This GT2 timing pully has no teeth and is designed to be an idler or used to create a belt tensioner. Unlike a standard GT2 pulley, this one has a set of bearings pressed into the center bore. The bearings have a center bore of 5mm and allow the pulley to spin freely on a stationary shaft or bolt. The pully is designed to work with all forms of 6mm wide GT2 belt we carry in stock and is intended to run on the smooth side of the belt.