To make a metal on metal connection with a swivel nut sets, such as on a CO2 regulator, you need a flare fitting washer. This washer fits between the two connections and helps to make sure the connection is tight, so that you do not leak CO2 or liquid. Flare fittings should be used on all metal-to-metal connections. Keep a few spares handy in case you have to rig a connection at a moment's notice!