I am in the final stages of doing an h22 swap on a Prelude si, and to my dismay, the cat bolts were so rusted, I had to cut them off. So I removed the lower exhaust manifold however, I still can not get the damn bolts off the cat. I attacked the other side again with a die grinder and I really don't see how they can be still in there. They look like they are pressed in there and have splines on them similar to lug studs. Other then just beating it with a hammer (which I have been doing with ZERO luck) what else can I do?

 

Splined like lug studs, that's exactly how they are. I had to drill mine out.

 

Heat the surrounding flange with a torch and hit the studs while they are hot.

 

Tourch and a BFH worked wonders.