On April 10, 2004, about 1030 eastern daylight time, a Cessna 206, N5019U, registered to a private individual, was substantially damaged during a forced landing in a ditch near Clewiston, Florida. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 personal flight from the Florida Keys Marathon Airport, Marathon, Florida, to the Vero Beach Municipal Airport, Vero Beach, Florida. The private-rated pilot and one occupant were not injured. The flight originated about 0945, from the Florida Keys Marathon Airport, Marathon, Florida. The pilot stated that after takeoff the flight climbed to 7,500 feet mean sea level (msl) and approximately 40-45 minutes into the flight, the engine started shaking. He repositioned the fuel selector, and the shaking decreased slightly. The flight continued and the shaking continued. He spiraled down to 3,500 feet msl, and the shaking became worse. He glanced at the cylinder head temperature gauge, and oil pressure and temperature gauges, and reported that at that time all were in the green. He looked for a place to land and due to cars on a nearby roadway, landed in a ditch adjacent to the road. Examination of the engine by a representative of the engine manufacturer with FAA oversight revealed that the No. 2 cylinder connecting rod and one of the connecting rod bolts were fractured. The other connecting rod bolt was not fractured and remained secured to the connecting rod cap and section of connecting rod. Additionally, the No. 2 crankpin was dry but not heat discolored, and the oil port for the No. 2 crankpin was free of obstructions. Metallurgical examination of the fractured components by the NTSB Materials Laboratory located in Washington, D.C., revealed the fracture surface of the connecting rod bolt exhibited features consistent with tensile overstress. The bolt was determined to meet specification related to hardness. Mechanical damage to the fracture surface of the connecting rod precluded determination of failure mode. Review of the maintenance records revealed the engine was last overhauled on March 29, 1977, and installed in the accident airplane on April 15, 1977. The engine was disassembled, inspected, and reassembled on July 15, 1977, or approximately 50.0 hours since the overhaul in March of that year. The engine was installed in the airplane, test run, and found to be airworthy. Cylinders 1, 4, and 6 were removed and replaced on April 4, 1981. The engine had accumulated approximately 1,652 hours since last overhaul at the time of the accident. The airplane was released, and the NTSB retained parts were released to the airplane owner on August 26, 2004.