Abstract: A lightweight high efficiency radiator core having a plurality of cylindrical metal tubes, and a plurality of fins with fin openings and fin collars fitting around the tubes in tight metal-to-metal contact, header plates having openings with collars, and grommets located in said header plate openings and fitting around the collars, the tubes being force-fitted through the grommets, and a method of assembling such a radiator core, by means of passing tubes through the fin collars, and thereafter expanding the tubes into tight metal-to-metal contact with the collars, inserting grommets in the header plates, and thereafter force-fitting the ends of the tubes through the grommets.