You will need new exhaust gaskets for the flanges that mount to the head. Any time a header is removed from the head it is recommended to change the exhaust gaskets. All the other joints (header to flanges, header to midpipe, midpipe to muffler) you need to use Ultra Copper silicone to seal those joints. You don't have to use an excessive amount either. Just a thin layer to seal each joint. You could try putting a small amount of Ultra Copper silicone on the flanges and see if they will seal against the old exhaust gaskets.
Are you just replacing the muffler? If so just use to stock exhaust gasket and it should seal just fine. It would take a thick bead of silicone to replace the stock gasket between the factory midpipe and a bolt-on muffler.
A forum community dedicated to Honda Fireblade motorcycle owners and enthusiasts. Come join the discussion about performance, racing, modifications, classifieds, troubleshooting, maintenance, and more! Find conversations about your favorite CBR 900 or 1000 series RR models.