Abstract: A method of affixing a plurality of tubes having oval ends into a plurality of corresponding oval openings of equal circumference in the header wall of a heat exchanger. The minor diameter of the tube ends is greater than the minor diameter of the header openings and the major diameter of the tube ends is less than the major diameter of the header openings. The tube ends are inserted into corresponding openings in the header wall whereupon the minor diameter of the tube ends is reduced and the major diameter of the tube ends is increased to create a substantially fully contacting fit around the circumference with said header openings. Ends of the tubes are welded to corresponding collar openings in the header wall to form a plurality of tube-to-header joints. Flux is applied to the air-facing side of the tube-to-header Joints, and molten solder is applied to the opposite liquid-facing side and flowed into any voids in the welded joints to substantially seal the joints against leakage of liquid.