Jim -

I'm a big fan of the Castwell knot for attaching lines to backing and leaders - the only knot for those applications that I have used for well over fifteen years now.

It has never occurred to me to try using it to attach tippet to a tippet ring. But I don't believe it will work. I think the Castwell knot works with flexible materials because it closes the materials to each other tightly and securely. I don't think that you can get that effect trying to tie a flexible material - the tippet - to an open, solid object like a tippet ring.

But - maybe it would work. I'd certainly be interested in what others think or have experienced, but a simple ( not improved ) clinch knot has always worked well for me for attaching tippet to a tippet ring, and the simple clinch knot is easier to tie than the Castwell, so I wouldn't be at all inclined to change even if the Castwell is an option.

John