ZS55 headset standard (zero stack, 55 mm)
What a ZS55 headset measures under the SHIS standard, and how to tell it apart from lookalikes.
Published dimensions
| SHIS designation | ZS55 (zero stack (ZS)) |
|---|---|
| Cup / bearing outside diameter | 55.0 mm cup outside diameter, 54.9 to 54.95 mm head-tube bore |
| Steerer diameter | 38.1 mm (1 1/2 in) |
| Crown race | 40.0 mm crown race seat |
Used on: the 1 1/2 in zero-stack lower on tapered 44/55 mm head tubes.
ZS55 is the wide zero-stack lower paired with a ZS44 upper on a tapered head tube. The 55 mm bore hides the lower bearing inside the tube for a 38.1 mm steerer. A 44 mm ZS44 lower cup will not fit a 55 mm ZS55 bore.
Frames and related headsets
EC34external cup, 34 mmEC44external cup, 44 mmEC49external cup, 49.6 mmZS44zero stack, 44 mmIS41integrated, 41 mmBSA bottom bracket shellthe frame's other threaded interface
SOURCES: Park Tool, Standardized Headset Identification System (SHIS) · BikeGremlin, Bicycle headset bearing standards (SHIS)