Maybe its also a function of how wide the cups are to then? I did the Phil Wood conversion, and the cups where more narrow than the UN7's cups. That might be an advantage of trying this with a plastic cup. It may tend to keep going, especially if your treads arent the cleanest further into your shell...
So you had your BB shell rethreaded sahadev? What width UN73 did you use? And what are you running in the rear? (gears?internal?hub width?) Im just asking because I used a UN72 that is 113mm wide, and it seems to be perfect for a single speed on a 115mm wide hub flipflop. But I cant see using the 113 UN72 for any hubs that are wider than 115mm. You wouldnt be able to move the chainline far enough over without your cranks making contact with the BB shell.
R-20's -
The thinkin' man's bike... and woman 
Come check the progress...
http://web.mac.com/phatatude/Green_S...enty_Blog.html