Its a captive nut. When you go to remove it, it will rotate around (clockwise in your picture) until it hits the manifold on the other side. Then as you continue to turn the bolt the nut unthreads.

 

ty

 

You take it off the normal way but from the other side.

 

It is welded to that piece of metal under it so it cant rotate once it hits interference. So when you turn the bolt, the nut may spin around until
it hits the pipe again. Then it will loosen.

 

I feel like I should provide an answer......

 

Also called a turbo nut. It's a form of locking fastener made that way to keep the nut from backing off. Similar things made for studding headers.

 

Yep, you have the same type nut on the left rear upper shock mount.

 

As was said, remove the bolt from this nut from the other side and don't get your fingers in the way of this nut while you do it. You'll not be happy if you do. This type of fastener arrangement can be found all over these trucks often used on suspension parts.

 

Yep...I was pissed when I first tried to put wrench on it and it appeared to be rounded off, the truck was exactly one day old. Turned out I didn't need a wrench then I felt pretty stupid.

Mike