Guys,

Thanks for the advice.

FYI, I downloaded a front and rear hub manual from Shimano for Ultegra hubs. This specified 11 x 3/16" BB for front and 9 x 1/4" BB for rear (per side) i.e. as I believe that 600 and ultegra were the same, it looks as if there should be no gap(?), although I hear what you are saying.