The 5/16″ coupler nut, the long one, should be tight in the middle assembly. That’s the only thing that keeps the tool head from falling down. There are two z nut locks. One for couplers that are 11mm across the flats and another for 13mm across the flats. Pick the one that best matches your coupler. While the coupler nut must be tight in the lock the regular 5/16″ nut, if you chose to use it, should float in the lock but not spin.