there was no debris or water visible. Disassembly of the fuel manifold revealed a clean screen; there was no fuel visible. The mechanic removed the fuel inlet supply line to the fuel manifold and activated the electric fuel boost pump. Fuel flowed normally out of the inlet supply line from the tank that was selected, all of which were selected. The reason for the loss of power was not determined.