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The next thing I tried was to use an AC evaporator core like the two used as coolant radiators.
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Careful with using AC cores for oil coolers. Rusty, who used to frequent this list, had one blow out on him. It seems that they are designed to handle more than the static pressure generated by the rotary's oil cooling system, but fail under the pulsations.
Of course, he could have just had a defective core.