Eccentric spacers are used to adjust the amount of pressure the V-slot wheels put on the V-slot rails. The hole drilled through the center is actually offset so the when adjusted, it will change the position of the wheels. Over time you may have to adjust these wheels if your extruder or aluminum bed feels loose. Most of our spacers can be adjusted with an 8mm wrench. Older versions, you will need a 10mm wrench.
If you are able to wiggle your extruder, the eccentric spacers on the bottom of the extruder will need to be adjusted.
The extruders eccentric spacers will be between the metal plate and nylon spacers under the extruder. The spacers can be turned either direction, but you can feel when it is getting tighter.
The aluminum bed's eccentric spacers many become loose after a while as well. You can tell by trying to wiggle the bed left to right. Some wiggle is ok. But if you can move the wheels out of the groove of the V-slot rail, it will need to be adjusted. There are 6 eccentric spacers located under the aluminum bed.
You can find the eccentric spacer between the nylon spacer and the angle corner connector on the wheel assembly of the aluminum bed.
Make sure not to over tighten the eccentric spacers. This may put to much pressure on the the V-slot rails. To much pressure on the V-slot rails may make movement on that axis difficult and result in binding or shifting.