The other day I needed to do a job that would involve a 5C collet with a stop.
Sadly my 5/16 collet did not have the internal thread for the stop to screw in. I do own the type that expands into a plain bore but don't like it.
I thought that I would have a go at threading the collet (a genuine Hardinge) but didn't think I would stand much chance.
I used a generic 60 degree 16ER type insert.
It was in the end very easy.
David.