Actually I think you were right initially... closer to 70. I just measured mine, it's ~73mm at the inlet, but it tapers a little. Maybe it's 71 or 72 at the throttle plate, but I don't have the tools to get in there to measure.75 mm ID is probably good enough, but there's always the chance of unlocking some more power there. Especially once cams are added to help the breathing out. ...or if someone is a glutton for ~200 cc more capacity and they choose to do some budget stroker action.