| Smooth idler pulley has free rolling bearings with the smooth side of the backside of a timing belt moves on a smooth idler pulley | Drive pulley sits on a motor shaft and is always teethed. Comes with two set screws that need a allen wrench to tighten the pulley onto the shaft | Heatset inserts have ridges on the outside and a female thread on the inside. They are pressed into printed parts using a soldering iron | Set screws are headless screws with external threads and a hex key on one side. Found on drive pulleys and in the L brackets to join two aluminium profiles |