Two options fit a range of MTB seat tube styles. Brass lever contact surface offers a more positive & consistent closure.> 34.9mm inner diameter fits all alloy Specialized mtn frame designs with 30.9mm posts & 34.9mm O.D. seat tubes (not available in all markets) > 36.9mm inner diameter fits all carbon Specialized mtn frame designs with 30.9mm posts & 36.9mm O.D. seat tubes (not available in all markets) > Low-profile curved lever follows the contours of the seat collar > Brass lever contact surface offers a more positive & consistent closure