If you are using a hollow-spindle crank, or any one in which one or both arms clamp to the spindle, you've got to be careful about adding spacers that reduce the clamp length (unless there is "excess" spindle length, like a spindle designed for a 73mm BB and you mount it in a 68mm BB shell). I did that, added 4mm spacers on the crank side on my bike, and the left crank arm came loose. (Chainline also suffered, was dropping chains. Solution was to grind off small part of FD inner lip that was preventing the FD from going 4mm further in; Then assembled crank with no spacers as it should be, did not come loose again.)