Summary Large is aprox. 45mm in center dia Basic is aprox. 21mm in center dia Both have 2.5mm lips on the end to hold spool in place so the hole in your spools need to be at least 5mm larger than the center Dia. Two spool spindles for filament .. both designed to use with a piece of 3 mm tubing, and 3mm(inner) x 6mm (outer) x 2mm (thick) bearings. (used a lot on RC aircraft) both are design so no support or inner structure is needed, to save filament and print time. Smaller one is solid caps as it's a small enough bridge that most printers should have no issue. the larger one is designed with 4 legs on top which are arced so the printer can build out without issue. my printer isn't larger enough (M3D printer) to print legs so I just cut some out of scrap plywood. I did have to knife out the bearing openings a tad as my printer's backlash sucks.. should be close if using a better printer though.. might even have to pad a bit.