Hi,

My best advice is to do just exactly what has been posted here, of which, you have done.
I agree with the choice of using heat for the nuts.
The studs can be heated but I would hold a rag on the stud for a seconds, to help it shrink as fast as that aluminum head is going to suck heat away!
Hopefully, you won’t need use it when you get that far in!

I can give you a little background on what you are dealing with.

When Removing the nuts, you are dealing with steel on steel and a fine crack between the two parts.
You can try squeezing the nut a little with Vice Grips to mash them a little out of round, but do not screw up the flats.
There are cracks that runs up and down in a 60 degree angled form.
Think of two opposing “V’s,” side by side, parallel to the axis of the studs. Squeezing shift those angles to possibly slide. We are talking pressure only at the molecular level.

The threads in a hex nut are holding the manifold in place, with its flat face shoulder, but the stud is still stretched!
Each thread on both ends is pressured to one side of their respective thread angles, thus making a metal to metal contact.
The aluminum threads are trying to hold onto the stud just as equally as the nut.

We need to work with those two forces to rattle open that fine crack in
both parts, since they have the same thread clearances we can move in!

On those mating parts there is a space at the top and bottom of the bodies of threads.
It was made this way by truncating the tops off creating root clearances.
It’s a space made to allow for debris, lubricants and most of all inconsistencies in dimensions due to material stress and distortion.
There are many more reasons for root clearances.
The main One is this!
The principal of threading is helical and is a theoretical 100% “V.”
That idea of thread fitting inside another thread cannot be made to work, especially on the internal side!
This is why a calculated maximum strength of a threaded hole is rated at 75% and the reason for a “tap drill size” chart. It does the most truncating.
This is because, It takes four times the torque, on the tool, to gain that extra 25%! Not practical nor needed!
The Rule of thumb is, at 75% the depth of thread a tapped hole needs not to be any longer, than one and one-half the times the diameter of the bolt or stud used to get its maximum strength.
This means, the stud or bolt will snap off before the threads in the hole will strip out!
I have never seen a tapped hole with stripped threads but a bolt thread in them, yes! All due to some sort of assembly error!

In actuality, you are working with the nuts first because you can apply axial torque to them.
But you can apply some lateral force to the stud by tapping in line with the axis or the center of both threads.
You want to shake any play in between those two fault lines a little bit.
You want to shift particles out of their “homely little pockets of togetherness” into those root spaces. That Vice-Grip squeezing stuff comes in about here. Just a little pressure is all to move in the tiny spaces.
You can do it hot or cold or in combinations while tapping them with a small hammer.

Ok now!
What it’s going to come down to, you are going to “vibrate” both the nut and the stud, working, within the two threads tensions, by striking against the stud and jolting the tensions. Just decent thumps at any speed you like.
This temporarily shifts the cracks and the crusty little varmints start falling out into the root cavity!
I suggest you add on an extra nut to the end of the stud, if you have any extra threads, to have something to beat on.
By having them double nutted, you can use either one to rotate the stud nut or the stud.

You want to work carefully at twisting the assembly “back and forth” with lots and lots of patience with “time” on your side.
It is the key to success!
Turning them consistently, each way, yes, I mean tightening too! Loosening with the same pressure that doesn’t snap anything.
Yes, tapping on them with the hammer eases up on the hands and tends to break up the monotony! Moving to another stud, helps!
All the while, spraying and letting the oils soak in.
These flakes and crystals need to shift around. They need to crunch a little, to slip a little.
A little space leads to a working that space of a little part of turn. After a bit of this several times, I then go back to pushing it a little more on the others, a few times!

Spark plugs use the same loosening and tightening procedures.
Oil and only the thoughts of any cussing words, will help keep your cool!

Phil