Dual Spool mounting on the top rail. 4mm for filament guide no hardware need it, unbolt the top rail and slide the part in and you are good to go. To be used with the factory spool spindle or you can use this https://www.printables.com/model/757853-anycubic-kobra-2-spool-spindle-kobra-2-plus-kobra- they both have to be glued inside.