I have a gate valve that shuts water off to outside hose bib, and it leaks through the valve stem when open. The valve was installed 25 years ago when the house was new, and is used seasonally to shut water off to the outside hose bib to prevent the hose bib from being damaged during winter freeze. I have recently acquired a replacement cartridge as this gate value is not the type that has a packing nut in order to fix the leak, and the bonnet or gland nut is a really odd size. It is larger than 16mm, but smaller than 17mm, and larger than 5/8" and smaller than 11/16". It is making me crazy. I've tried spanners (crescent wrench) of all these sizes, and stilsons (pipe wrench), and mole grips (vice grips), and even a deep 17mm socket on a compact drill ... and yes, I managed to strip the nut. I used WD-40 on it, three times, and let is sit over night each time. I've tapped on it. I put a pipe wrench with opposing force on the body of the valve when doing all of this so I wouldn't twist the copper pipe it sits on. I have not tried heat yet, but I guess that is next. I also will try to file the stripped nut to try to re-construct flats for the spanners. I am also thinking of buying 10" Knipex Cobra pliers. Any other ideas? I'd really rather not cut the pipe above and below the valve and just replace the whole thing, especially as I think the valve body is fine and I have the replacement cartridge sitting here just waiting to be installed. Any thoughts on how to unstick this stuck bonnet / gland nut?