1503 Series M4 x 0.7mm Shoulder Standoff (5-6mm OD, 10-16.5mm Length) - 4 Pack

These shoulder standoffs allow you to mount 3600 Series V-Wheels, holding the center of the wheels 21.5mm off the surface the standoffs are fastened to.  Given the 43mm inside width of 1120 Series U-Channel, mounting the standoffs inside the channel puts the wheels equidistant from the two sides. This setup aligns the goRAIL® right down the center of the channel so you can connect it to a drive such as a lead screw.