All material is copper so cutting, forming and soldering is relatively easy. The core is made of 4 pieces of domestic fin and tube baseboard hot water heating radiator material. The bottom tank is made from 1-1/4 inch or 1-1/2 inch copper tubing. The two-piece top tank is made from 0.020 copper sheet. I used a flattened piece of copper rain gutter. The bottom tank has four 3/8-inch holes drilled and extruded from the inside out so as to form a coned-in lip (see illustration in the Image Gallery). The top tank holes are punched out with a wad cutter with the material placed on the end grain side of a wooden block so the holes will be coned in slightly toward the outside of the top tank (see illustration).
Try to make all holes with a slight friction fit between the tubes and the holes. Make the holes in the top tank before bending it into shape. Do not bend the perimeter tabs of the top tank until after the core tubes have been soldered to it. To hold the parts in alignment for soldering, use a tray of sand about two inches deep. Before soldering the core tubes in place, make a tin shield to protect the thin aluminum fins from the direct flame of the propane torch. The thin aluminum fins will not withstand the propane torch flame even a few seconds. Be sure to tin the mating surfaces before soldering the parts together so as to facilitate the flow of solder between the joints.
Soldering is done in this order: first the plug and elbow of the bottom tank; radiator tubes to the bottom tank; radiator hose connector to the top tank; filler neck to the top tank. Now bend the tabs of the top tank and solder the tank top.