He isn't talking about the gasket between the manifold and the head.

One thing I've noticed is that most muffler shops will make you an exhaust and will use a 3/8 bolt, which is a no-no because the hole in the manifold is something bigger (I think 7/16 but it's been awhile, I no longer run stock manifolds). The small bolt in a big hole allows everything to move around too much which causes gasket failure. But now that I typed all that, it will probably end up being something else.