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