Abstract: The use of relatively heavy, large profile headers in a heat exchanger are avoided through a header construction wherein headers (20, 22) are formed of header plates (34) tapped with tank plates (36) having a concave surface (38) facing the header plate (34). The header plates (34) include a plurality of flanges (82) directed away from the tank plates (36) surrounding elongated slots (48). Each of the flanges (82) includes a pilot surface (80) extending about the associated slot (48) for receiving the end (90) of a flattened tube (20). The construction prevents the ends (90) of the tubes (28) from entering the header chambers (42) to a location whereat they could disrupt the flow of a heat exchange fluid therein to prevent uniform distribution of the same throughout the associated header (20, 22).