It happens more than it should. An exhaust nut welds itself onto the head. Even with the proper wrench, the risk of destroying the threads on your precious BMW Airhead exhaust spigot is very high if you try to turn a nut that doesn’t want to turn. One solution is to cut the nut and more or less relieve the ‘tension’ so the nut can be spun off. Often heat and a good penetrating fluid (ATF and Kerosene 50/50) are both still needed. Below is one method for removing a frozen nut that just might save a trip to the machine shop. Note this is for information only and I don’t guarantee results. Good luck.
A frozen exhaust nut. The devil’s work.
You’ll need one of these tiny hacksaws. Very carefully choose a spot where the blade can go between fins on the head itself as the blade goes back and forth. Note the cut does NOT have to be straight on the nut. All you are trying to do is remove a kerf of aluminum so you can split the nut off. See the next picture or two.
Here you can see the kerf starting to take shape. Note is is not straight and does not have to be, so don’t get all OCD at this point. Note also that the thread we are interested in is the bit of the nut toward the head, so concentrate your cutting efforts there. The idea is to cut just enough material (depth, I mean) that the threads are lurking right under the hacksaw teeth…but no deeper. Make sure the cut does go all the way across the nut so the overall strength is reduced in this locality, but be careful not to cut into the head pipe. It’s pretty easy to get carried away so I advise to take a few strokes, look, take a few strokes, look again. Etc.
As unnatural as this seems, take cold chisel and put it in the keft at the point where the thread would be connected to the head. Give the chisel a few hard raps with a hammer.
The results of the hammer blows. You can just see a little black line in the bottom if the kerf. This is where the nut has split from being whacked with the chisel. Now spray some penetrant into this little crack and go have a cigarette or cup of coffee. Kiss your wife.
Now put your BMW Airhead correct exhaust nut wrench on the nut and gently try to turn it. If you feel resistance, you may have to split the nut a little more or use heat and some more penetrant. In general, the more resistance, the more likely there will be some damage. Just do your best.
The frozen exhaust nut has been removed. Here you can clearly see the the third thread from the ‘top’ is completely ruined. The one above isn’t real pretty either. Don’t despair, it can still be saved. A proper threading die should give you at least the ability to save the threads that are still there. If you try to put a new nut on without cleaning up the threads first will surely result in a worse situation that you have before, so don’t do that. Again, if you feel resistance, stop!
There is no shame in getting a machine shop to help at this point. The main thing is YOU tried.