A heat exchanger and method of making a heat exchanger which includes providing a header having openings adapted to receive a plurality of tubes. The tubes are inserted into the header openings, then, substantially uncured fluid sealing material is applied to at least the inner surfaces of the header openings. The inner surfaces of the header openings and the outer surfaces of the tubes are connected by the sealing material. The method further includes curing the sealing material after the tubes are inserted into the header openings. The sealing material thus provides a flexible, bonded, liquid tight, tube-to-header joint.