Water Temp Sending unit size?

[slick6732]

I know this is a stupid question, but I am installing a new temp gauge and the sending unit I got is 1/2 inch. Which is way to big, don't want to pull it until I have the correct sender!!! So is it 1/4 - 18 , 1/8 - 27, 3/8 - 18 ? Or none of the above? Can not find a size listed anywhere.... 1987 Turbo-T
Thanks in advance for the help!!!!!