A method for producing a heat exchanger core (40,40') defining a plurality of exterior faces and an interior region, comprising the steps of: stacking a bottom end sheet, a plurality of alternately stacked individual hot and cold layers, and a top end sheet, each of the individual hot and cold layers including: a fin element forming a plurality of parallel open-ended channels adapted to pass a fluid therethrough; a parting sheet (18,48') separating the individual hot and cold layers from the next individual hot and cold layers; and two closure bars positioned on opposite sides of the fin element, parallel to the open-ended channels and extending a length of the open-ended channels; wherein: each closure bar has an original closure bar width and a closure bar height, the original closure bar width at least as great as the closure bar height; each closure bar defines an inner face and an outer face, the inner face directed toward the fin element and the outer face directed away from the fin element; the open-ended channels of the individual hot layers are parallel to each other; and the open-ended channels of the individual cold layers are parallel to each other; brazing the bottom end sheet, the plurality of alternately stacked individual hot and cold layers, and the top end sheet in a brazing furnace; characterized in that the method further comprises the step of: removing material from each of the plurality of exterior faces by precision material removal, thereby removing material from each closure bar outer face to produce a final closure bar width; and characterized in that the precision machining produces two vertical core bands on each closure bar outer face, each configured to be joined to a manifold by welding, thereby forming a plurality of welding heat-affected zones that are isolated from the interior region. stacking a bottom end sheet, a plurality of alternately stacked individual hot and cold layers, and a top end sheet, each of the individual hot and cold layers including: a fin element forming a plurality of parallel open-ended channels adapted to pass a fluid therethrough; a parting sheet (18,48') separating the individual hot and cold layers from the next individual hot and cold layers; and two closure bars positioned on opposite sides of the fin element, parallel to the open-ended channels and extending a length of the open-ended channels; wherein: each closure bar has an original closure bar width and a closure bar height, the original closure bar width at least as great as the closure bar height; each closure bar defines an inner face and an outer face, the inner face directed toward the fin element and the outer face directed away from the fin element; the open-ended channels of the individual hot layers are parallel to each other; and the open-ended channels of the individual cold layers are parallel to each other; a fin element forming a plurality of parallel open-ended channels adapted to pass a fluid therethrough; a parting sheet (18,48') separating the individual hot and cold layers from the next individual hot and cold layers; and two closure bars positioned on opposite sides of the fin element, parallel to the open-ended channels and extending a length of the open-ended channels; each closure bar has an original closure bar width and a closure bar height, the original closure bar width at least as great as the closure bar height; each closure bar defines an inner face and an outer face, the inner face directed toward the fin element and the outer face directed away from the fin element; the open-ended channels of the individual hot layers are parallel to each other; and the open-ended channels of the individual cold layers are parallel to each other; brazing the bottom end sheet, the plurality of alternately stacked individual hot and cold layers, and the top end sheet in a brazing furnace; characterized in that the method further comprises the step of: removing material from each of the plurality of exterior faces by precision material removal, thereby removing material from each closure bar outer face to produce a final closure bar width; and characterized in that the precision machining produces two vertical core bands on each closure bar outer face, each configured to be joined to a manifold by welding, thereby forming a plurality of welding heat-affected zones that are isolated from the interior region. removing material from each of the plurality of exterior faces by precision material removal, thereby removing material from each closure bar outer face to produce a final closure bar width; and characterized in that the precision machining produces two vertical core bands on each closure bar outer face, each configured to be joined to a manifold by welding, thereby forming a plurality of welding heat-affected zones that are isolated from the interior region.