Nut Size for stud between h pipe and header

I was wondering if anybody could help me to find an oem replacement for a nut that bolts the H pipe to the header collector. I bought a new X pipe and installed in and heard a leak. I guess I forgot to tighten one of the nuts and now it's gone. I can't figure out the thread size. i know what socket wrench will fit it but the same socket wrench fits the nut between the X pipe and mufflers but that nut wont fit the header stud. Thanks

Thanks importwho, but nothings stripped. I tried one of the other nuts and it went on fine. I just think I forgot to tighten it. If I had the $$ right now I would definetly be buying LT