I've been trying to seal up the leaks in my new exhaust system and am having NO LUCK. I finally managed to get the mismatched cat and header pipe to seal (cat is spin flange, header pipe is fixed. Long story.), but now I think I have a leak from the header pipe to exhaust manifold connection.

I used a new gasket the first time, but I had to remove the header pipe to work on it, and not having another gasket, I re-used the week-old gasket. It don't seem to be sealing this time. Do they only seal properly the first time?

BTW, I got the mismatched cat and header pipe to seal using the steel donut gasket, by tapering the inside of the fixed flange header pipe. Short work with a rotory file. I believe I also did not ruin the header pipe in case I want to use it with a fixed flange cat in the future.