Morning all, I have only last week cleaned out a nut to 9/16 x26tpi whit and that fits a treat ( pure luck I had a tap ). I bought a couple of nuts from PVN Racing Vincent,But they would not go on to the Douglas type amac bowl with the fitting integral with the bowl. However the pvn fitting would go on to the amac bowl I have with a screwed in fitting. I have asked Pvn what size he has made his threads but as yet have not heard back.
Hope this helps
Jonathan