This is a bushing with five different diameters that match the four diameters I currently use plus one extra (31mm, 37mm, 43mm, 49mm, 54mm). It reduces them all to fit on a 3/4" PVC pipe as used on the Solidoodle 4. I also created a version with a 23mm opening for 22mm dowels as an option. For narrow spools, the two inserts can slide into each other.Print two. Insert into spool. Insert PVC.