David
The thickness of the center washer can vary by how much the center rib on the V wheels is out of spec. Look here for what I found when I measured all the wheels that came with my machine…
Most users of X-Carve machines will not have the measuring tools or metal lathe to take measurement of each wheel and determine the exact thickness of the spacer washer. That being the case the use of a fiber washer that has a little give to it is a good choice for replacing the metal washers in wheels that have movement.
Hope this helps
Dave