These are my 5/16" tubing insertion plyers. They have a pair of vice grips with a split, threaded rod adapter to grip the tubing and a pair of fingers to hold the fitting. I’ve modified them to make them line up a bit better for the way I use them. I’m thinking about taking off the plate with the fingers and making the slot smaller to better fit the fittings I use. This tool is an absolute must have if you’re going to build a vacuum tube system.

I bought these at CDL (catalog)but it doesn’t look like they sell this model any more. The closest one I see in the catalog is pn 6623157 which has an added knife for some reason.