be used furthermore in at least some applications the tubular insert structure 108 may be omitted from the bypass flow conduit 38 such that the calibrated bypass passage 102 is defined soley by the inner surfaces of the plates 54 56 at the flow restricting region 100 in such an embodiment the bypass flow conduit 38 could for example be similar to what is shown fig4 7 but without the tubular insert 108 in some example embodiments the plates 54 56 are stamped or roll formed to provide the configurations described herein in example embodiments the relative dimensions of the calibrated bypass passage 102 to the remainder of the flow channel 65 through the bypass conduit 38 is such that the total amount of fluid flow through the entire bypass flow channel 65 is substantially determined by the dimensions of the calibrated bypass passage 102 rather than the dimensions of the remainder of the bypass flow channel 65 the length l and diameter di of the calibrated passage bypass passage 102 are selected to allow a desired amount of fluid to bypass the main heat exchanger core area 200 during cold flow conditions without substantially reducing heat exchanger performance during normal operating or hot flow conditions by way of non limiting example in some configurations the length l of the calibrated passage bypass passage 102 is substantially in.