This v-band flange has a 3.0" I.D. and a 3.55" O.D. and it meant to be used with our 3.55" v-band clamp. This is the correct flange to use if you are making a downpipe or O2 housing to mate to a Xona 3.0" discharge Tial V-band housing, Evo SS housing, or Tial GT28/30/35 housing.