Fsa is the world leading headset manufacturer and the core of their product line with many years of experience they understand that multiple dimensions and exacting requirements are a must when it comes to headsets the fsa orbit ita internalexternal tapered headset is mix of internal and external headsets it fits 1 18 head tube with a outside diameter of 50mm and 1 18 to 15 tapered forks the lower part fits the same as a conventional headset with a 15 36 x 45d angular contact