You did not understand my reference to a "7/16th" wrench. That would be the designation of size of a 7/16th 8 point socket -- in other words the square end of the plug is a 7/16th" square. An 8 point socket is meant to fit onto a square plug. 11mm is very close to 7/16th" - I don't recall which is the slightly smaller dimension - so one might work better than the other. Also - as I mentioned - there are-or were boxed end wrenches made with a square opening. My guess is you are over tightening the plugs. A quick online search turned up numerous 8 point sockets (not metric as far as I went) and also ratcheting 4 point box wrenches. - Dave
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