I have three plugs out of one bank, but one plug is being especially difficult: it's about half way out; I am pouring the PB Blaster to it, but I am afraid it is going to twist off. I would try to warm the head a little to expand the threads, but how to do this if the engine can't run? Maybe pour hot water in the radiator? I think I might get it to go back in; being male, I hate to go "the wrong way", if you understand what I mean.
Once the other three plugs finally came out (with varying degrees of difficulty), the new plugs went right in, threaded in by hand, and were fine.
When we have similar problems with industrial engines, sometimes it is caused by carbon fouling the thread on the end of the plug, then trying to drag back through the other threads during removal. Advice?

Gently turn the plug back and forth as you slowly turn it out of the hole.
(male euphemism).
It it entering the head at the same angle as the other three?
If not it may very well be cross-threaded in the head.
dpk

Angle looks right; all the plugs were tight to remove; this one is the worst so far.
I will wait to remove the other four until I have run the engine enough to warm it a little. Maybe the goo or whatever is making them tight will soften and the threads in the head will expand some with heat. The plugs aren't stuck, I can move them, just tight enough to be scary. They have been in there a long time (too long-they're burned up); I know the PO, but I doubt they remember the last service the car had.

I would just keep moving it back and forth with a lot of liquid wrench sprayed on the threads. Maybe a little heat from a torch?

On new plugs, especially with alum cylinder heads, I always apply a little bit of thread anti-seize compound to the spark plugs, not too much. I have never had a problem getting the old ones out. (Of course do NOT over torque)

Good Luck.

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1994 E320 Cabriolet
1992 300CE (Sold)
1991 300SE (Sold)
1986 560SEL (Sold)

Thanks for all the advice; my first aluminum heads, and they have to be Mercedes-Benz heads on top of that! I am treading very lightly.

I had a devil of a time with my 86 420SEL...they were super-tight, but just going back and forth seemed to break them loose eventually. Best of luck to you.

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2002 Ford ZX2
2 x 2013 Honda Civics