Actually, a castle nut is used when the bolt does not need to be super tight. Not sure of torque specs on that bolt, but I am guessing very low. Tighten, then back off till the cotter pin goes into slots in castle nut. By design, they are not meant to be very tight. the cotter pin acts as a nut keeper if you will. Same as we commenly use in many aviation applications btw.
Thanks everyone. The nut is turning, so I am unable to tighten the castle nut unless I use lockjaws on the stud under the boot, which I did. It still turns though.
Sorry for the pic, I was on the cell phone and didn't see how the pic turned out.
no.interesting idea (i wish i could claim credit but it was my buddy who thought of it): Possible reason that bolt turns with the castle nut is because the socket has been worn down and allows it to move freely. I'm going to get in touch with the manufacturer and see if i can order a replacement.
There is one more. It was driven with the castle nut loose and the pin wallowed out the tapered hole. That will let the castle nut bottom out on the threads where they meet the pin and not tighten up any further.Only two reasons I can think of where the castle nut will turn the shaft: #1-The threads are boogered up on the nut or the shaft, or #2- the tapered part of the balljoint is not fully seated into the matching taper on the arm.