This is an external filament holder that fits the small 120mm spools. The double bearings allow the narrow spools to ride in the gap between the two bearings. This helps with keeping the spool centered. You can print bolts and nuts here if you want. http://www.thingiverse.com/thing:193647I printed this at 100% fill. Parts You can remove the washers between the bearings if you want. It helps reduce friction on the spools without the washers. Installation I put washers on the outside of the base too, but you don't need to do that.