How to stop hot air from engine compartment?

dibbons said

I think maybe in this case some good old-fashioned pipe dope may be the ticket, being the fact the valve will be opened/closed from time to time.

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Nope, the threaded rod/valve is solid. Around the threads only. The adapter to body threads and the valve threads can leak. I used to run these on my tool trucks. 28 years and 5 trucks.