[Healeys] 1959 BN4 temperature probe threads?

[Chris Scholz]

Crazy question. What are the threads for the female nut on my temperature probe?  My thread gauge says 18 per inch and Nissinger Instruments mailed me a coupler to fit this temperature probe into the aluminum block of my chevy LS1 V8. The coupler in the photo below is 3/8" British Standard pipe on the left side which supposedly fits into my brass nut.   This coupler screws in 2 turns and then I meet lots of resistance. I am nervous to crank on it, in fear that I will ruin my brass nut on the probe.  I am putting this coupled probe in a machined frost/casting plug on my block.  Any advice?  Thanks. 

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