An aluminium weld-on radiator neck with a 40mm outlet, the larger of the two sizes we list. Forty millimetres suits main radiator top and bottom hoses on most engines, where coolant volume is high enough that the 32mm neck would become a restriction and hurt cooling performance. Weld it into a radiator end tank, a fabricated header tank or a custom alloy coolant pipe to place the outlet exactly where the hose run wants to go, which is usually somewhere the donor radiator never provided. Aluminium construction means it welds directly to alloy tanks and pipework without dissimilar metal issues. Prepare the surface properly before welding and finish the joint with a conventional hose clamp.