I wish people would quit trying to quess at the fix on things.
I have removed many plugs like this. If the plug broke off, using
a regular spark plug socket, there is little or no chance of removal
with anything but heat. The only way is the heat the plug remains
to the point of melt down, and imediately insert your "easy out"
or thread extractor tool. This is almost a guaranteed to work.
I have used this on 50 year old plugs. SO lets try to keep the
list to some real experience and hard facts.

            Eric                   86-VW4x4
            vw4x4@fyi.net          72-240z
            Pittsburgh, PA USA     1936-Chrysler