Hello there-
Exhaust studs very often do bad things due to the extreme heat they see.

Replacing them can work one of two ways. If they pull out with the nut and don't break, they can be reused. You can simply reinstall the nut and stud.

You could clamp the stud in a vise and try to remove the nut but you'll often break the stud.

Replacement studs are available at virtually any auto parts store. Volvo uses M8 exhaust studs (8mm). There are also copper nuts to go with them, which reduce the chance of repeating the problem of the stud pulling out.

If the stud breaks off, it really should be drilled and tapped and a new stud installed. This process is usually difficult and frustrating. Nearly impossible to get it right with the head on the car, unless you're a good machinist. Removing the head is really the way to go for proper repair. There are helicoil kits to replace damaged threads, as well as "step studs" with a larger thread on one part, and a standard M8 thread on the remainder. These allow you to make good repairs in aluminum heads as with the Volvo engines.

When reinstalling, I recommend using Never-seize compound on the threads.
If you can't find the right nuts and studs locally, FCP Groton sells them, though they're probably not listed on the website www.fcpgroton.com. They carry the M8's and the M10's for the manifold-to-header flange as well.
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Rob Bareiss, New London CT ::: 87 244DL- 249K, B230F/M47, Turbo sways, Bilsteins, GT braces, Virgos, Turbo exhaust, Gislaveds for winter!