I had to be brutal with this one even after having the alloy collars red with a blow torch it was resort to a ten inch cold chisel persuader in the end!
If you click on the image you can see it much more clearly.
So, one down the other to go and exhaust studs to replace, one to drill out so far but likely at least one of the other side will break off too.
I was amazed that the nuts (also steel and should ALWAYS be brass) came undone but I had been applying Plus Gas to the collars liberally for months as I knew the day was coming whereby I had to set about getting them off.

