I noticed that the nut had fallen off of the stud that connects the bottom of the exhaust manifold to the downpipe (which then connects to the cat).

I went to reinstall the nut but realized that the entire stud was loose and only hanging on by a thread or two. The stud won't thread back in as the threads on the stud are damaged however. I was able to flip the stud around though (just as a test) and start threading the other end into the manifold which confirmed to me that the threads in the manifold side seem to be OK still.

A couple of questions for the group:

1.) Does anyone know the best source to purchase this stud and what type of metal I should be using?

2.) Assuming the threads are still good, how does one tighten the new stud in place? I assume that maybe I need to lock two nuts together and tighten the stud in place, and then loosen the nuts off. Is this easy to do without turning the stud loose again in the process of backing the nuts off?

3.) What type of nut should I be using? The nuts seem different on each of the two studs.