Originally Posted by
shelbyfv
elaborate?
There is probably enough of a gap between the ends of the splines between the left arm and the spindle to allow the spindle to protrude some more before bottoming out. Each crank might be different, but I saw a video where a guy took one of the 2.5mm shims out of his MTB BB for his 68mm width shell, and the left mtb crank arm did not bottom out. He then took out the 2nd 2.5mm shim, but had to shave down some of the splines in the left crank arm to prevent bottoming out.
If the same applies to road cranks, then the left arm could probably eat the 2mm extra protruding spindle length, when you switch to the 2mm narrower mtb bb. If it does bottom out, you could add 1 or 2mm of shims to the left cup.