Overheating of various parts of a hydraulic torque converter between the engine and the transmission of the power train in a motor vehicle, wherein the torque converter employs a bypass clutch which generates heat when it is operated with slip, is counteracted by establishing one or more paths for the flow of hydraulic converter fluid from a first plenum chamber at one side of the axially movable piston of the bypass clutch to a second plenum chamber. A first portion of each such path is defined by at least one groove in a friction surface of at least one part of the bypass clutch, and a second portion of each such path is defined by one or more throttling orifices which offers or offer to the fluid flow a resistance several times that of the groove(s).