You use an OEM donut gasket for the header/cat connection. You use universal 2 or 3 bolt(depending on the flanges on your exhaust) universal style gaskets for the piping. The OEM piping uses a donut style gasket similar to the header/cat gasket to seal the midpipe to the muffler section. That style gasket wouldn't work with your new midpipe since i t uses flat flanges.