Technical flathead temp sending uint thread size??

Hey guys, it's been awhile since I have been on. I have a friend that has been helping me on my 37 ford, I'll post pict later, we have been going at it pretty hard he is one that says do it now and right and you wont fight it later. We have the front end off front end out and rebuilt, rear end out and rebuilt all new gaskets, I'm learning a lot I think its been about a month now. It is coming along good. My question: I would like to run two temp gauges with under dash temp. gauges, I would like to clean the threads for the sending unit in the heads and would like to know for sure the correct size. I'm thinking 3/8 pipe?? I hope someone here knows the correct size and lets me know. Thanks

 

My 8ba was 3/8 pipe if that helps.

 

Thanks