I looked up his info he owns: a 1979 240d
If it is the head of the bolt is messed up so much you cannot get a wrench to hold on it everyone has said that Sears sells a set of External Eze-out type nuts that fit over whats left of the bolt and grips it so that you can get it off. I have seen them myself but so far have not needed to buy them.

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84 300D, 82 Volvo 244Gl Diesel