Grade 5 or 8 with never sieze on both the stud/bolt into the head and on the threads for the nuts. I recomencd studs because if the nut siezes and it breaks off the stud, you can still remove the header and the shank of the stud. For whatever reason bolts tend to break off flush with the head and are usually harder to remove.
My shop usually does 1 or 2 broken exhaust bolt/stud removal per month, and most of the heads that come in here have 2 or more broken off.
And the first question is, "can you do it on the car". No, and if you try to do it on the car, most often you end up having to remove the head anyway because you usually wallow out the hole, requiring a helicoil, timesert, or larger insert.
Most often the customer tries to drill it first for an easy out and the hole is not centered all the way down and gets into the threads, locking it into the head.
Then they break off the easy out flush with the head too.
We have ceramic and carbide mills to plunge those out too, but again it's done in a bridgeport mill, threading for an insert and more$$$.
We usually get 4 or 5 calls about the same job from different shops after the owner has called us for an estimate, and we are told "thats too much, if I had the tools I'd do it myself. Go buy a mill and you can do the next one too!
The other shops mark up our work, so we always get the job anyway. Call a machineshop first. Never sieze is cheaper.
BTW, never sieze won't make the bolts/studs loosen up in use. The reason you have to periodically tighten the fasteners is because the gasket crushes with heat against the fasteners and when it all cools the gasket gets smooshed with every heat cycle. Check/ tighten, but not overnighten them, it won't help.
Remember that the exhaust pressure is about twice or more than the boost is, you will get to a point where gaskets won't work and then just clean the surfaces and use hi temp silicone instead. You will still have to retighten the fasteners as they will stretch with the heat cycling.
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