Exhaust nut/ bolt material

[May Pop]

I do a fair amount of exhaust work at a shop that does work for single moms and low income folks. Many vehicles near the end of their life. Most of the time I can remove most extra crusty nuts and bolts. Some of the exhaust systems use fasteners that have a silver/ bronze tint to them. If they are heated they gall and self destruct. I’ve learned to use a 3/8 air impact with what ever socket I can hammer onto to the fastener. By rattling it at low power those will often com off. Or heat with an oxy/acetylene torch. The special bolts don’t even like being burned off.
Any idea what those different fasteners are made of. Perhaps inconel?
Any ideas on making it easier?

 

[RoninB4]

-I would seriously doubt inconel or any of the other nickel based monel metals were used. On aerospace applications perhaps but not on a production automobile. The monel metals are expensive and more difficult to machine (especially threaded) than ordinary SAE steels. More likely they're just a plated/coated treatment. Also possible that someone replaced some of the OEM rusted fasteners with SS from the hardware store. Do you coat the fasteners before re-installing them?

 

[rockettauto]

Most likely just grade 8 s. The coating is yellow zinc.

 

[finn]

We had one particular engine series that was hell on exhaust manifold fasteners, primarily school bus applications. We tried a number of alternate materials and designs, but ended up with a stainless material, even though the tensile properties were inferior to our typical grade 8 hardware.

School bus throttles have two positions for the most part. Idle/coast, and full fuel. They undergo a hellacious number of thermal cycles, even though they don’t accumulate a lot of miles or hours.

The thermal cycles would pop the heads off the fasteners, since the manifolds and flanges would heat up and expand before the heat transferred to the fasteners.

The stainless had a higher coefficient of thermal expansion, so it would “grow” faster than the cast manifolds during the cycle.

Never had another failure after the switch.

The fact that in your case, you observed galled threads points towards stainless alloy of some sort.

 

[May Pop]

Yes to some kind of alloy. A grade 8 or 10.2 metric equivalent burns off just as fast as a cheap fastener.
Just yesterday I was assisting removing the bolts for a catalytic converter on a mid 2000s Volvo and the also were similar. Next time ill see if they are magnetic.

 

[rustyzman]

A number of automotive exhaust bolts are stainless. Older Honda manifold outlet nuts and studs were. It was a challenge to remove them at times because they would gall if you worked them too much or heated them excessively. Nature of the beast for stainless unless you get into Nitronic. A solid indicator of stainless was when you tried to burn them off with a torch. They don't oxidize so they won't burn away, but instead king of crumble from the oxygen flow pressure alone.

The good news was that if you were careful, they came apart and did not corrode away over time. They were excellent quality fasteners.
Anti-seize on the threads makes a world of difference on reassembly.

 

[rustyzman]

Yes to some kind of alloy. A grade 8 or 10.2 metric equivalent burns off just as fast as a cheap fastener.
Just yesterday I was assisting removing the bolts for a catalytic converter on a mid 2000s Volvo and the also were similar. Next time ill see if they are magnetic.

Remember that 400 series stainless is magnetic. These are the martensitic series stainless', so a magnet test does not help much. 409 stainless is (or at least was) the stainless used for catalytic converter shells.

 

[Zeke]

I have used silicone bronze. It seems to me that new nuts for every install is wise whatever you use.

 

[finn]

Remember that 400 series stainless is magnetic. These are the martensitic series stainless', so a magnet test does not help much. 409 stainless is (or at least was) the stainless used for catalytic converter shells.

It was used on everything from the manifolds to the converter.

Pretty sure the entire systems are usually 409 now, since they don’t rust off the after the converter anymore.

 

[BillK]

One thing I have learned is to heat the nuts up red hot with the torch but then let them cool off almost completely. Then give them a shot of Deep Creep or your choice of penetrating lube. After that they will almost always come off. Same goes for pipe plugs in engine blocks etc.

 

[nadogail]

My 1973 Toyota used Brass Nuts to hold the Exhaust Pipe flange to the studs on the Exhaust Manifold.

 

[finn]

My 1973 Toyota used Brass Nuts to hold the Exhaust Pipe flange to the studs on the Exhaust Manifold.

