The new two chain covers that replace the old plastic covers with larger footprint. The ballscrew nut should not have the cotter pin in order to use this new chain cover. You can replace the cotter pin with the new set screw before using these covers. Simply remove the two screws from the old covers and put the old covers aside. There are four magnets to go on the large mounting bolts. Snap the new covers on the bolts. This is a spare part and no need to buy along with the machine.