If there has been other front end work done on the car, a possibility exists that nuts could have been accidently mixed up. Here is what I ran across. The Upper Control Arm ball joint takes a different thread count than the rest of the similar joints on the front end. Consequently, if it is used on the wrong threaded end, you end up with stripped threads. Believe me, almost nobody carries the correct nut except the dealer. (Yes, I tried Fastenal, who has about the biggest selection around.)

The best approach is to get a thread pitch guage and measure the threaded end. Also, check other areas of the suspension to see if something is amiss.

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Sam

84 300SD 350K+ miles ( Blue Belle )