The easiest way of which I am aware that works would be to make a new "repair" fitting out of a piece of hex brass.

Internally thread the remains of the projection on the airhorn.

Make the repair fitting with male threads to fit the female threads you made in the airhorn, and "burr" the male thread slightly. Thread the other end of the repair fitting with the proper thread for the cap on the heat tube. Leave enough of the "hex" to hold with a thin wrench.

Now screw the repair fitting into the airhorn. The "burr" will cause the thread in the fitting to "seize" in the airhorn and stay in place.

As an insurance that it stays in place, hold the fitting with a thin wrench when installing/removing the heat tube.

Jon.

Good carburetion is fuelish hot air
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