I am in the middle of replacing the tie rod end boot. I got the new boot on but when I try to tighten the castle nut the threaded shaft just spins. What is broken or what am I doing wrong here.

TIA

 

Tap the treads on the castle nut to clean them up/straighten them out. Clean the threads on the balljoint to remove any debris that the nut might snag on while tightening it.

If it really comes down to it, use a pair of pliers or vise grips to clamp the outer tie rod down onto the knuckle, then impact it or tighten it with hand tools.

Use anti-seize.