Do I need a 68mm or 73mm bottom bracket, and how do I measure it?
By Crankworthy · Published · Updated
It depends on your frame's shell width. Threaded (BSA) shells are 68mm — common on road and older mountain bikes — or 73mm on most mountain bikes. Many external mountain bottom brackets fit both using spacers, so check that yours lists the width you have.
The numbers
| Standard threaded shell | 68 mm |
|---|---|
| Oversize threaded shell | 73 mm |
| Difference | 5 mm (2.5 per side) |
| Thread, both widths | 1.37″ × 24 tpi |
Where on your bikeBottom bracket shell, between the cranks
Why
The bottom bracket shell is the tube at the bottom of the frame that the bottom bracket threads into. Threaded BSA shells come 68mm or 73mm wide [1]. The width decides how far apart the bearing cups sit, which in turn sets the chainline.
Both widths use the same 1.37″ × 24 tpi thread [2], so the thread will not tell you which you have — only a measurement will.
External-bearing bottom brackets built for mountain bikes usually come with spacers so one part covers both 68mm and 73mm shells. The spacer layout differs between the two, so follow the instructions for your width.
Some cranks are sold in versions for particular shell widths, or with a spindle length to suit one. Check the crank's listing covers your shell as well as the bottom bracket's.
How to measure it
- Measure the shell
- With the bottom bracket cups removed, measure straight across the shell with a caliper. It will read very close to 68 or 73mm.
- Square-taper spindles
- A square-taper cartridge is sold by spindle length as well as shell width. The length is usually printed on the cartridge body — replace like for like unless the crank maker says otherwise.
- Check the frame spec
- The manufacturer's spec sheet often lists it, sometimes as "BSA 73" or "English 68".
Skip
- Don’t buy: a bottom bracket whose listing does not name your width
- Instead: one that lists your shell, 68 or 73mm, outright.
Shell widths on real bikes
From our research, the shell width each maker lists for these bikes.
- 2024 Electra Townie 7D Step Thru — 68 mm shell maker’s spec page
- 2024 Mongoose Title 24 — 68 mm shell maker’s spec page
- 2024 Mongoose Tyax 29 — 73 mm shell maker’s spec page
- 2024 Santa Cruz Hightower C R — 73 mm shell maker’s spec page
- 2024 Santa Cruz Hightower C S — 73 mm shell maker’s spec page
- 2024 Santa Cruz Hightower CC X0 AXS — 73 mm shell maker’s spec page
- 2024 Santa Cruz Hightower CC XX AXS RSV — 73 mm shell maker’s spec page
- 2024 Santa Cruz Tallboy C R — 73 mm shell maker’s spec page
Threaded (BSA) bottom brackets
From our catalog. Check each listing covers your shell width and your crank's spindle.
- Shimano BB-MT501 (BSA)Threaded BSA 68/73mm external bearing BB, 24mm Hollowtech II, alloy cups
- SRAM DUB BSA Bottom BracketThreaded BSA 68/73mm BB for DUB 28.99mm spindle cranks$37.82Amazon
- Wheels Manufacturing BSA Bottom Bracket, Shimano MTB (24mm)Threaded BSA 68/73mm BB, ABEC-3 bearings, for 24mm Shimano Hollowtech II cranks
Prices were last checked when the catalog was built and can change at any time. Retailer links are affiliate links; we may earn a commission at no extra cost to you.