Stepper motor couplers

[Nidalap]

Hi, I purchased a Chinese CNC machine a while back and have had some issues with it, swapped over the table for a different one because I believe it was dropped during shipping. Anyway, the problem I'm having is the stepper motor coupler for the Z axis just doesn't tighten. I crudely measured the diameter of the screw and it appears to be around 8mm more or less dead but the coupler (after you tighten it is around 8.25 or so (Eye balling from a ruler here). Needless to say, when you put the motor on and tighten the screw, it doesn't grip no matter how tight I try to tighten it (have used a glove to get it super tight but still no love).

What are my options here? I got some plumbing tape

plumbing tape - Google Search

but I don't think it's going to help. After putting on the stepper motor I could feel that the fit was no tighter than before and after taking it off again, the plumbing tape was merely ripped and pushed down to where it was no longer doing its job.

Maybe purchase a new stepper motor coupler? (If I can find the same one on the Internet somewhere that is).

Any other ideas on how I can get a snug fit?

I haven't tried interchanging the motors, so not sure whether it's the thread of the screw that's worn off or whether it's just that motor that is the issue. (Not too keen to be taking the motors on and off in case I get the same issue on other axis all of a sudden... I figure in a case like this, if it ain't broke, don't fix it).

When I swapped over the table, I unscrewed everything and after assembly the same issue crept up - not sure whether it was a poor manufacturing run on the Z axis screws or whether one of the couplers is improperly drilled/sized and I managed to put the same stepper motor on the same Z axis with both installations, probability of that is obviously 1/3 so quite likely that it is just that 1 stepper motor coupler. The other axis appear to be working well from what I can see.

P.S. Both times that I put the motor onto the screw of the Z axis, the feel was different to the other axis i.e. it slid straight on whilst with the other axis I had to push the motors before it would be flush with the mounting plate so it was immediately apparent to me that something was wrong both times.