Muffler gasket. Waste of money, they are so fragile that you can break them picking them up.If there is play where the new muffler slips on, wrap the header pipe with some beer can aluminum until the slack is taken up. It will still leak, but, it won't rattle. Mine has a pipe coupler welded to the header pipe, the HD spec universal muffler slips into it, sheet lead gets pounded into the joint to seal it up. Used lead since i ran out of asbestos rope (wood stove shops sell it).