Find one of the online W123 manuals and read up on rebuilding the cylinder head and then you can also print a copy of the specs to show the person who would do the rebuild.

When I changed my Valve Stem Seals if the Piston is at top dead center and the whole rocker arm assembly is removed or perhaps even with only the Rocker Arm removed and the top you you could perhaps file 20-30 thousands off the top of the Valves.

I have no Idea if the metal on the top of the Valve Stems is extremly hard or not.

They do have diamond dust coated files and sharpening stonse that you can use by hand to file down even hardened metal but they are slow.

But does anyone know any reason why the top of the top nut could not be filed instead of the top of the valve stem?

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84 300D, 82 Volvo 244Gl Diesel