Pretty common on British cars at one time, and one of my old chainsaws has them.

Not very strong, but they don’t gall in high heat applications.

 

[maplewood]

-I would seriously doubt inconel or any of the other nickel based monel metals were used. On aerospace applications perhaps but not on a production automobile. The monel metals are expensive and more difficult to machine (especially threaded) than ordinary SAE steels. More likely they're just a plated/coated treatment. Also possible that someone replaced some of the OEM rusted fasteners with SS from the hardware store. Do you coat the fasteners before re-installing them?

Doesn't matter for this thread at all but... When we visited a disused nuke silo turned museum, they showed us one of the nozzle's fueling plates (not the correct term) - it was made of inconel, which apparently was a big feat at the time. The thing weighed a huge amount given its size. I was surprised to learn later on that there are aftermarket inconel exhaust studs you can buy for certain engines (usually for turboed applications where the damn thing falls off no matter what you do). They are cheap enough that everyday people can get them, but certainly still expensive enough that I'd doubt they are bring used anywhere its not really, really needed.

 

[yhprum]

I used to work in the turbine engine world. Almost always one part of a threaded fastening system was sliver plated in the high heat areas and they never seized when you had to unscrew them.

 

[sz0k30]

There is No way they are Inconel. And there is no way auto manufacturers are using grade 8 fasteners on exhaust. Grade 8 is structural - suspension & steering.

 

[finn]

There is No way they are Inconel. And there is no way auto manufacturers are using grade 8 fasteners on exhaust. Grade 8 is structural - suspension & steering.

Nope. We used grade 8 for engine fasteners for years, as far back as I can remember, ie the mid seventies.

When we eventually converted to metric, grade 10.9 became the common fastener. It’s cheaper to keep non standard, low strength fasteners out of the factory than to chase failures in the field.

We we’re making 400000 Diesels annually at the time.

Casually looking at modern car engines, it looks like they do the same thing. Smaller, lighter high strength fasteners are common.

 

[roc_on_the_rocks]

We we’re making 400000 Diesels annually at the time.

My guess: Navistar

PS.: As a fellow engine engineer, I like your posts Finn.

 

[finn]

My guess: Navistar

PS.: As a fellow engine engineer, I like your posts Finn.

I’m in a self imposed witness protection program.


Cat and Cummins fastener strategies were the same, and probably still are.

Very rarely do I find cheap fasteners on automobiles. Corrosion is the biggest problem, not material strength.

 

[RoninB4]

I’m in a self imposed whiteness protection program.

-White-ness? Your Freudian slip is showing but that's your privilege to do so.

 

[2ndGearRubber]

They're stainless. Toyota loves to use them.

Gun them off "cold" no heat, that's my best advice. Cobalt spot weld bits can drill out the studs if required.

 

[finn]

-White-ness? Your Freudian slip is showing but that's your privilege to do so.

No, it’s Apple autocorrect striking again. I guess I should just turn it off. Never had that problem with my old android devices or the windows stuff.

I fixed it

 

[May Pop]

2nd Gear
I’m glad you replied. I have been doing that for the last few years. I rattle them with a 3/8 air impact then they come out. Any advice burning once they have been mutilate. I’ll have to try cobalt bits.
I think my next thing is to practice burning the bolts out of junk exhausts and converters.

 

[CraigStu]

I know access is usually going to be difficult but cutting though a nut as much as possible w/ a die grinder w/ cutoff wheel can sometimes work. RE: high end fasteners on production cars. I changed out the front swaybar on my 19 Stingray. Surprised the **** out of me to find out the chassis mount bolts are 8mm - one use - stretch bolts that are tightened to 22# plus X(?) degrees. On a swaybar mount???????? They thread into an aluminum frame piece which I assume had helicoil inserts. But I am not going to be held back needing a one use bolt replacement one day so I installed 10.9 studs and flange nuts.

 

[MBfreak]

Many MB´s from 60/70s used copper plated mild steel exhaust nuts. Always come off easy. And for just a few $´s I always put on new ones at reassembly.

Ola