Hi Jon. The threads aren't specified but the "O" ring is. It looks like 1/2 inch ( 12.42 mm ) with a 3/4 inch nut for the outside ( it says " hex flats 3/4" ) and a 11/16 nut for the inside ( "hex flats 11/16" ).The reason I brought it up is that 1/2 inch diameter OD seems awfully small for a 4 prong connector. I don't know how thick the sleeve is but that doesn't leave much room inside.It makes me think I'm reading the drawing wrong.Then too, if it's metric threads/sizes it would take an adapter bushing on an American made sub.Maybe we can find someone who has one......anybody out there ?Frank D.