On June 25, 2016, about 1800 Alaska daylight time, a Stinson 108 airplane, N9063K, sustained substantial damage during a forced landing, following a loss of engine power near Big Lake, AK. The airplane was registered to and operated by the pilot, as a visual flight rules (VFR) flight under the provisions of 14 Code of Federal Regulations (CFR) Part 91 when the accident occurred. Of the three people on board, the certificated private pilot and one passenger sustained minor injuries and one passenger was uninjured. Visual meteorological conditions prevailed, and no flight plan had been filed. The flight departed Merrill Field Airport (PAMR), Anchorage, Alaska, at about 1725. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on June 25, the pilot stated that the purpose of the flight was to take two family members, who were visiting from out of town, on a sightseeing flight. About 35 minutes into the flight, while circling a friend's cabin at about 550 feet above ground level, the engine began to "sputter" followed by a total loss of engine power. He made a forced landing in an area of densely populated spruce and birch trees. During the forced landing, the airplane sustained substantial damage to wings and fuselage. On October 12, 2016, the NTSB IIC, along with a Federal Aviation Administration (FAA) safety inspector from the Anchorage Flight Standards District Office examined the airframe and engine at the facilities of Alaska Claims Services, Inc., Wasilla, Alaska. The propeller remained attached to the engine crankshaft. Both propeller blades exhibited aft bending with minimal torsional "S" twisting. Examination of the Continental O-470R engine revealed no anomalies, contamination, or evidence of malfunction in any of the engine accessories. The cylinders, pistons, valve train, crankshaft, and other internal components were all without evidence of anomaly or malfunction. The engine was rotated by the propeller. When the engine was rotated, blue spark was observed on the top ignition leads. Examination of the airplane's wing fuel tanks revealed that the tanks had been modified. An additional section had been welded on to the factory fuel tank with lightening holes drilled in the factory end. Each tank was placarded near the filler cap on the exterior of the wing "FUEL 80/87 MINIMUM GRADE 20 GALLONS." The fuel selector inside the cockpit was placarded "18 GAL." No FAA form 337 (major repair and alteration) or logbook entry was located in the airplane's maintenance records for the modification of the fuel system. The examination of the airframe and engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation. The closest weather reporting facility was Wasilla Airport, Wasilla, AK, about 19 miles east of the accident site. At 1756, a weather observation from Wasilla Airport was reporting, in part: wind from 080 degrees at 4 knots; visibility, 10 statute miles; clouds and sky condition, few clouds at 4,600 feet, scattered clouds at 5,500 feet, broken clouds at 7,500 feet; temperature, 66°F; dew point 48 °F; altimeter, 29.89 inHG. After repeated attempts, the pilot did not submit an NTSB Pilot/Operator Accident Report form (NTSB Form 6120.1) as required.