Ditto a hole saw, but get a "bimetal" hole saw (as opposed to a cheaper plain steel hole saw, but both should be available at Lowes/HD) and do back it up with wood as suggested. I'm assuming you are doing your cabin heat valve? If so (and assuming you have not yet riveted on your firewall recess), you can clamp a piece of wood to the firewall to serve as the back up. Good luck.