@DavidB I donāt really think that it matters if the metal parts are tight against the red plastic. I donāt think that would prevent the shaft from bottoming out at the red spacer. You should really measure the gap between the two shafts without the coupler in place.
1.5mm according to my measurements
I think I figured it out! I was going to try to mount just that orange/red plastic piece and see that the two shafts still fit and the plastic does not fit over the ball screw shaft so that alone is pushing it out about 5mm. If I really try to push hard I might be able to push it onto the ball screw shaft but no way could I do it when it would be in the coupler itself. This might be because of that roughness on the end of the shaft preventing the plastic from slipping over the end.
@DavidB Interesting. What is the diameter of the shaft? It may just need some persuasion to fit. Maybe a bit of silicone lube would help.
I didnāt check diameter of shaft yet but did put slight bevel around the hole of the plastic and was able to push it on then with some force. Will bevel it out a bit more. Makes me wonder if others donāt realize their motors arenāt sitting flush because of this and/or that is why they made such a point in the assembly instructions.
Beveled the edge a bit more and after putting the coupler back together and test mounting the motor the motor is now fully flush on both the X and Y axis. 
Since the coupler has two different sized ends, I would have thought the center indent of the red spider would have two different sized indents. I Wonder if these spiders are meant for couplers with two same sized shafts ???
I would run this past Sienci tech support. Maybe they got a bad batch of couplers?
