Instead of a 14mm Allen Wrench you can go the the Hardware store and by a 9/16 or if you can find one a 14mm coupling nut (they are several longer than the normal nuts). It will need to be long enough to go into the plug and have enough sticking out that you can use a Socket or other wrench on the part sticking out.
Some of the Threads idicate that it may be helpful to use some heat to help loosen the plug.

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84 300D, 82 Volvo 244Gl Diesel