Thru-axle length and thread pitch: keep every endpoint and interface explicit
Read axle diameter, source-defined shaft length, pitch, threaded extent, head contact and receiving system separately, without using a matching hub label or trial-tightening as permission.
Published 11 October 2026 · OpenBikeFit
Pitch counts axial intervals
- Fictional 10 pitch intervals span 15 mm → P = 1.5 mm; another 10 intervals span 10 mm → P = 1.0 mm.
- Count spaces, not boundary marks. Pitch is independent of total threaded extent and shaft diameter.
- Source-defined L = 170 mm from head-contact plane to tip is only this fictional datum. Head seat, receiver and permitted engagement are unresolved.
Original schematic; not to scale. Not a real part drawing, tolerance, conversion, installation or physical inspection.
On this page
- 01Identify the exact retaining part
- 02Diameter is independent of length
- 03A length needs two named endpoints
- 04Thread pitch is a spacing, not threaded extent
- 05Threaded length and engagement still need their own evidence
- 06The head contact and operation are separate
- 07Do not silently repair a conflicting declaration
- 08Keep a checkable reference hand-off
Free blank worksheet
Keep supplementary paper references
The worksheet provides general question spaces, not dedicated axle or brake fields or installation approval.
Open toolIdentify the exact retaining part
Start with the axle identity and the receiving frame or fork, including year, model and variant where applicable. Keep front and rear records separate. An axle label may combine several numbers, but a shorthand is not a complete drawing or approval statement. Preserve the original units and field definitions, plus the source revision, rather than searching for a visually similar replacement and filling gaps from it.
This guide helps read an existing label or manufacturer document. It does not direct wheel removal, thread testing or tightening. If the retaining part cannot be identified without physical work, keep that unknown and ask the appropriate mechanic or manufacturer. A part that appears to enter a receiver is not a safe measurement instrument for establishing an unidentified interface.
Diameter is independent of length
A diameter such as 12 mm describes a transverse shaft reference under the stated convention. It says nothing by itself about overall shaft length, thread pitch, threaded extent or head seat. A hub label such as 12×142 can name axle-system diameter and hub contact span; it must not be silently reinterpreted as a complete diameter-by-retaining-length specification.
The linked Robert Axle Project product makes this hub-standard versus axle-length distinction explicit for its own item. Keep that limited fact separate from its particular fitment recipe. Two descriptions sharing 12 mm may still differ in every other retaining-part field. No tool on this path supplies a universal length from diameter or hub spacing.
A length needs two named endpoints
Manufacturers can specify shaft or overall lengths with different head, shoulder, seat or tip references. Copy the exact definition, including whether the head and any separate contact piece are included. Do not assume a universal under-head measurement just because one diagram uses it. A photograph without a readable datum cannot settle where a dimension line begins.
For the fictional schematic only, L is explicitly the axial distance from a labelled head-contact plane to the shaft tip, 170 mm. W is a separate 142 mm hub contact span. The difference is 28 mm, but it is not thread engagement, receiver thickness or a replacement allowance. Different endpoints can change L while the hub span stays unchanged; record that distinction before comparing numbers.
Thread pitch is a spacing, not threaded extent
Pitch P is the axial spacing between corresponding points on adjacent thread repeats. It is distinct from the total length of the threaded section. The Robert Axle Project text guide illustrates separate 12×1.0, 12×1.5 and 12×1.75 cases. Those examples show why equal nominal diameter does not determine pitch; they do not identify your receiver or give a universal choice.
An original fictional line contains ten pitch intervals spanning 15 mm, so P is 15/10 = 1.5 mm. A second ten-interval span of 10 mm gives 1.0 mm. Count intervals rather than confusing the number of boundary marks with the spaces between them. These are abstract arithmetic examples, not instructions to turn an axle, compare printed profiles at an unverified scale or force a thread into a receiver.
Threaded length and engagement still need their own evidence
The axial extent of visible or specified thread is another quantity. Permitted engagement depends on the actual receiving design and exact part requirements; it cannot be recovered by subtracting hub span from total shaft length. Equal pitch and nominal diameter also do not establish the full thread specification, tolerance, usable contact, material or retention condition.
Keep a documented threaded extent in its own row, with the source definition. Record permitted engagement or receiver conditions only when the exact applicable documentation establishes them. Unknown is not zero, unlimited or adequate. Do not use a minimum from another axle family or infer that a longer shaft is acceptable because it has more thread.
The head contact and operation are separate
The head, washer or other contact arrangement can have a specific seat and operating design. A flat-looking photograph does not establish the specified contact geometry, and a visible lever does not identify the correct mechanism or closure procedure. Preserve the exact head-seat and retention-system description alongside the dimensional fields instead of collapsing them into one bolt-size name.
Park Tool distinguishes general retention types and their operation. That is terminology support only here, not an instruction to apply its generic procedure to a proprietary axle. No universal torque, turn count, lever direction, lubrication or tool requirement is supplied. The manufacturer source and current physical inspection remain separate from a completed reference checklist.
Do not silently repair a conflicting declaration
If the frame source, axle marking and hub description disagree, preserve the disagreement. Do not choose the most common pitch, round a length into a familiar standard or use an adapter shown in a unrelated listing as a default. Distinguish a transcription error that a source actually resolves from an assumption that merely makes the numbers look compatible.
A complete comparison can show two candidate documents with some equal fields and other unknowns. It should not rank them, select a substitute or infer approval from the count of matches. The geometry comparator models other declared dimensions and does not resolve these physical interfaces. Its numeric results must never override missing axle evidence.
Keep a checkable reference hand-off
Record axle identity, receiving assembly, position, diameter convention, length endpoints, pitch, threaded extent, head contact and all exact source conditions. Note which fields are published and which were independently observed, with method and condition where known. Keep the hub contact span and end-cap configuration beside them, but do not relabel them as axle length.
Continue to the rotor guide because an identified axle does not settle hub brake mounting, rotor diameter, thickness, lockring clearance or caliper requirements. Supplement the free blank worksheet with technical paper notes; do not add rider contact or health information to part labels. Reading this page neither installs a part nor verifies actual retention or riding readiness.
Practical questions
Frequently asked questions
Does a 12 mm axle always use 1.5 mm pitch?
No. Diameter does not determine pitch. The exact axle and receiving system need their own documented thread specification.
Is total length minus hub spacing the thread engagement?
No. Those dimensions have different endpoints and do not allocate head, receiver, contact or usable threaded regions. Actual permitted engagement needs its own source.
Can a longer axle replace a shorter one if the pitch matches?
Not on that evidence. Length datum, threaded extent, head seat, receiver, exact part permission and physical conditions remain independent.
Can I test an unknown thread by starting it in the frame?
This guide does not direct trial-fitting or tightening. Preserve the unknown and obtain the exact identification and physical assessment from the applicable source or mechanic.