I've measured three of them and they all seem to be in the 23.6 mm range. I use a Zipp TT Quickview Computer Mount (which is specifically made for the newer standard aero bar diameter of 22.3 mm) on Profile aero bars. I tried it out on a Syntace without the rubber insert and it fit fine. One turn of black electrical tape would make an adequate mounting surface. Barfly also makes a "4 TT aerobar mount " that they say fits 22.3 or 24 mm bars.