Pololu 42x19mm Idler Wheel/Sprocket Pair - White Overview These idlers have 3.5 mm diameter circular holes that allow them to spin freely when used with the included shoulder bolts or similar mounting hardware. The plastic hubs are the same as the idler sprockets included with the Pololu 22T and 30T track sets, and they can be used with the 30T track set as additional idler sprockets: They can be used with the included ruer tires as idler wheels that have the same overall dimensions as our 4219mm wheels: This product includes two idler sprockets/hubs, two ruer tires, and mounting hardware consisting of two shoulder bolts, two M3 nuts, and two washers. The idler sprocket/wheel can be mounted with the provided shoulder bolt as shown in the diagram below. (The red block represents a bracket or chassis that the sprocket is being mounted to, and is not a part included with this product.) Our nylon spacers can be used to offset the idlers from a flat mounting surface. Dimensions Hub diameter: 1.38 (35 mm) Wheel diameter: approx. 1.65 (42 mm)The diagrams below show the dimensions of the hubs and the included shoulder bolts. (Note that the hub diagram is actually the diagram of a drive sprocket the labeled dimensions are all the same for the idler version, but the center hole is circular for the idler hubs, not D-shaped, and there is no strain-relief slit.) The threaded portion of the shoulder bolts can be cut to the desired length. Documentation on producer website.