"cylinder head removal kit"

[Phil Kingdom]

Sorry I sort of misread your post a bit, from your description, I should imagine the bottom half of a plug would be to tall so only the threaded section of the spark plug could be used, drill it out and tap it to say 10mm and weld in a peace of studding or lock in place with a nut then just use a flat plate with a hole in it across the head studs with nuts on and use a nut to tighten down on, then a couple of sharp taps with a soft mallet should work but I think I would use at least two of then to spread the load, sounds like a good idea, think I will knock up a couple with some plates. though shaped plates may be needed to clear the water inlets depending on the type of engine you have.

This may well work on a cast iron head, but not for a really tight aluminum head as it could strip out the plug threads.