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The threads on these ball joints are VERY shallow and easy to cross thread. I would be willing to bet that they ball joint was crossed threaded when installed. I spoke with a mechanic who has years of experience working on older cars. I had him remove my old ball joints from the control arms and he suggested that I add a few spot welds to my ball joints just to be safe.
I think a couple of tack welds would be the way to go.