Maybe it's different on the W123, but I replaced just my driver's side lower ball joint. Didn't seem to affect alignment, but then again, my steering is pretty loose. I guess I could just replace the passenger side one too, but I'm a broke and lazy S.O.B. and I've got other far more pressing issues.

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1983 240D 3.0T 4-speed manual, now sold

1989 Subaru GL Wagon 5-speed Touring Edition