Join Date: Aug 2018
Location: Central Virginia
1. Always a new gasket for new headers - they won't be exactly the same shape as what you took off.
2. The primary concern for exh manifold bolts/studs is corrosion - all the other bad things that happen derive from that. If it's going into an aluminum head, the clamping loads aren't high. The ideal choice is to change to stainless hardware. A second choice is to go to a high strength steel alloy - much stronger than needed, because these also tend to be high chromium alloys and are thus more corrosion resistant (if not loaded to a structural limit). The high strength steel will be more resistant to galling than most stainless fasteners. Don't forget the alloy of the nuts and any washers - most split lock washers are carbon steel and don't retain their springiness after a few header heat cycles. Automotive OEMs tend toward flanged self-locking nuts in this application with no washer. If they don't 'lock' anymore, replace them.
If clamping force is low enough, you can use long brass nuts with steel studs - Kawasaki did this on two strokes back in the '70s with the specific goal of protecting all the thread on the stud from the environment. It works very well in combination with copper antiseize.
You always want to use an anti-seize compound on these fasteners. Chemistry matters, so you want a different one at the steel-aluminum interface with bolts into the head or putting head studs back in (I use 'silver' permatex in the tube - originally 'Aviation Permatex') from the one at the steel-steel interface (I have a tube of NAPA-branded copper anti-seize for this, but it's just copper particles in an oil base -all brands created equal). There are different recommended anti-seize compounds for stainless fasteners of particular alloys. Because of the high temperatures involved, the chemistry matters more than it does on most things.