I am replacing my o2 sensors and broke one of the studs at the exhaust manifold removing them and stretched two to failure re-installing them. My JTIS guide calls out a torque recommendation of 60-80 Nm for this connection. Is this too high or have the existing studs just been weakened by 15 or so years of temp cycles? I don't want to replace the studs with a higher grade stud just to put too much stress on the cast iron manifolds if this torque rating is too high in the first place (getting to the cats was hard enough, I can see that replacing an exhaust manifold would be a big job).

 

You don't mention model or year but it has to be at least seven years old and 'weakened by 15 or so years' means it could be a 1997?

Cat to manifold studs typically corrode badly as in the right hand side example:



If undisturbed the joint generally remains perfectly gastight but once cracked open it's difficult to tighten again effectively.

Given the minimal cost and chances of a leaking joint, I always replace studs and nuts whenever the joint is split for any reason.

Graham

 

It's the downside of OEMs using small studs and pinch nuts. Even when using a torch to heat them cherry red before removal one has to be extremely careful that the tool they're using remains exactly square to the flange. Some Toyotas are also notorious for the same problem.

When I removed the cats during my trans r&r I found a previous person had only reinstalled two nuts per side, 180° apart. However, I had no leaks. Of course I replaced the missing studs once the cats were out.

You don't need to change the stud grade, chances are if you were to reassemble with the exact same parts and remove the cats five minutes later you'd run into the same problem. A long time ago I worked at Midas for several years and had run into the same problem. When we wrote estimates for cars that used studs like these we anticipated half of them would strip or break. If you want to reduce the chances of repeat problems the best method is to not use pinch nuts or Loctite. Either use lock washers or nuts alone.

 

Yes, 1997 XK8. I will replace the studs with a similar grade, just wanted to make sure I wasn't missing something and make things worse down the line.

 

you ALWAYS replace the nuts/ bolts and studs when working on cats. ALWAYS