Headset bearing dimensions and angles: keep diameter, height and contacts separate
Build a precise bearing reference without treating one diameter, an angle pair or a headset family as an interchangeable-part certificate.
Published 11 October 2026 · OpenBikeFit
Headset reference · not to scale
Size is a record, not a single number.
Fictional A: OD 42 mm, ID 30 mm, axial height 7 mm, inner contact 36°, outer contact 45°.
B may keep those three lengths but declare inner 45° and outer 45°. Equal lengths do not establish equal contacts or compatibility.
The schematic names contacts without reproducing a part profile, angle-datum convention or tolerance. Exact source definitions are required.
On this page
- 01Identify which bearing the statement belongs to
- 02Separate OD, ID and axial height
- 03Name both contact surfaces before reading angles
- 04Avoid a universal contact-angle assumption
- 05Keep construction and service context independent
- 06Record declaration and observation without merging them
- 07Carry the bearing record to the crown interface
Free blank worksheet
Keep the named reference questions
Use controlled supplementary paper notes. There are no dedicated headset fields or compatibility verdicts.
Open toolIdentify which bearing the statement belongs to
Start with upper or lower position and the exact headset assembly, not a bag of anonymous similar rings. Keep the manufacturer, model, part identifier, applicable frame/fork variant and source together. A bearing photographed next to a bicycle is not proof that it was installed in that bicycle. Likewise a general catalogue listing may describe an available replacement family rather than the presently fitted part. The first task is to locate the statement in the assembly and establish what it actually claims.
This guide is a reading and record-keeping aid, not a removal or service procedure. Do not dismantle a headset to follow an example. Use accessible markings, exact documentation and an existing controlled technical record. When a hidden quantity is not established, preserve that limitation for a qualified mechanic. Marking every row unknown can still produce a useful question because it identifies the specific evidence needed. Filling those rows with common dimensions would instead create an invented component identity.
Separate OD, ID and axial height
Outer diameter describes the outside circular reference, inner diameter describes the opening under its stated definition, and axial height describes a separation along the bearing axis. They are three dimensions, not alternative names for size. Preserve the source order and units rather than assuming that every shop writes triples in the same sequence. A nominal upper steerer label is not automatically the measured bearing opening, and a frame family code does not become the bearing outer diameter merely because the numbers sometimes resemble one another.
Use a fictional record A with declared OD 42 mm, ID 30 mm and axial height 7 mm. Record B keeps OD 42 mm and ID 30 mm but has height 8 mm. These abstract records agree in two dimensions and differ in the third. They demonstrate why a search for a 42 mm bearing is incomplete; they do not describe products or establish interchangeability. Even record A repeated exactly would still lack contact definitions, construction, tolerances and permission. The dimensions organise evidence rather than approve a substitute.
Name both contact surfaces before reading angles
A cartridge reference may include more than one seating angle. Identify which declared angle belongs to the inner contact and which belongs to the outer contact, and preserve the source convention used to define the angle. Do not infer the order from a two-number string without the associated drawing or explanation. The contact surface itself is different from a convenient flat edge visible in a photograph. An unclear angular reference stays unclear even if a protractor overlay supplies a visually convincing number.
For an abstract comparison, suppose A declares inner 36° and outer 45°, while B declares inner 45° and outer 45°. Their OD, ID and height can remain identical in the fictional record, yet one contact declaration differs. No physical seating is modelled here. The example does not show that a nine-degree numerical difference can be corrected with preload or a shim. It shows that the missing angle cannot be discarded just because the ring looks similar or the three length dimensions happen to match.
Avoid a universal contact-angle assumption
Current Cane Creek Forty product specifications list different contact-angle pairs within their stated assembly scope. That is enough to show why a single universal angle is not an adequate record. Preserve the exact applicable product statement rather than generalising it across every frame, year or brand. Older explanatory pages can be useful for terminology while containing historical statements about what was then current; the date and scope matter when reading such a claim.
Do not resolve contradictory sources by voting for the more common number. First determine whether the sources describe the same bearing, position, generation and angle convention. If they do not, retain them as separate records. If they do, preserve the conflict and ask for clarification. A corrected reference should identify its evidence, not merely replace an inconvenient value. This approach avoids presenting a tidy angle pair that has never been established for the actual assembly.
Keep construction and service context independent
A headset family does not establish that every bearing is an independently interchangeable cartridge. Manufacturer-specific service arrangements remain part of the exact component context. Chris King’s FAQ, for example, describes its own bearing service and replacement route; that is not a universal procedure for other headsets. A dimensions-only record cannot tell the browser which components may be separated, which parts belong to a matched system or whether a particular operation is permitted.
The complete question also retains seals, seating profiles, compression interfaces, materials and applicable product instructions where those are relevant. Do not erase them because an online search returned a ring with similar dimensions. A fit finder or product table can support a scoped documentation check without proving the installed state, wear or damage. This guide supplies no bearing pressing, extraction, cleaning, preload, torque or lubrication instruction and no cross-brand substitution recipe.
Record declaration and observation without merging them
Keep a manufacturer-declared dimension in its original form. Keep a separately observed value with its location, method, units and stated precision. A rounded label and an instrument reading need not have the same meaning. An observed outside edge may include a seal or another component rather than the intended bearing reference. Identify that surface before interpreting a discrepancy; do not silently round the observation until it looks like the catalogue number.
Repeated readings can describe the span of those observations but do not establish an allowable bearing tolerance. If a fictional accessible reference gives 42.0, 42.1 and 42.0 mm, its observed span is 0.1 mm. That span is not a specification, an uncertainty guarantee or permission to choose a 42 mm family. Keep all readings and the method. Never use a simple average to hide a conflicting identity, missing contact angle or damaged surface that requires qualified inspection.
Carry the bearing record to the crown interface
The lower bearing’s contact belongs to a separately identified crown interface; the upper bearing has its own supporting and centring parts. Continue with the crown-race guide instead of assuming that the lower race-seat diameter supplies all bearing dimensions. Then read the external-stack guide, where component height and installed axial contribution remain distinct. A bearing height is not automatically the complete upper or lower headset stack because its position within the receiving system also matters.
Use the blank worksheet’s general reference and question spaces for a clearly named supplemental bearing record. It is not a compatibility result, purchase list or installation work order. Keep unknowns visible and direct unresolved questions to the exact manufacturer or qualified mechanic. The separate cockpit calculator cannot infer missing bearing contacts from head angle or spacer height; do not invent a calculator input to absorb a dimension it intentionally does not model.
Practical questions
Frequently asked questions
Is the SHIS head-tube number always the bearing OD?
No. Preserve the nominal receiving-interface code separately from exact bearing dimensions.
Do identical OD and ID establish the same bearing?
No. Height, contacts, identity, construction and applicable permissions remain independent.
Can I assume every headset contact is 45°?
No. Identify both contacts and the exact source convention for the applicable part.
Does a 0.1 mm observed span establish a fit tolerance?
No. It only describes those fictional readings, not manufacturing limits or verified seating.