How do I measure my head tube?

Measure the internal diameter at the top and bottom of the head tube. Also measure whether it is threaded (external cups screw on) or smooth bore (internal/zero-stack cups press in).

What is a tapered head tube?

A tapered head tube has a 56 mm bore at the bottom and a 44 mm bore at the top, matching a tapered steerer fork that flares from 1⅛" (28.6 mm) up to 1½" (38.1 mm). This provides more stiffness for braking and steering loads. Remember the SHIS numbers (44/56) are the head-tube BORE — the cartridge bearings that fit are smaller: 41.8 mm OD (ZS44, upper) and 52 mm OD (ZS56, lower).

Why is my ZS56 bearing 52 mm and not 56 mm?

The "56" in ZS56 (or IS52) is the head-tube bore, not the bearing. The cartridge bearing that presses into a 56 mm zero-stack bore has a 52 mm outer diameter. Likewise ZS44 takes a 41.8 mm bearing and IS41 takes a 41 mm bearing. Always order by bearing OD × inner diameter × angle (e.g. 52 × 40 × 45°×45° for ZS56), not by the bore number.

What is the difference between ZS and IS headsets?

ZS (Zero Stack) has cups that press into the head tube. IS (Integrated) uses bearings that sit directly into the frame with no separate cups. EC (External Cup) has cups that sit on top of the head tube.

Can I use a straight fork in a tapered frame?

Yes, with a reducer crown race. A 44/56mm tapered frame can use a straight 1⅛" fork with a crown race adapter that fills the larger lower bearing space.

How often should a headset be serviced?

Service headset bearings every 6-12 months: disassemble, clean, re-grease, and replace bearings if rough or pitted. A well-maintained headset lasts many years.

BikeCalcs

Headset Size Calculator — Find Your Headset Type

Identify the correct headset type and bearing sizes for your frame.

Head Tube TypeMeasure the head tube bore: is it threaded (cups screw on), one diameter top & bottom (straight), or wider at the bottom (tapered)?

Steerer TypeYour fork's steerer tube: 28.6 mm all the way (straight) or flaring to 38.1 mm at the crown (tapered).

Frame TypeSelect your frame category to get the most likely headset standard for that type of bike.

How We Calculate This

Headset Standards

EC (External Cup): Traditional cups sit on top of the head tube
ZS (Zero Stack): Cups press into the head tube, flush with the surface
IS (Integrated): No cups, bearings sit directly in frame bore
Numbers (e.g., ZS44/ZS56) indicate the head-tube BORE in mm — not the bearing OD
The cartridge bearing OD is smaller than the bore: ZS44/EC34/IS41 → 41–41.8 mm bearing; ZS56/IS52 → 52 mm bearing
Tapered: upper bearing (41–41.8 mm) is smaller than lower (52 mm), to suit a steerer flaring from 1⅛" to 1½"

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify results and consult a professional bike fitter where appropriate.