I am not sure. Eccentric spacer must be turn with wheel or its stable and just make pressure wheel?
It should not be able to turn with the wheels. It just allows you to adjust the pressure the vwheel applies to the extrusion.
Once you have it adjusted to where you want it, it needs to be tightened in place so it stays there
the eccentric nut should not turn with the wheel. It must be fixed.
Turning the eccentric spacer will change the vertical distance between the V-wheels slightly. So, when building your machine, turn the eccentric nut so the distance is at the max and slide the assembly onto the rail. You’ll notice there is some play when you yank it up and down.
Then adjust the exxentric spacer until there is no more play. Don’t overtighten as well or the wheels will not move freely and wear out much faster. You’ll need to find the sweet spot.
After fixing it, it should not move anymore
Thank you 
question… there is a small divot on one side of the spacer. Does this indicate the “open” spot as in the lose position of the v-wheel? Just wondering
When turn it . its basicly push screw rods 1 side.