Heater Core bypass valve.

You can simply remove both hoses at the firewall and join them with an appropriate copper fitting (I have used two 90 degree fittings soldered onto a short piece of straight pipe). Select the fitting/pipe size that is closest to the ID of your heater hoses, and be sure and use hose clamps on the fittings.

Since the input and output fitting on my heater core are of different sizes, a single pipe fitting would not work. Figured out how to do it using pipe fitting joined together. All the bypass valves I saw were one pipe size both ends. Have to put all this back to original since winter is coming. Do it while I'm replacing the antifreeze.