Here ya go

The threads go half way down the spacer, this allows the O-Ring to seal against the stud, (threads are out of the way). I have 3/8-24 on one end and 10x1 into the end housing all threads are rolled.

Tony

You are on the cutting edge with those things Tony to save half
a pound. Pardon the pun. I don't think a simple O ring is going
to work. What you need is some of these McMaster sealing
washers.