Abstract: A heat exchanger such as a radiator for an automotive engine is produced by fixing lengths of flattened tubes and corrugated fins to each other by brazing within a furnace. The flattened tube is formed by bending a strip and welding the opposing edges of the bent strip to each other to form a tube of a substantially circular cross-section, and applying pressure to the tube along the weld line to form the flattened tube while depressing the tube wall along the weld line to form an elongated recess or groove along the weld line. Then, a brazing material is applied to the outer surfaces of the flattened tube to cover and fill up any minute weld defects which may exist in the flattened tube whereby the weld defects are repaired.