There is a machine shop here that has successfully removed stuck GPs from the 606 heads, the dealer sends them there occasionally.
The fellow showed me how he gets them out. He sets the head in a flat bed then mills them out using progressively larger boring tools (not with a hand operated tool!) then when he finally gets close to the aluminum thread he stops tha process and then uses a tap to push the remaining GP material that is in the threads out. Its a very slow process. But you don't wind up buying a new head.
As I said elsewhere, if I owned on of these engines I would be replacing GP's every 18 months.
And I would use some Alunox on the threads of new plugs when I installed them.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!