Interesting tool. Well I've just tried an older spark plug (cleaned up), and it does the same -it will torque up but not a crisply as they others. And It does need some winding in for the last couple of turns using the ratchet bar (whereas the others are turn by hand then nip up, in the usual way). So maybe I do have some messed up threads towards the bottom.
It has, however, led me to look at the old damaged plugs more carefully... and it does seem that apart from one cylinder the originals did not naturally seat - the plugs that were damaged by the water sitting in the wells have corroded seats - which is odd.
