I think the issue you are having with a direct thread coupling is that you are clamping dissimilar materials in shear which have a different rate of thermal expansion. Using a bolt might allow that little more compliance but then it's only anchored in a clamping capacity. I'd really like to hear some examples of success stories using a girdle on a B/RB wedge. You'd think there would be some data because they've been around for plenty of time now.