I am just wondering if there is supposed to be a gasket between the round part of the exhaust on the back side of the cat and the intermediate pipe that goes to the muffler? I am asking because it seems like it wouldn't seal so well with metal on metal for this type of fitting. The cat has a roundish end and the intermediate pipe has a bowl on it that sits over the cat pipe. I was thinking of maybe using some of that exhaust sealer you can buy that dries flexiable?
I was thinking super high temp copper RTV or some of the paste that is sold, I will see which is a higher temp and go with that one.

 

If the end of the cat output is rounded, then no - no gasket needed. The flange on the intermediate pipe holds the 2 parts together tight, so it doesn't use a gasket. Never hurts to throw some Permatex Ultra Copper RTV on there though. Stuff is good up to 700°, so it's perfect for that application.

 

I figured I better make sure it doesn't leak. I went with the permetex tail pipe sealer that is apparently catilitic convert safe temp wise. I've read plenty about copper RTV but it seems like it should be used on the engine, not rusty exhaust parts. Fleet farm has it for $2 and no one else carries it that i've seen. Hopefully this cures my neverending I own a blazer vortec engine that has no power and the fuel regulator always fails problem? I'm thinking an exhaust leak is killing my power on mountain passes at altitude, I use 85 octane gas at around 11-9000 feet.