I am using a Mega Exo with my XT crank, and have not had any problems. If you want to make sure it works, put in the external cups--just snug, and see if the non-drive side arm seats correctly. It should be up against the bearing, but not under any pressure. If you have a gap, check to see which spacers fill it. If it takes two spacers (if I remember right they are 2.5mm), put one on each side. For spacers, remove crank and cups, and install. For no spacers, remove crank and tighten the cups, then reinstall crank.