Heat exchanger, in particular radiator for motor vehicles

A heat exchanger for a motor vehicle, comprising:

• a radiator block including a plurality of cooling tubes,
• at least one header box, wherein the plurality of cooling tubes are attached to the header box,
• wherein the at least one header box includes a plurality of chambers which are divided off from one another by partitions,
• a movable actuating member fluidly connecting the plurality of chambers via connecting orifices arranged in a connecting duct,
• wherein the actuating member is a piston adjustable axially between an open position and a closed position, and cross sections of the connecting duct and of the piston decrease in size from a first outermost connecting orifice to a second outermost connecting orifice,
• wherein the connecting duct is a stepped duct and the piston is a stepped piston,
• wherein the stepped duct has, in a region of the connecting orifices, chambers of different cross sections, and the stepped piston has piston portions of different cross sections,
• wherein annular gaps are provided between a wall of the stepped duct and the piston portions which, in the closed position of the piston, are sealed off with respect to one another and, in the open position, form spaces communicating with one another,
• wherein the annular gaps are sealed off by sealing rings arranged on the stepped piston.