An improved viscous fluid heater is disclosed. The heater has a rotor that is operably coupled to a drive shaft. The rotor is disposed in a heating chamber filled with viscous fluid. The rotor is rotated with the drive shaft to shear the viscous fluid and generate heat in the heating chamber. The rotor has a flat rotor body and a boss. The boss has an axial length greater than that of the rotor body. A mechanism is provided with at least the boss. The mechanism mounts the rotor on the drive shaft and transmits torque of the drive shaft to the rotor.
