These guide wheels are a type of bearing with a deep groove. This groove is designed to keep whatever is being run through aligned and moving smoothly. The guide wheel has a 4mm inner diameter with an overall outer diameter of 13mm. The grove is 1mm deep and 2.1mm wide. The overall width of the guide wheel is 4mm. These can be used for rope, cable, wire, 3D filament, or just about anything else that will fit.