Abstract: A lethal service header box for use in manufacturing an air cooled heat exchanger having a front and a back plate, a top and a bottom plate and a left and a right end plate, each of the six plates having a planar external surface and each having an internal surface configured such that the header box is assembled by welding together the six plates, all welding being accomplished exteriorly of the box and within a wedge shaped cross-sectional environment in which the inner terminus of each weld cross-section is at a narrow weld ledge of at least one of the plates.