My exhaust manifold stud are so rusted that the thread is dead.
I purchased new studs, but the I have a hard time taking the old ones out of the casted iron of the manifold.
I've put some of this product that is supposed to help loosen rusted screws and waited a full day and night; doesn't help much.
I tightened two bolts one against each other on a stud to be able to use a wrench, but the thread is too rusted to hold the torque.
I saw a slit on the end to be able to use a screw driver, but I can't apply enough torque with a screwdriver.
I shaped the end with a file to be able to use a monkey wrench. The stud would still not turn.
Any idea ?

Spray it with a product called PB Blaster. You will need to give it a spray everyday for a few days. Take a hammer and gently hit the stud a few times everyday too. This will help loosen it up and allow the oil to work its way in to the rusted area.  Good luck.  It worked for me. indy992009-07-05 09:39:40

If the stud is not brass or stainless you can burn it out with a cutting tourch, WHAAT!?! you say, yep,  I've done it many times, what I do is hack saw  the stud flush with the flange , chuck the manifold on a drill press and drill a small hole through the center of the stud all the way through so I can see daylite, take a small burning tip and burn the stud out, carry the heat in the center of the hole you drilled, the sl*g will exit through the the other end of the hole you drilled, as you watch the stud melt away from the center of the hole you will be able to see the threads appear, untoached, the reason why this works is because the melting point of cast iron is way higher than carbon steel, all you have to do is run a tap though the threads to clean them,....I USE THIS METHOD AS A LAST RESORT..... the other way to do it is take nut that will slide over the stud and weld it top and bottom to the stud with a wire machine, try not to undercut the stud, let it cool stone cold, clamp the manifold in a vise take a small  oxy-acc. welding tip size 0 or 00 and heat the boss of the bad stud while trying to move the stud with a box end wrench, if you get it to move work it in and out gradualey till it comes out, don't over heat the boss to where the stud gets hot, .....NO GUTS, NO GLORY.... if your not comfortable with these methods theres always the ole muffler shop. anips2009-07-05 10:04:59

I am impressed!

"Not comfortable" with these methods is an understatement given I am not equipped to do that. I'll look into it anyway. In the meantime, I'll keep following Indy99's method for a few more days.