minor_LEGEND
Senior Retro Guru
Cranks tend to have a optimum b/b width & is specc'd by Shimano. From memory old MT60 (DX) & XT M730 are 127mm for example. Black LX 560 is 110mm as modern cranks took on a lower profile. Throw in 68mm/73mm shell widths & differing frame design & things are not set in stone, but it is a starting point for most applications. The correct question should be which axle length do I need for 'X' crankset?