That headset is really an item that eventually wears and will never be capable of being tightened properly as the main outer ring wears and expands over time. It is and always was a poor design, and we no longer use it. Parts are not available for it, much like parts are not available for anything at all nowadays - as everything is meant to be modular and disposable/replaceable as a unit.
I recommend having a LBS replace the entire headset with a better cartridge bearing one. In place of the Diatech "adjuster" have them use a thin headset spacer and a Problem Solvers adjustable headset spacer to tighten the headset.
This is what we use now and also what RANS and Lightning (that used to use the Ditch) has moved on to a similar setup. The Diatech design was cheap and not capable of lasting very long.
Robert
I also had problems with the BI-3 headset on my QX-100 (it kept loosening) and after seeing (but not fully understanding) this post, I bought the Problem Solvers expanding and locking spacers, thinking I could just replace the BI-3 adjustable spacer. When I removed the BI-3 adjustable spacer, I found that it was cracked (right above the white screw in photo), which explains the loosening. But I also realized that the BI-3 includes an attached gasket that seals the top of the headset (at bottom, in the photo).
So my question is: Do I need to have the entire headset replaced, or is there a way to add a gasket, below the Problem Solvers spacers, that would seal the bearings, without replacing the entire headset?