A heat exchanger core including a header and a plurality of tubes, each of the tubes having a tube end secured in an opening in a wall of the header to form a tube-to-header joint, a plurality of outer fins capable of transferring heat between a fluid passing through the plurality of tubes and the exterior of the outer fins, the plurality of outer fins being attached between the plurality of tubes and centered about the midpoint in the length of the tubes, the distance from the ends of the plurality of outer fins to the header being a free-of-fin area, and a plurality of resilient tube inserts having a substantially similar cross-section to the cross-section of the plurality of tubes. Each of the resilient tube inserts is inserted into one or more of the tubes at the tube end and secured in the free-of-fin area for the purpose of increasing the tube strength in the free-of-fin area and where the tube passes through, and is attached to, the header. Each of the resilient tube inserts has an outer cross-section dimension greater than the inner cross-section dimension of each tube prior to insertion to enable the tube inserts to remain in place in the free-of-fin area during assembly of a heat exchanger.