These eccentric spacers, available in lengths of 6mm. are used with  Openbuilds V-Slot aluminum extrusions to create simple, affordable linear motion systems. The spacer has an off-center 5mm bore, which allows you to tighten the wheel against the extrusion. 

The spacer sits in a 7.3mm hole and allows for an adjustment of up to 0.79mm. There is a divot on the spacer that indicates where the offset bore is closest to the edge.