53mm at the top triple clamp for the S(Ohlins); Base (Marzocchis) is 58mm. They take different top triple clamps and clipons. Diameter at the lower triple (and the clamps/steerer) are the same on both. Plenty of stuff out there to fit the S with the Ohlins, but very little so far for the Base. Ducabike make a separate steering damper mount if you change clipons, which you'll need since the damper mounts to the right clipon clamp. They are tooling up for 58mm bits, supposedly on market in Sept. S was first to market, first to the aftermarket.
