Rascal: I used to deal with tube fittings all the time and I don't recall them as being threaded. But in order to know what size fitting to go with a given size tube I think we always referred to the manufacturers catalogue or just tried them all out. You might look in some of the catalogues such as the Granger Cat. or the Thomases Register. I think your best bet though is to look up the tube manufacturer, find the ID of tube you are using and see what they say about fitting it.
Sincerely,
DS ME
