Exhaust manifold broken stud ... again (F134)

Last year youall may remember I posted a thread about breaking the rear-most stud when I went to replacing my exhaust system. Drilled it out, retapped it, all was good. I just happened to notice today that the nut and washer had dun falled off that stud. (thought that was odd) Went to putting another nut on and the stud STUB fell off. No torque more than with my fingers

I know the job at hand (all too well) but, what the heck caused it, and, how to prevent it from happening again???

There is NO exhaust washing visible around the entire port (with manifold still on) nor carbon traces so I guess the gasket is still sealing.

When I installed the studs, I only torqued them, with double nuts, to just 'feel' as though the [final] nut would not turn the stud ... not much torque. I used anti-seize on all.

Any thoughts?

 

Just for clarification, I am talking about the manifold-to-block connection not the manifold-to-exhaust pipe connection ...

 

I would have guessed exhaust erosion, but you wrote that there is no washing... :?

 

Yeh, me too. Not only no signs on the block or manifold but the stud looks brand new ... satin black oxide finish.

 

The last time you worked on this, did you get a good removal on old stud and a good re-tap of threads? Possibly have too loose of hole with new stud? Just thinking?

 

Thanks for the thinking Walt. I really think I did get a good cleanout last time. Though I don't remember running a regular bolt thru after the tap, to check it, the tap felt really [properly] tight after finishing. Of course, with the interference fit of the stud, it was tight.

I'm leaning towards a poor heat treatment on the stud. (?)

Because I have no noticable leak, I'm inclined to leave it be and watch out for a future leak and repair then (?)