The large knob allows you to make tension adjustments easily and accurately. The diameter of the barrel is exactly 17 mm. The diameter of the shaft is 19/64-5/16". To remove the shaft from the barrel you will need a small standard screwdriver or a small 2mm Allen wrench-depending on the model. I can do this for you if you do not have this size of tool- Let me know at time of purchase.
I just remove the barrel and place the shaft in a 19/64" hole if I'm using a softwood like Pine, add a drop or two of glue, tap it in with a hammer, and your good to go. If you are using a hardwood like Oak or Maple, drill a 5/16" hole and you can firm it up with a little more glue.
These tensioners have been used to build or repair 1,000's of rods and they work great-they make a big difference in the ease and quality of your wrap.
The Fourth and Fifth pictures show how I place them in my Rod Wrappers. International orders use eBay's Global Shipping link.
This is what we do for fun--Enjoy your build