Originally Posted by
dminor
^^ Don't most of those square-tapers have the same amount of spindle beyond the face of the cups?
No. As already mentioned, the length of the spindle is largely determined by the design of the crankset, and to a lesser degree the frame. For any square taper crankset, the crankset manufacturer will have recommended spindle length(s), usually giving a couple of different lengths.
Also, as already mentioned, 113mm is common for mountain bikes with square taper cranks, but it will vary between about 110mm and can need to be as long as 120+ (121, 127 aren't very common anymore, but they're out there). 118mm is common, too.