i have a cyberdyne digital water temp gauge, i'm going to tap into that thing on the drivers side head, on the outside,,,i have headers and its right between the tubes closest to the front.
the things is, i know that hole is bigger than my sensor, so i need to buy an adapter fitting. my question is,,what size is that hole in the head?

 

The parts book says it's 3/8-18 (and should be a pipe thread), but it sure seems bigger than that (maybe not).

 

3/8-18, what is the -18 ?

so a male 3/8 fitting should fit in there?

 

ttt