The old nut is not hard to remove. Usually a chisel and a hammer on the edges can do it. The 12 point nut is horrible though. I tried looking everywhere for a an ultra deep 27mm star socket but eventually welded my own together. The dealer sells them though but their super expensive. Replace the rear seal while your there too and put a thin coat of RTV grey on both sides of your new gasket (and youll have to figure out which gasket to use, idk if you can use the 300d one or the sd one or if you can use either one. Just look at the edges and passages.

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1981 300SD 512k OM603