Eccentric spacers are the key to getting your carriages and wheels properly pre-loaded. They let you ensure that the wheels fit the extrusion snugly enough that there's no wobble, but not so much that they can't spin smoothly.
These spacers should be used with the wheels on one side of any carriage or gantry plate - the other side should use a matching length standard spacer. We're carrying these in two lengths, 6mm and 1/4" (6.35mm) - apart from length they're functionally identical, and should be paired with either 6mm or 1/4" aluminium spacers for the other wheels.
Each spacer has a visible notch that indicates the side nearest the bolt - these should be facing 'outward' - away from the extrusion - at first, to make it easy to slide the gantry onto the extrusion. Once installed, these eccentric spacers can be rotated to adjust the distance between the wheels and the extrusion they're riding on. The easiest way to do this is with a 10mm spanner, otherwise a pair of pliers should do the trick.
Note: For convenience, these are sold individually.