From a friend's low-mileage 2018 GMC V8. Hopefully, not too much damage to the engine's threads. Both exhaust manifolds needed replacing — severely warped.
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Stretched exhaust bolts on GMC exhaust manifold
- Thread starter Mr Boochie
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They seemed to have used soft steel alloy in those bolts. Being tweaked like that made them hard to undo. If he ever had to do the job again, he may need to rethread the cylinder head.
The driver's side manifold was warped and leaking exhaust gas.
tom3
Super Member
I've read there is a special torque value for those bolts to allow the iron manifold and aluminum head to slide just slightly to protect both parts from cracking. Those bolts almost always fail at some point, usually the end ones. Good idea to replace them, check on the correct torque.
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There are some measures that various people claim work well, to stop this problem.
One, is that there are the aforementioned grade 10.9 bolts.
Another, is to slightly drill out the bolt holes in the manifold, to allow for a bit more manifold expansion and contraction. This will reduce the bending force on the bolts (you can see the ones in the OP are slightly bent, as well as stretched).
Regards,
Gordon.
One, is that there are the aforementioned grade 10.9 bolts.
Another, is to slightly drill out the bolt holes in the manifold, to allow for a bit more manifold expansion and contraction. This will reduce the bending force on the bolts (you can see the ones in the OP are slightly bent, as well as stretched).
Regards,
Gordon.
My thoughts too, one should never re-use bolts that are designed to stretch, and observing the correct torque setting is obviously very important.
yeah stretch bolts are one-time. No idea if the exhaust bolts on these engines are that. Haven't had to replace mine, so I never bothered to learn that detail. Even if they aren't sometimes its just obvious that new hardware is the way to go. Last water pump I did on something I got new bolts for because I knew the bolts were in questionable shape. Friend needed pump bolts. I gave him the old ones with the warning that I didn't trust them. One broke on install, which is exactly what I was concerned about. Glad I bought new ones.
Actually, all bolts stretch to a degree. It's how they tighten or wedge. It's those cheaper bolts or ones under a lot of torque that stretch irreversibly, to the point that they can't safely apply the same torque and tightness — the second time.
I was taught to replace all the cylinder head, flywheel, and main crankshaft bolts — amongst others.
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Some bolts are designed as Torque To Yeild (TTY) Bolts.
They are meant to be stretched to/just past their yield point, where they just start to deform. It's at this point that they achieve their maximum clamping force. These bolts will have a torque spec and then an angle spec. Basically, you torque them down and then turn them however many degrees further that is called for. IE; 110 ftlbs and 90 degrees.
TTY bolts are one time use.
If they look necked down in the threads, then they've been over tightened past spec and are definitely not reusable.
Modern engines are chock full of TTY bolts. Heads, big end caps, etc, etc.
All bolts do "stretch" slightly when torqued to spec, it's a matter of wheter or not TTY is called for and the metallurgy in the bolt itself.
They are meant to be stretched to/just past their yield point, where they just start to deform. It's at this point that they achieve their maximum clamping force. These bolts will have a torque spec and then an angle spec. Basically, you torque them down and then turn them however many degrees further that is called for. IE; 110 ftlbs and 90 degrees.
TTY bolts are one time use.
If they look necked down in the threads, then they've been over tightened past spec and are definitely not reusable.
Modern engines are chock full of TTY bolts. Heads, big end caps, etc, etc.
All bolts do "stretch" slightly when torqued to spec, it's a matter of wheter or not TTY is called for and the metallurgy in the bolt itself.
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I anti-seize NGK;s all the time. I use copper anti-seize compound though and just a dab, which is all you ever need anyways.
I usually don't differ from OEM recommendations, but this is one spot where I do. First time you gall a steel thread into an aluminum bore is one time too many...
I usually don't differ from OEM recommendations, but this is one spot where I do. First time you gall a steel thread into an aluminum bore is one time too many...
Be careful with over-torqueing!
krautmaster
68 Chrysler -440 Magnum !
Yep, drilling the manifold holes 1/8" oversize is an old trick to deal with this problem, and most engines with aluminum heads & iron manifolds have this problem. Just do your best to properly align the manifold to the port when tightening up the bolts.
Some people leave the center-most bolt hole factory diameter, and drill out all the other ones. It's more unlikely to try to move in the center, anyway.
Regards,
Gordon.
Not a problem.
Even if the shell does get stretched enough to change a heat range by one value, the engine is not going to really care. Thats an insignificant change.
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