Alex, the one in my intake manifold is seized in place and will be destroyed upon removal. it takes a 14MM hex key and seems to be 1 1/8" across outside thread to outside thread. Does this help at all?
Alex, thanks for the help.
Don, when I had some work done on the Manifold, I suppose in the cleaning of it the inner seat of the plug was a little too thin and subsequently it now has a few holes in it.
It was a spare manifold and quite likely was like this when I sent it out for the work. On my existing Manifold the plugs appear to be new, but again will not come out of the manifold itself. I do not want to destroy these plugs as they are now my only back-up.