trying to fix this problem instead of dream about it, i got the viton cap gasket and new filler aprons. here are the dimensions of the former: 58mm OD, 38mm ID, 3mm thick. on inspection, my old gasket did not appear damaged at all, but it was harder and just slightly smaller than the new one. i think PERHAPS shimming it from behind could have addressed this problem. wish me luck....

the filler neck is not really centered in the hole in the bodywork, but the apron still fits OK.

thanks all for your suggestions and ideas!

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'77 240D, 504H, OM617.952, etc.