ZS44 headset standard (zero stack, 44 mm)
What a ZS44 headset measures under the SHIS standard, and how to tell it apart from lookalikes.
Published dimensions
| SHIS designation | ZS44 (zero stack (ZS)) |
|---|---|
| Cup / bearing outside diameter | 44.0 mm cup outside diameter, 43.9 to 43.95 mm head-tube bore |
| Steerer diameter | 28.6 mm (1 1/8 in) |
| Crown race | 30.0 mm crown race seat |
Used on: the common 1 1/8 in zero-stack upper on modern road, gravel and MTB frames.
ZS44 is a zero-stack (internal) 44 mm cup: the section holding the bearing drops inside the head tube, so it stacks lower than EC44. The 44 mm bore is shared with EC44, but the two cups are different heights, so confirm zero-stack versus external before buying.
Frames and related headsets
Headset on a Specialized Stumpjumper (2019 on carbon)zero stack, 44 mmHeadset on a Santa Cruz Hightowerzero stack, 44 mmHeadset on a Santa Cruz Megatowerzero stack, 44 mmEC34external cup, 34 mmEC44external cup, 44 mmBSA bottom bracket shellthe frame's other threaded interface
SOURCES: White Industries, ZS44/EC44 headset · Park Tool, Standardized Headset Identification System (SHIS)