322 temp gauge sending unit issue

hi there, im having an issue with the thread size on the temp sending unit on my 322 nailhead. its seems to be really close to 3/8 thread but smaller. im having a hell of a time finding a new one that will work. i bought a cheap gauge and the sending unit thread piece does not seat the sending unit. and the adapter at the local parts store is 3/8 and is too big. anyone have any suggestions? any help would be great

 

never thought about it being metric at the block( or head in this case), especially since i have never ran into anything but standard on the whole car. but that would explain the close but no cigar situation. thanks for the tips 440roadrunner.

 

make sure you are using a pipe thread
Mark