Removing Stuck Nuts

exhaust bolts are close to impossible, especially in the north east. If they're on there that well, the studs will probably be in poor shape when you do get the nuts off.  My go to reaction to anything exhaust is to just cut it off and replace the hardware.

That said, get some good penetrating fluid. Not PB blaster. Kroil is amazing. Or try home made recipes. My favorite is 50:50 mix of ATF and Diesel. Soak, then hit with heat (there will be a little fire, be careful), then soak again. Repeat many times. Once you get the nut to start moving, do not go for full removal immediately. Back it off a little, then tighten again. Add penetrating fluid, then back off a little and repeat. You want to work the stuff into the threads before you trying to force it completely off. If you're lucky you'll slowly get it to back off.

Another option is to just break the nut. They sell nut splitters that will just break it in half. Then throw a die on the stud to clean it up. They look like this. They may or may not fit on the nut depending on how close to the exhaust they sit.

OK, just did this job over the last three weekends on my 940.

If it is just the back one you cannot get off because you cannot get a clear shot with an impact (you can barely get a shallow socket on that one with a wobble!) you honestly are better taking off the the exhaust manifold assuming it is a non-turbo.  If it is all three you cannot get of with a quality impact wrench, you MUST take off the exhaust manifold.

I got two of three out "easily" by grinding the tapper off a 9/16" shallow impact socket (my hardware was 15mm but yours could be 17mm).  I had already soaked them for three days in penetrating oil AND took a wire wheel on a dremel to all the areas I could get to removing rust from the threads.  I then put a long extension on the modified socket and pounded it on with a 3# hammer.  The 9/16" is technically a little too small for a 15mm nut but rust had eaten away enough it went on after about 5 whacks.  Then I hit it with forward 3 seconds, backward 3 seconds, rinse repeat until it all at once let loose in both cases.

The third one was not to be so I ended up pulling the manifold anyway.  I did end up rounding off the nut enough I had to weld another nut to it but with it out of the car I could get 90% weld around the nut.

A nut splitter does not fit on that one by the way...in or out of the car.  The infamous guru of the Brickboard, Art Benstein, maintains 11 of these cars and by his own admission, he does not even try to take them off in the car anymore and drills them after to replace with stainless hardware.

I think that is what I am going to try.

I hear a mix of ATF and acetone works well as a penetrating fluid. 

I don't think a nut spliter is going to fit in there unfortunately.

I would get it out, drop a larger nut down on there and weld it to the old nut.  It it does not work and your weld is not what breaks, the studs will break off flush.  Drill them our and either just drop a bolt in there or or re-tap it for a stud.

FYI, the "standard" nuts for the 10 mm studs are 17 mm.  If you can thread one on, weld it to the original nut, it solves two problems in one.  Cleans up the exposed threads and gives you a much bigger bearing surface for the socket.