Abstract: A simple, reliable bypass valve construction (50) is provided for use in conjunction with a donut oil cooler (16) having an aluminum multi-piece housing (28). The bypass valve construction (50) is easily accessible and readily replaceable so as to lend itself to serviceability. The invention is incorporated in an oil cooler (16) having a housing (28) with a coolant inlet and outlet (42) and (44), and a surface (26) adapted to be sealed against an oil filter (14). The oil cooler (16) includes a plurality of interconnected heat exchange units (21) located inside the housing (28), with an oil inlet to one of the units and an oil outlet from another of the units. An oil bypass is provided through at least some of the interconnected heat exchange units (21) and communicates with a bypass outlet (62) in the housing surface (26).