It comes with two different adapters. I have mine but I dont have anything to measure the threads for you.
Look it up on the autometer site, I think tehy tell you what sizes it comes with. I think I used the bigger one and tapped the water neck for it.
Also if I were you Id put the autometer on in the stock location since its more accurate, the tap the neck and rethread the stock sender. Alot of people say that the water neck will crack, but if you rethread the stock sender then you will have no problems.