Best material for exhaust manifold studs?

My Volvo V50 has several broken exhaust manifold studs. There are 12 studs (M8x1.25), and each gets a concave washer and a flange nut. This is a shockingly common problem on these cars, apparently, with the nuts loosening or the studs simply snapping. It manifests itself with a squealing noise when the engine is cold (gases blowing past the manifold gasket, it seems). I don't know if it's more common on the T5 turbo cars, but given the increased heat it wouldn't surprise me. Right now, I'm not worrying too much about it, but eventually I'll want to tackle it. There is a fair bit of discussion on SwedeSpeed about trying other types of studs, mainly stainless steel or titanium. If you could choose any material for the studs, what would you use? Would you use thread locker, or anti-seize, or something else? I know we have some people knowledgeable in metallurgy here, and I'm curious to hear thoughts on this.

The washers are supposed to allow the manifold to slip around under the fasteners, since the manifold might be a peak 1300-1500F while the head is 220F so it out-expands the aluminum by a fair amount.  When the manifold rusts, this locks the washers to the manifold and also makes the fastener connection tighter, making it more difficult for the manifold to slide around, so it breaks the studs instead.

The stock studs are a pretty high quality piece, it is just the nature of the joint and Volvo's chunderheaded decision to make engines that last a very long time.  Lessor engines fail before they have a chance to have age related manifold stud failure 

New correct spec studs, nuts, and washers torqued to the shockingly low correct spec, and they will be good for another 15 years.

I would go with stainless and a good slathering of anti seize. The stainless can gall if it is not lubricated with anti seize. I would think that Titanium would be expensive and not really do anything better than stainless steel. 

Stainless is the devil.  Buy the oe studs and nuts from Volvo.

In reply to Streetwiseguy :

Inconel is kind of like stainless, no? 

tester (Forum Supporter) said:

The stainless can gall if it is not lubricated with anti seize. 

Stainless steel will gall.  I'd simply go with a decent quality steel stud, ARP must have some in the correct size.

Streetwiseguy said:

Stainless is the devil.  Buy the oe studs and nuts from Volvo.

Stainless steel is worse than bolt steels at everything other than corrosion resistance and strength at high temperatures. It's far weaker, so you can't get the same clamping load at room temperature. No matter what you use, coat everything in good antiseize. T8E is better than anything off the shelf, but it's expensive and difficult to find.

Pete. (l33t FS) said:

In reply to Streetwiseguy :

Inconel is kind of like stainless, no? 

 

Inconels are nickel alloys. That family is excellent for this if available. I believe 718 should suffice. I'm not sure how available it is though.

You could start with some basic information like this:

https://www.extreme-bolt.com/products-inconel-bolts.html

At the higher temperatures a turbo manifold sees, stainless steel hardware will be susceptible to carbide precipitation, which is a bad thing. It leads to intergranular corrosion and bolt failure. OEM spec bolts will do a better job. I would still use high temp anti-seize, though... it'll help with proper torque and keep things from getting stuck. 

stuart in mn said:

tester (Forum Supporter) said:

The stainless can gall if it is not lubricated with anti seize. 

Stainless steel will gall.  I'd simply go with a decent quality steel stud, ARP must have some in the correct size.

I agree with this

-B.S. Materials Science Engineering. 

Good quality fastner with proper forming, meaning attention to stress risers, with the proper antisieze for the temperature of application torqued properly is the correct answer.  ARP tends to make good stuff. 

if you are not going with a purpose made stud for the application, it is worth considering that higher grade (meaning using a grade 8 instead of a grade 5, or the metric equavilent) hardware may be more prone to fracture from the cyclical stresses that result from the thermal cycling as discussed above. 

Note that any frictional deviation (material, coating, lubricant, etc.) will alter the 'correct' torque used to achieve the intended clamping force.

Recon1342 said:

At the higher temperatures a turbo manifold sees, stainless steel hardware will be susceptible to carbide precipitation, which is a bad thing. It leads to intergranular corrosion and bolt failure. OEM spec bolts will do a better job. I would still use high temp anti-seize, though... it'll help with proper torque and keep things from getting stuck. 

The torque assumes clean, dry threads and new locknuts.  The issue with antiseize is that it is a lubricant, and you will overtorque the nuts, which will prevent the manifold from sliding around easily, which will lead to broken studs.

It's actually pretty fascinating how they engineer a sealing surface to handle 40-50psi of internal pressure and still be able to move around laterally.

You sure the manifold isn't warped? Audi had a lot of this with their 5 cyl exhaust manifolds warping and breaking studs. I used to just have them resurfaced and replace the studs with new stock Audi ones. They usually lasted pretty long before I had to do it again - it wasn't a yearly maint. type thing.

Lots of very useful information here, thanks. I do not know if the manifold is warped, as I haven't had it off the car. I do know that putting new flange nuts on the studs broken out past the manifold stopped the noise; these were gently snugged down, out of fear of breaking what was left. It's a temporary solution, I know.

One question, given what's been stated already: if not on the threads, what about anti-seize on the manifold surfaces the washers contact?

I went through every permutation imaginable on this theme in my Miata.  Inconel is the right tool for this job.  It's more slippery than the factory studs so using those funky star nuts with the fingers you can add to keep them from backing off is a good idea.  I would use an OEM gasket and avoid antiseize.

There's really nothing you can put on the manifold that won't burn off the first time the thing is glowing red under boost.  I wouldn't stress over it too much.

Audis were fun because the manifolds themselves would also break.  I managed to find a rare 2 piece 10VT exhaust manifold, not because of reliability but they are supposed to flow a lot more than even a ported 1 piece manifold.

Pete. (l33t FS) said:

There's really nothing you can put on the manifold that won't burn off the first time the thing is glowing red under boost.  I wouldn't stress over it too much.

I don't have the data to back it up at this point, but I have done testing to prove that this is incorrect. As I stated, T8E is the best anti-seize I have tested. I have a small amount in my toolbox but it's really expensive.

Can't be that bad if it works.

I'm not sure what it will do to prevent the manifold from rusting, though.  But I'm interested in it all the same.  Who makes it/where can it be sourced?

Shazam, not cheap is right!

https://www.mrosupplyshop.com/products/tiodize-t8e-paste-155-gram

But it's certainly got some specs.

https://www.tiodize.com/lubricative-anti-seize-greases/

paddygarcia said:

Shazam, not cheap is right!

https://www.mrosupplyshop.com/products/tiodize-t8e-paste-155-gram

But it's certainly got some specs.

https://www.tiodize.com/lubricative-anti-seize-greases/

Yup that's the stuff. The water based stuff is difficult because of shelf life. That's what I have. If I can bring it back to life I can just ship some to the OP. It really is great, at least in my testing.