This is a screw chuck/mandrel #2 morse taper with drawbar for turning bottle stoppers. The thread size is 3/8 x 16 tpi and matches the threads on my bottle stoppers also listed on this site. This also includes a drawbar that screws into the back of the chuck/mandrel for holding it securely while you turn a bottle stopper. Begin by drilling a 21/64 hole in your blank. In softer woods I drill a slightly smaller hole since the threads don't hold as well. For woods that are more dense I drill a slightly larger hole. Some turners like to run a metal tap into the wood. This is especially helpful for denser woods.
The length of the drawbar can be sized to your particular lathe. You don't want this rod sticking out too far from the headstock. This bar is easily cut with a hacksaw and the edges filed smooth. A tip when cutting the drawbar to length is to put a nut on the drawbar where you want to cut it off. Then make a cut with the hacksaw right next to the nut and you will get a cleaner cut on the rod and there will be less clean up to get the knob threaded onto the drawbar.