On September 21, 1995, about 1415 central daylight time, a Bell 47G, N616X, force landed following an in-flight engine vibration, near Union City, Tennessee. The helicopter was operated by the pilot under the provisions of 14 CFR Part 91, and visual flight rules. Visual meteorological conditions prevailed. A flight plan was not filed for the solo, instructional flight. There were no injuries to the private pilot, and the helicopter was substantially damaged. Origination of the flight was Union City, Tennessee, about ten minutes prior to the accident. The pilot reported that while enroute to his farm, he noticed a vibration in the flight controls and initiated an autorotation. The pilot stated that he touched down in a flat bean field with forward speed. The skids caught on the beans, the helicopter rolled over, caught fire and burned. The pilot held a private pilot certificate with airplane single engine, multi engine, and instrument ratings. He also had a helicopter solo endorsement. During the ensuing engine teardown, it was noted that the connecting rod from the #4 cylinder was found separated from the crankshaft with the rod cap wedged between the rod and the case. Only 1 connecting rod bolt was located for the #4 cylinder, and it was located in the cap, with the threads stripped from the bolt. The separation of the rod resulted in the fracture of the camshaft, case cracking, and lifter separation.