While attemting to tighten a loose header nut, the entire bolt on the #4 exhaust header pipe broke in half. Now, is this a small roblem, or is this a big problem? For one, that part of the header is rattling terribly on the engine and the smell of exhaust is seeping into the interior. Not a lot, and not always. But when I come to a sudden stop after driving "briskly" the smell will enlighten the occupants. Now, this could also be coming from the loose Cat-gasket under the passenger seat. So, any suggestions anyone?