Abstract: A viscous fluid clutch wherein a fluid medium functions in response to ambient temperature changes to transmit torque between relatively rotatable input and output members by variably filling a predetermined shear space therebetween with a viscous fluid medium, the shear space consisting of interdigitated concentric annular grooves formed on adjacent faces of the input and output members, each groove being approximately 0.060 inch wide and 0.270 inch deep, with the lands separating adjacent grooves being approximately 0.040 inch wide, and the fluid medium used in conjunction therewith having a viscosity of approximately 3,000 centistokes, the combination producing improved results with respect to inhibiting boiling of the radiator coolant and prolonging fluid medium operational life.