Abstract: A method is disclosed of forming a header assembly in which a plurality of tubes transition into a single header passage. The method includes forming a blank extrusion comprised of a plurality of juxtaposed tube elements interconnected by web sections of a thickness approximately double the wall thickness of the individual tube members in the blank. The blank is placed in a female die and a wedge-shaped male forming tool is advanced into the blank for a portion of the length of the blank, the male forming tool being configured to split the interior of the blank along the center of the web sections as the tool advances thereinto, forcing the web sections and connected tube portions apart and into contact with the wall of the female die cavity to form the header section.