I'm not sure if this pertains to all J-pipes but you may find this helpful.
Here's a quote from the thread below on Jpipe stud size:
"I ran into some trouble on the j-pipe studs. I read that the j-pipe studs were the same as used for the manifold (M10x1.25). Sure enough, the 10x1.25 nuts threaded right on."
"It took me forever to figure out what was going on… the original stud was two totally different threads! The “anchor” end was M10x1.5 and the “nut” end was M10x1.25. WTF?! "
http://www.sr20-forum.com/general-maintenance/729-how-replace-manifold-j-pipe-studs.html