Im in the middle of rebuilding a black rear gs wheel.
the one I bought had a bit of heavy corrosion on the hub,there was a small amount of evidence of corrosion around each spoke where it comes through the rim,
a quick strip down and repowder coat I thought, no chance spokes were mostly totally seized into the rim had to cut them and then drift each one out,its a bastard of a job, just about managed to salvage the rim.
no wonder wheel specialist are avoiding them, took me 8+hours.
Thanks to this thread I have managed to find replacement spokes and stainless nipples from Doug Richardson £100 for the set,the spokes are a bmw only size so are not he norm, they would cost about that from bmw for the spokes only.
As for needing a special jig for Truing the rear as far as I know its only for the mounting of the wheel in the truing jig, as in it has no bearing and needs an adapter,like you would need for balancing.so i made this,
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Doug Richardson@s a busy man, quoted me 3months for the rebuild, I'm not desperate so will have a go myself.