Heat on the head! Great idea. Get a heat gun and heat that sucker up! (I thought maybe a blow torch, but then again, not a good idea) Then let it cool for an hour and heat it up again. Keep trying that. Also, WD40 is a penetrant. Let that do it's magic. Some patience and some heavy duty tools should do the trick. I also like your idea of using a magnet to pull the debris out of the chamber. Good luck with that project also.
Remember, we're trying to help.

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Kyle De Priest
2007 e320 Bluetec
2013 GLK 250 Bluetec