Oops, my bad.
I'm using a threaded rod for shaft, which is not ideal, there's a slight wobble. I just measured and the distance is 1/4" on one side and 5/16" on the other. No matter how I turn the nuts, I can't get the wobble out. Will have to use something else for shaft.
HA!
Same kind of problems I have!
Except I don't follow those measurements and the pics.
Looks like 4 neos (2 on each disk), and they are 1/8" thick.
The distance from 2 on one disk to the 2 on the other disk should be less than 1/2", so I said 7/16".
You said 1/4", which sounds like a whole lot less than it looks like in the pics.
The magic number is all the magnets for that coil (4 x 1/8"), then go a little less.
9/16" is crap, but 7/16" is great.
(less than 7/16" does not help much in this type of PMA, within these parameters)
You might try all-thread couplings, or whatever they call them. You know, those nuts that are an inch long.
The extra length should get rid of some wobble.
One nice thing about speaker plates is the tall section in the center, like the all-thread couplings.
Drill it a hair under thread specs and tap it, and it runs much truer than flat disks between nuts.
Most speaker plates are manufactured to decent run out tolerances, though I can not imagine why.