Excuse my ignorance if I'm missing something, but wouldn't soaking them overnight with some penetrating oil help? If I ran into the same problem, I think I'd warm up the engine then use penetrating oil and let it sit.

Isn't there a risk of striping threads? I've done that once removing a seized spark plug from an aluminum block engine, and have been paranoid ever since. I use a lot of anti-seize these days.

Maybe it's not an issue with a cast iron block.

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1979 240D, 4spd manual, Power Sunroof, manual windows, 147k miles, Pastel gray/Black MB Tex.
1991 300D 2.5 - Smokes like it's on Crack!