On August 13, 1996, about 1615 Alaska daylight time, a float equipped Piper PA-18, N7797B, crashed during a forced landing about 11 miles northeast of Skwentna, Alaska. The airplane was being operated as a visual flight rules (VFR) cross-country personal flight when the accident occurred. The airplane, registered to and operated by the pilot, sustained substantial damage. The certificated private pilot and the sole passenger were not injured. Visual meteorological conditions prevailed. The pilot reported that he departed a remote area known as Quiet Lake. About 3 to 4 minutes after departure at an altitude of 400 to 500 feet above the ground, the engine began to lose power. Emergency procedures failed to restore engine power and the pilot turned back toward the lake. The pilot made a "mayday" radio call and then the engine quit completely. The airplane touched down about 40 feet short of the lake shore and the front portion of the left float assembly dug into the ground. The airplane received damage to the fuselage and left wing. The pilot indicated that both he and his passenger were wearing helmets. On August 15, 1996, the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) examined the airplane after it was recovered. The airframe gascolator contained a small amount of blue colored fuel and several large particles. The carburetor bowl contained a small amount of green colored fuel with several small particles. The exhaust tube from the number 4 cylinder was horizontally split, adjacent to the point where the tube is attached to the muffler. The number 4 cylinder push rod retainer tab was broken. A cylinder compression test revealed the number 1 cylinder was 75/80 PSI; the number 2 cylinder was 76/80 PSI; the number 3 cylinder was 74/80 PSI; the number 4 cylinder was zero. Examination of the bottom spark plug from the number 3 cylinder revealed the insulator was broken. The bottom number 4 cylinder spark plug was heavily fouled with metallic particles. The upper number 4 cylinder spark plug was fouled and the insulator was broken. On August 21, 1996, an engine examination was conducted at Alaska Engines, Anchorage, Alaska. The examination revealed the paint on the number 4 cylinder was discolored and darkened. The right magneto was timed to 21 degrees before top dead center (BTC). The left magneto was timed to 23 degrees BTC. The proper magneto timing is 25 degrees BTC. Removal of the number 4 cylinder and piston revealed the top piston compression ring was frozen in its slot. The outer and upper edge of the top of the number 4 piston was eroded and scored. Tiny particles of the piston material were attached to the spark plugs. Slivers of metal were located in the oil screen. The magnetos were removed and examined. The right magneto points were found to open 5 degrees late. The carburetor was removed and examined. All internal parts conformed to appropriate part numbers and all airworthiness directives had been applied. Sealant was found around the threads of the inlet fuel coupling. The pilot reported that he fueled the airplane from a portable tank that he owned. He last obtained fuel for the tank on August 8, 1996, with the purchase of 100 gallons of 100LL octane fuel. The pilot provided a sample of blue colored fuel from his portable tank and it was submitted for testing. The examination revealed the fuel sample met the specifications for aviation fuel. The lead content of the fuel was 1.63 milliliters per gallon. The maximum lead content for 100LL fuel is 2.00 milliliters per gallon. Examination of the airplane's maintenance records revealed the engine was converted from an O-320-A2B, 150 horsepower, to an O-320-B2B, 160 horsepower, under a supplemental type certificate (STC) on February 7, 1992. The engine was disassembled, inspected, and repaired on February 6, 1996, due a propeller strike.