Ball Joint help

I am replacing a ball joint in my Crown Vic and I am having problems putting it back in. As I am tightening the nut on the ball joint the bolt is also turning so the nut wont tighten all the way down. I dont know how to keep the ball joint still while tightening the nut down. What am I doing wrong?

You are doing nothing "wrong". You do often need to place a jack under the balljoint and apply upward pressure to it so the ball-stud will not spin as you tighten the nut. Sometimes, this takes a lot of pressure (like lifting the control arm a few inches).

Thanks for reply. I ended up using the press to get it fitted up in the knuckle and then it pinched and I was able to tighten it.