Originally Posted by
BlazingPedals
Check before you do anything, but quite often you can cut the boom - the inner piece - so it'll slide in further. This only works if the end is bottoming out in the frame, for instance at the crucifix or on a bottle brazeon. Otherwise, you may have to get shorter cranks.
If I do need to shorten the distance, I might go with shorter cranks anyway - first of all I have the tools to swap them but no tools to cut a boom, and also shorter cranks would benefit my knee. On my df bikes, I ride with 160mm.