a combination structure interconnecting and sealing means for joining an air duct with the end plate of a heat exchanger core to allow for variations in spatial dimensions resulting from thermal expansion . the coupling arrangement comprises a circumferential flange attached , as by welding , to the circular duct . the periphery of the flange is provided with radial slots which are engaged underneath t - shaped clips attached to the heat exchanger plate . within the circumference of the flange and generally in line with the duct and associated manifold section of the end plate is a u - shaped bladder member extending entirely around the joint and welded to provide a seal between the end of the duct and the plate . similar bladder members are provided between adjacent units making up an overall heat exchanger core .