This is an end cap that attaches on to a 1.5" Schedule 40 Pipe (PVC, SS, etc.), which can then be held together by a 1/4" x 20 hex bolt and nut. - Socket Diameter: 1.90" - Socket Depth: 0.25" You might ask, *What's the point?* Well, for IC3D filament spools, the spool hole is slightly bigger than an piece of 1.5" pipe, so two end caps can turn a piece of pipe into an nifty spool holder.