This ball-bearing v-wheel kit works in conjunction with?X-Rail?to create smooth linear motion. The Acetal construction of the V-Wheels offers high durability and wear resistance. The internal ball bearing has an ID of 5mm and the overall v-wheel diameter is 0.60?. ?Four 5mm to 1/4? tapped standoffs are included to make it easy to attach the?V-Wheel?to channel or other Actobotics? components. The 0.675? standoff length makes it ideal for more compact projects.