On March 10, 2000, at 0945 Alaska standard time, a Cessna 207 airplane, N75703, sustained substantial damage during an off airport forced landing about two miles northwest of the Delta Junction Airport, Delta Junction, Alaska. The solo commercial pilot received minor injuries. The airplane was operated by 40 Mile Air Service, Ltd., of Tok, Alaska, under 14 CFR Part 135 as an on-demand cargo flight. The flight departed Fairbanks, Alaska, about 0900 for Healy Lake, Alaska, with 50 pounds of mail. Visual meteorological conditions prevailed at the time of the accident, and a VFR flight plan was filed. The pilot of a second airplane, who was approaching Delta Junction at the time of the accident, told the NTSB investigator-in-charge (IIC) during a telephone interview on March 10, that he overheard the accident pilot transmit a "mayday." He located the distressed airplane while it was still in flight and followed it toward Delta Junction. The witness pilot said he observed thin puffs of white smoke, intermittently coming from the area of the engine exhaust. He said the accident pilot transmitted the engine was losing power, and he was going to attempt to reach the Delta Junction Airport. The witness relayed that the pilot then transmitted the engine had quit, and he was going to land. The witness told the IIC the airplane contacted trees and then cartwheeled into a clearing. He said the pilot got out of the airplane, turned off the Emergency Locator Transmitter (ELT), and was assisted by a nearby homeowner. The accident pilot told the IIC, during a telephone interview on March 13, that while in cruise flight at 5,500 feet msl, he detected engine roughness and a slight power loss, so he diverted toward Delta Junction. About 10 miles northwest of the airport, the engine power began to alternate between idle and half power. The pilot stated that varying magnetos, throttle, mixture, and the electric fuel pump, had no effect. He noted that oil pressure, oil temperature, and cylinder head temperatures all registered in the low end of the green range, which he felt was consistent with low power. When the engine lost all power, he said he pulled the propeller control to the high pitch / low rpm position, and the propeller responded. He said he turned the fuel and electrical power to the airplane off prior to impact. On March 10, two FAA inspectors inspected the airplane at the accident site and observed fuel in both wing tanks, and no preaccident anomalies with the airplane or engine. Initial shop inspection of the engine on March 13 by an FAA airworthiness inspector revealed detonation and disintegration of the number two piston, and metal particles in the induction system and seven spark plugs. The fuel manifold, fuel distributer lines, and fuel nozzles were tested on a calibrated flow bench under supervision of the NTSB IIC on May 8. All components flow tested within manufacturer's tolerances. The magneto to engine timing, internal magneto timing, and ignition harness integrity, was checked on May 12. No anomalies were noted. A review of the engine logbooks revealed that the day prior to the accident flight, the fuel nozzles, and spark plugs, were cleaned. The number two cylinder was replaced on February 12, 1999, at 4,014 tachometer hours, 817 hours prior to the accident. Since that time, the numbers three, four, five, and six cylinders were also replaced, due to stuck rings and burnt exhaust valves. The number one cylinder had been replaced 181 hours prior to the replacement of the number two cylinder. The manufacturer's service bulletin, SB 91-8, recommends a time between overhauls (TBO) for the IO-520 engine of 1,700 hours. The engine had accumulated 1,560 hours since overhaul.