Abstract: A heat exchanger is provided including a first aluminum extruded header spaced apart and arranged essentially parallel to a second aluminum extruded header. The first and second headers include a plurality of parallel passageways extending therethrough along a longitudinal axis thereof. The passageways are fluidly interconnected by at least one transverse cross-drilled bore extending essentially perpendicularly to the longitudinal axis. A core assembly is secured between the first and second headers including a plurality of fluid conduits and heat radiating fins surrounding the fluid conduits. A by-pass valve assembly fluidly communicates with the headers and core assembly while providing an alternate pathway independent thereof. Preferably, the by-pass assembly is responsive to fluid temperature and/or pressure for directing fluid flow either into or exterior of the heat exchanger core assembly.