Internal headsets are available for 1 1/8”, 1 ¼”, 1.5”, and tapered steerer tubes. Internal headsets are sometimes referred to as Zero Stack (ZS) because both cup and bearing is housed entirely inside the headtube.
For 50mm OD headtube
Ø5/32"x20 caged ball bearings
Alloy custom top cap
Buy FSA No.10 Internal Headset from Chain Reaction Cycles, the World’s Largest Online Bike Store.