5/16 UNC is correct. I bought one a few weeks ago. Rick - do you do anything to make sure yours doesn't rattle loose (i.e. loctite or a lockwire)? I notice the original plastic one is made so it cannot turn freely - so I was worrying about the stainless one coming loose on corrugated roads.
Brian -
I am sure Racor make some good stuff and I am not familiar with their water separators. However I have seen some dubious products made by them over the years... I know someone who works for them in the US - I have my doubts about a company that proudly claims that they don't do R&D, they just do engineering...
The effectiveness of centrifugal type collectors depends on the size of the water droplets - modern developments in hydrophobic and superhydrophobic (low energy) surfaces have enabled the development of filter media which is amazingly efficient at coalescing water dropets so they settle out under gravity. Some hydrophobic filter media works so well that water droplets will not pass through it. So all I am trying to say is there have been a lot of new developments in recent years.
I am sure the "oil burning" equipment you describe would be illegal in many places now as it wouldn't pass emissions regulations - not 100% sure though.

