It's a threadless pin, more like a big rivet. I think a 14mm bolt or rod would do, but the inner half, on the bracket, seems smaller than that. Of course, it could be tapped out some. I was thinking I could whip up something out of Delrin or Teflon, but from the looks of it, that pin has to carry a lot of weight and the plastic would probably break.

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1983 300SD 467,000 km - 473 champagnermetallic

location: 34°5' N 106°53' W (Socorro NM, USA)