I had a similar situation with a om603 intake.

The last allen bolt was rounded out by a previous owner.

To slove the problem i took a cheap torx socket slightly bigger than the rounded out hole and bashed it in with a hammer, and took a breaker bar and was able to loosen the bolt.

Might be worth a try.

I also did a similar thing with a w126 lug bolt that had its head ripped off. So i took a socket slightly smaller then the shank and bashed it in, and managed to loosen it.

Hope this helps

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1974 240d
1983 240d -sold
1989 420sel with om606
1994 s350 turbodiesel - sold