I had a lower ball joint fail two weeks ago, stud/ball pulled out. I had replaced all ball joints about a year ago when I did a front brake drum-to-disc conversion. No problems until recently.
Ball pulled out of socket? Extreme wear, cheap-junk defective parts, or operating angles screwed-up due to excessive suspension travel. (Modified ride-height, and/or missing bump stops.)
When my ball joint failed I replaced all four ball joints, inner and outer tie rods and cut a coil off the the stock springs because I had been unhappy with the ride height. Brought it in for a front end alignment and was back on the road.
Yesterday, I had a lower ball joint nut come off. I heard a click on the way to work when I had to brake abruptly, was shortly at work and while in the parking lot had the ball joint come out and control arm come down. Bad luck... or good luck that in both cases I was barely moving.
So I must be doing something wrong. The cotter pin was broken and still sitting on the lower control arm. After reattaching, spinning a new nut on and using a new cotter pin, I checked the other side... missing the cotter pin but nut still in place! So I made sure to tighten and add a cotter pin.
Wrong torque on nuts, and/or WRONG TAPER ON BALL JOINTS or STEERING KNUCKLE. If the wrong taper was used, or the joint spent any time loose, the taper in the knuckle is now worn/damaged.
Looking through the forums, I did not follow the torque specs listed in this thread, more like the "umphhhh" method and then a little bit to line up the nut. I don't think I'm nearing the 125 max for lower ball joints but I'll doublecheck them tonight. Maybe I've gotten stronger?
So my questions ... if you have not reached the 70-90 ft/lbs, should you tighten the nuts more even if you can already get a cotter pin through? or should you only tighten just enough to get a cotter pin through? I feel like if I tighten it too much then the cotter pin won't sit snuggly in the castle nut or maybe won't catch enough of the castle nut and maybe that little bit of play is causing pins to break.
Lower ball joint = 80 ft/lbs, plus only enough extra to line up the hole for the cotter pin. 80 is entirely sufficient if the cotter pin can be installed. As stated previously, other sources suggest a minimum of 70 ft/lbs, and a maximum of 125.
First Guess: your tapers are incorrect/damaged, and nothing is going to fix this until you get correct male and female tapers--which probably means replacing the knuckle(s) and maybe the ball joints.
There was a long thread a few years ago about aftermarket steering knuckles, and how the Communist Chinese couldn't cut the proper taper into them--and how the Communist Collaborators in this country couldn't do proper QA on the part.