Yeah, that's a nice plug. Way nicer than using a normal bolt. The shoulder provides a much better sealing surface (as it was intended). So, is the head metric? Or is it (like so much of the other "metric" stuff available here) metric threads, but an English sized head (like 1/2 inch)?

Also, I wouldn't use the O-ring. I'd use an aluminum or brass crush washer designed for that. I don't think the O-ring is the right seal for that type or twisting/crushed sealing application.

That O-ring was probably designed for an application that had a counterbore cut in the female portion as a groove to hold the O-ring. It's nice that it came with, but I wouldn't use it.

Thank you all for the information you provided.  I think I've got my answer!!

What I would have used, had I known it was M16 x 1.5, 

?

47 minutes ago, zKars said:

What I would have used, had I known it was M16 x 1.5, 

Perfect!  Haha!! 

"Crank" that sucker in there!