I replaced my ball joints on my 71 and for some reason the nut keeps eating the pin and backing out. Seemed like a pretty simple install but I must be doing something wrong.

 

Is it a stock type set up? Is the ball joint turning in the tapered knuckle?

 

Yes stock mood ball joints in stock control arm. The ball joint isn't turning its the damn nut!!

 

How the hell is it getting enough torque to cut a cotter pin?

 

In order for that nut to turn, it has to be turning in the taper joint. If they are torqued and fit properly the ball joint shaft will only turn in the socket.

 

Ok so if it is, how can I fix it??

 

As noted, the ball joint stud is a tapered wedge locking fit in the spindle. Once the nut is properly torqued, the stud cannot turn in the spindle and thus the nut cannot back out. The only way for the nut to loosen is if the stud is turning in the spindle, which means that the taper is not seated properly. This is usually caused by incorrect torquing of the nut initially or a worn or damaged tapered seat in the spindle, preventing the stud from seating. The latter is more likely. The only other option is an incorrect ball joint or one with an incorrectly machined taper angle on the stud. All of these are dangerous to drive with.

 

As Joe stated above, you need to figure out which is causing your issue.