This is probably a dumb question, but do you have the original Z-axis motor or a motor sourced from somewhere else? The old coupler I have has different sized openings on the ends. Here is the only reference I found to an old post @Bar made on the original Z-axis coupler.
Also, The instructions for the original kit on github, maybe of help.
Personally, I modded my original Z-axis following the instructions in the Community Garden and recall a visit to a drill press to widen the opening on one end a few thou.
http://maslowcommunitygarden.org/z-axis-for-AEG-router.html
Good luck,
Jamtek