The fsa orbit xtreme pro threadless headset comes in two sizes 15rdquo amp 1 18rdquo the 15rdquo size is one of fsarsquos onepointfive headsets and itrsquos been designed specifically to fit steerer tubes that are 15rdquo in diameter the internal diameter of the headtube determines if the frame is compatible with the headset the orbit xtreme pro has cups that have been constructed from forged cnc machined 6061t6 alloy this fsa headset also has two lightweight aluminium