Lower control arm castle nut

To be more specific if it helps, its the lower control arm meets the ball joint. The ball joint thread is stripped and so was the nut. The cotter pin fell out both times. Thank you for the reference though. Not sure why its falling out. I replaced the cotter pin on both sides, and only the pin on the driver side fell out.

If the thread is stripped such that the nut isn't grabbing and you aren't developing clamp load from torquing it then nothing you do will work short of getting a new ball joint. Can't recall if that's something doable diy or buy new knuckle/hub carrier and it comes with fresh ball joint.

And the cotter pin won't "fall out" if it's the right size and installed properly. The head is too big to go through the hole and you flare the legs out when you bend them so they won't go back through. Only way it comes out if it's the right size is you un bending the legs and pulling it out or the pin gets sheared by the nut and ends up in pieces.