On September 16, 2017, about 1715 eastern daylight time, an amateur-built Exec 162-F helicopter, N123XZ, performed a forced landing to a field near Plymouth, Indiana. The pilot and passenger were not injured, and the helicopter was substantially damaged. The helicopter was registered to North Indiana Rotor LLC and operated by a private individual under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which operated without a flight plan. The local flight departed from Plymouth Municipal Airport (C65), Plymouth, Indiana.The pilot reported that while approaching the airport, the helicopter's engine rpm increased, and the rotor rpm decayed. Unable to maintain altitude, the pilot conducted an autorotation to a field. The helicopter landed hard, the skids bent, and the tail boom buckled, resulting in substantial damage. An examination of the helicopter by the responding Federal Aviation Administration inspector revealed that the helicopter had a Pro Drive system driving the main rotor blades. The main rotor drive belt/cog belt began losing teeth resulting in a failure of the belt to drive the main rotor blades. The Goodyear Falcon PD belt had recently been installed, about 3.5 hours prior to the accident.