Finally got my down tubs off, one stud came out with nut, one left helicoil from nut on stud, easy to pull off. Can anyone confirm stud thread size for me, very rusty and I'd like to run a tap over them or replace studs and nuts. 91 XJS V12. Appear to be 3/8ths fine.

 

If you're talking about the downpipes,,, I would recommend just getting the double ended bolts and brass nutts that can be had and R made for the application. I admire your worksmanship at getting them things out (if that's what you mean) of the manifolds. Damn! I did the downpipes on mine recently and the scariest part of the whole thing was a mishap with those bolts. I feel I got lucky.

They are like these but there are many more sources.

 

More interested in replacing nuts with stainless, was just unsure on thread size.

 

I understand the want for SS. I was leaning in the same direction, to. But was advised otherwise. There is a reason that jaguar used kinda extra long brass nutts on those bolts that hold the downpipe to the manifold. Had to do with heat, expansion, contraction and corrosion - I think. Unlike most any other place on the vehicle, the conditions these bits deal with are pretty extreme. I say, for each his/her own, but in some cases I try to do what is suggested. Jus say'n

 

Originals were steel with some kind of Helicoil in them to make them act like a lock nut.

 

And you struggled with those? I don't know it had been since mine had been removed but I put a 9/16ths socket on them and they turned off like butter. Didn't even turn out the bolt end. I was left with 4 brass bolts that looked pretty much new after a clean up... I'd stick with an oem style fastener... that's just me