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I have it on good authority that my nut is too thin (!). Anyone have a spare one?

Sorry I have none spare but I did buy some new in Stainless. A quick measure with a caliper for the stub diameter and check of the thread pitch, then I was surprised to find them available online in varying thicknesses but including the ones we need. So measure the stub length protruding from the chain adjusters yoke-you'll want a nut slightly less than that length.