1503 Series M4 x 0.7mm Shoulder-Standoff (5-6mm OD, 10-8mm Length) - 4 Pack

These shoulder standoffs allow you to mount 3600 Series V-Wheels, holding the center of the wheels 13mm off the surface the standoffs are fastened to.  The 13mm height allow for 1mm of clearance between the mounting surface and the adjacent surface of the goRAIL.