I ran into this on my intake manifold, the head was on my bench. No amount of chiseling, grinding slots for a screwdriver, nor impact screwdriving could loosen the bolt.

I ended up buying a dremel (finally) and grinding off the head like a dentist. Once the intake manifold was free, I could remove what was left of the bolt with my fingers. Overtorqueing really locks the bolt in the hole!

It was also the bolt closest to the firewall and would have been the hardest to get at had it been on the car.

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95 E320 Cabriolet, 169K