Bottom-bracket shell standards: BSA, T47 and press-fit are receiving interfaces
Read the frame shell, cup or bearing interface and axial width separately from the crank spindle. Keep nominal family labels, physical observations and exact assembly permission distinct.
Published 11 October 2026 · OpenBikeFit
Name each axial pair of reference planes
- Fictional shell planes z = 0 and 68 mm → shell width 68 mm.
- Independently declared outer planes z = −9 and 78 mm → outer span 87 mm. All four stations share the same axial scale.
- 87 − 68 = 19 mm does not locate bearing centres, allocate spacers or establish engagement. No universal cup projection is implied.
Original schematic; not to scale. Not a real part drawing, tolerance, conversion, installation or physical inspection.
On this page
Free blank worksheet
Keep supplementary paper references
General reference and question spaces, not dedicated bottom-bracket or chainring fields or assembly approval.
Open toolBegin with the receiving frame
The bottom-bracket shell is the frame region that receives the specified bottom-bracket arrangement. Start with the frame manufacturer, model, year and variant, then preserve the exact term used in its documentation. A crank label, visible external cup or bicycle category is not a substitute for that receiving identity. This guide is a reference-reading workflow, not a procedure for removing a crank, probing threads, pressing bearings or preparing a shell.
Keep three records: the frame shell, the complete bottom-bracket assembly and the crank with its spindle variant. A statement about one record must not silently fill a missing field in another. Write unknown when a source cannot be matched to the actual part. A sales photograph may conceal the bearing arrangement, thread or adapters, so visual resemblance alone cannot resolve the interface.
Thread labels name their own geometry
Park Tool identifies the common BSA or English cup thread as 1.37 inches by 24 threads per inch and distinguishes that nominal thread reference from the smaller bare internal shell bore. The nominal major diameter is approximately 34.8 mm; it is not a 24 mm crank spindle. The number 24 in the thread designation counts threads per inch, whereas 24 mm in a spindle declaration is a transverse length. Keep units beside every number.
T47 names another threaded interface: M47×1.0 in the same terminology reference. Its nominal thread major diameter is 47 mm and its pitch is 1.0 mm. Neither is a crank axle diameter. A bare bore observation near 46 mm cannot by itself establish the presence, condition or exact identity of a T47 thread rather than another receiving arrangement. Do not test that inference by starting an unidentified cup in the frame.
Press-fit labels do not supply a crank match
A press-fit family refers to specified receiving dimensions and construction, not a universal spindle. Park Tool separates shell references such as the 41 mm family from several crank-spindle arrangements. Wheels Manufacturing also lists shell, crank and installed-width columns independently. Read the exact assembly that the source names rather than promoting one familiar bore number into a complete compatibility statement.
Likewise, BB30 can refer to the receiving shell family while a named bottom-bracket product supports another spindle interface. Wheels Manufacturing describes its BB30OUT products in their own receiving-shell and crank variants. That product-specific distinction is not permission to install any 24 mm or stepped spindle in every BB30 frame. A family name can begin a document search, but cannot finish it.
Shell width has two frame endpoints
Give an axial width its two named reference planes. Shell width, installed outside span over seals, cup projection and bearing-centre separation are not interchangeable. A chart may include dust seals in an overall dimension; that is a different convention from a drawing through bearing centres. Retain the original heading and configuration next to the number, including which parts the endpoints include.
The original schematic uses fictional shell planes at axial coordinates z = 0 and z = 68 mm, giving shell width 68 mm. Independently declared outer reference planes are z = −9 and z = 78 mm, giving outer span 87 mm. This common-axis subtraction names what was declared; the two outer offsets are not a universal cup projection or a prescription for a real assembly.
Do not infer bearing stations from overall span
The fictional 87 mm outside span does not locate either bearing centre. A seal face, cup edge, shoulder and bearing load reference are different surfaces or stations. Subtracting the 68 mm shell width gives 19 mm, but does not allocate it to spacers, engagement, bearing separation or permitted projection. Missing locations remain unknown even when all the subtraction is numerically correct.
Wheels Manufacturing explicitly identifies its over-seal dimension as including dust seals. This is useful evidence for keeping endpoints explicit, not a source of a universal bearing-width formula. The next guide separates spindle length and named bearing stations. A longer overall label alone cannot tell you which crank variant, axial stops or retention arrangement belongs in the receiver.
Preserve product-specific conditions
An exact source may distinguish shell widths, crank families, frame variants and associated bottom-bracket products. Preserve those conditions as a scoped statement. Do not combine a frame row from one chart, a cup row from another and an axle photograph into a newly approved assembly. A matching nominal number can coexist with different contact surfaces, axial arrangements or manufacturer exclusions.
This path supplies no universal fit tolerance, spacer thickness, preload, lubricant, torque or pressing instruction. It does not compare frame quality, bearing life or power transfer. The exact manufacturer and appropriate mechanic still need to resolve physical condition, materials, component identities and all installation requirements; a completed reading checklist is not that physical inspection.
Keep a checkable shell record
A useful supplementary note retains frame identity and source revision, shell family, thread or receiving-bore description, source-defined axial width and complete bottom-bracket product identity. Separate published nominal values from observations, with the observation method and endpoints when available. If a source has an exception for your variant, keep the exception visible rather than copying only the attractive matching number.
Continue to the spindle guide, then the chainring guide, before returning to the existing whole-crank compatibility questions. Use general paper reference spaces beside the free blank worksheet; it has no dedicated shell inventory. The crank-length and gearing tools remain separate conditional calculations. Neither a calculated radius change nor a favourable ratio establishes the shell, bearing or chainring interface.
Practical questions
Frequently asked questions
Does BSA 24 mean a 24 mm crank axle?
No. The 24 in the BSA thread designation counts threads per inch; a 24 mm spindle diameter is a different quantity with different units and endpoints.
Is a 47 mm T47 label a spindle diameter?
No. It names the nominal threaded receiving interface. Identify the exact bottom bracket and crank variant separately.
Can outside cup or seal width replace shell width?
No. Preserve the source-defined endpoints and included parts. Outside span does not automatically locate bearing centres or define required spindle length.
Do matching shell and crank labels approve installation?
No. Exact assembly applicability, axial references, physical condition and all manufacturer requirements remain independent. This guide does not perform or approve installation.