Originally Posted by
cyccommute
External bottom bracket cranks have a fixed spindle length. They are made to fit 73mm bottom bracket shells. They are adjusted to fit on a 68mm bottom bracket shell by using 5mm of spacers which are arranged on the outer part of the bottom bracket bearings to provide the correct chain line.
Some are, but tons of road Hollowtech II cranks have no room for spacers and work for 68mm shells only. Which seems like something you already know, so maybe you didn't mean this as it is written?