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.