Parts guide
MTB Headset Sizes Explained: ZS44, ZS55, Tapered and Integrated
Headsets are sized by standard, not by brand. Here is how to read the code, measure your frame and pick the right one, whoever made your bike.
- By
- Collective, London
- First published
- Updated
- Reading time
- 3 minutes
A headset is the set of bearings that lets your fork turn. Get the size wrong and it simply won’t go in. The good news is that headsets follow an industry standard, so once you know your frame’s code you can fit any brand’s headset in that size, ours included.
This guide explains the SHIS codes you’ll see on frames and headsets, how to measure if you don’t know yours, and which HX1 headset matches which frame.
Chapter 01
How to read a headset code
Two halves: the top, then the bottom.
Most frames and headsets now use SHIS codes (the Standardised Headset Identification System). A typical code reads ZS44/28.6 | ZS55/40. The part before the bar is the top of the head tube; the part after is the bottom.
- The letters are the type. ZS is zero stack: cups pressed into the frame, so the bearings sit inside the head tube. EC is external cup: the cups sit outside the head tube. IS is integrated: the bearings drop straight into the frame with no cups.
- The first number is the inside diameter of the head tube in millimetres, 44 or 55 for example.
- The second number is the steerer diameter at that point. 28.6 is 1 1/8 inch. 40 is the 1.5 inch base of a tapered fork. 30 is the crown race seat on a straight 1 1/8 fork.
Chapter 02
No code? Measure it
Calipers and two minutes.
- Look first. Check the frame maker’s spec sheet or the headset already in the bike. The size is often printed on the cups.
- Measure the head tube. With the headset out, measure the inside diameter at the top and bottom. 44mm at both ends means a straight zero-stack frame. 44mm at the top and 55mm or 56mm at the bottom means a tapered zero-stack frame.
- Measure the steerer. 28.6mm all the way down is a straight 1 1/8 fork. 28.6mm at the top widening to 1.5 inch (38.1mm) at the crown is a tapered fork.
- Check for cups. If the bearings sit straight in the frame with no cup, it is integrated (IS). If the cups sit outside the tube, it is external (EC).
55 or 56? Measure carefully, or check the frame spec. One millimetre is the difference between a press fit and a cup that won’t go in.
Chapter 03
Which HX1 headset fits?
Match the frame, then the fork.
| Your frame and fork | Headset | Code |
|---|---|---|
| Tapered fork, 44mm top / 55mm bottom | HX1 Tapered | ZS44/28.6 | ZS55/40 |
| Straight 1 1/8 fork, 44mm head tube | HX1 Straight | ZS44/28.6, straight |
| Integrated BMX frame, 1 1/8 steerer | Integrated BMX Headset | Integrated, 45x45 |
| Tapered, 56mm bottom | Needs a ZS56 lower cup | ZS44/28.6 | ZS56/40 |
| External cup (EC) or other sizes | Not covered by HX1 | — |
On our own bikes, the C3 uses the HX1 Tapered and the C100 V3 uses the HX1 Straight. But that is just two of the many frames built to those sizes.
Chapter 04
Why it’s worth upgrading
Quiet, tight, and in colour.
A worn headset knocks when you brake, creaks on landings and makes steering feel vague. HX1 headsets are CNC-machined alloy with sealed cartridge bearings and come in black, chrome, gold, purple or oil slick. Coloured headsets in these sizes are hard to find, and HX1 matches our ST1 stems, Hub 2, PX1 pedals and SC1 clamps.
Fitting needs a headset press and a crown race tool. If you don’t have them, any bike shop can fit it in minutes.
Chapter 05
Questions
Will the HX1 fit a bike that isn’t a Collective?
Yes, if your frame and fork use the same sizes. Headsets are made to a standard, not to a brand.
What is the difference between ZS55 and ZS56?
The lower head tube diameter: 55mm or 56mm. They are not interchangeable. The HX1 Tapered is ZS55.
Can I fit a straight fork in a tapered frame?
Sometimes, with a reducer lower cup, but the HX1 Straight is made for straight 44mm head tubes. Ask your bike shop.
What is a zero-stack headset?
One where the cups press into the frame, so the bearings sit inside the head tube and add almost no height.
Will the BMX headset fit my mountain bike?
Only if the frame is integrated with 45x45 bearings and a 1 1/8 steerer. Most mountain bikes use zero-stack cups: see the HX1.
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