+1 to what the old mechanic said. I use nothing but 6 point sockets. I use the one with the tightest fit and many times using a metric on a standard bolt or a standard on a metric bolt. A little squirt of pen oil and some cases I find it works if you go to tighten it an 1.8 of a turn, then go to loosen it if not rock it several times.
Plan B is a small cut off wheel in a dremel.
