upper ball joint
 

I was replacing the upper control bushings on a 1999 suburban and I took out the ball joint and messed up the threads on the ball joint so the nut would not go back on so, I just cut 1/16 off the bottom of the damaged threads and the nut went back on ,does this weaking the ball joint ? I was able to install the cotter pin.

Welcome to the forum.

Not if you're talking about the end of the threads where you start the nut.

as long as it is tight and you secured the castle nut with the cotter pin you should be alright BUT NEXT TIME!!! Put the nut back on the ball joint before you hit it with the hammer. This way if you hit the ball joint you have only smacked the nut protecting the threads.

Thank you for the reply, I will be changing out the driverside's upper control bushings and this time I will put the ut back on before I hit the ball joint.

DONT HIT THE ACTUAL BALL JOINT!
you use a method i like to call shock breaking, u heat up the spot the ball joint goes into, then u hit the sides and all around it and watch it drop out before your very eyes