Bicycle Component Interface Standards
Every machined bike part bolts to something that already exists. These are the dimensions that decide whether it fits, collected in one page so you are not reading six forum threads to find a bore size.
Last checked September 27, 2026. Free to use and to quote with attribution, CC BY 4.0.
Download the tables: CSV or JSON. Same data, same sources, one file.
Contents
Thirteen tables. Jump to what you need.
Steerer tubes and headsets
Sets the stem clamp bore. The steerer diameter is the fork maker's dimension, so the stem is always the part that adapts.
| Standard | Steerer outside diameter | Notes |
|---|---|---|
| 1 in threadless | 25.4 mm | Older road and touring frames |
| 1 1/8 in | 28.6 mm | The default for stems. Most common by a wide margin |
| 1 1/4 in | 31.8 mm | Uncommon, some road and gravel frames |
| 1 1/2 in | 38.1 mm | Lower bearing on tapered steerers, rarely a stem clamp |
| Tapered 1 1/8 to 1 1/2 | 28.6 top, 38.1 bottom | Stem still clamps the 28.6 mm section |
| Tapered 1 1/8 to 1 1/4 | 28.6 top, 33.0 bottom | Less common tapered variant |
Checked against: Sheldon Brown, headset crib sheet, Wikipedia, Headset (bicycle part).
Handlebar clamp and grip diameters
Two different diameters live on one bar: the clamp area at the stem and the control area at the ends.
| Diameter | Where it is used | Notes |
|---|---|---|
| 25.4 mm | Stem clamp, older MTB and some flat bars | Still current on some touring and city bars |
| 26.0 mm | Stem clamp, older road bars | Largely superseded by 31.8 mm |
| 31.8 mm | Stem clamp, current road, gravel and MTB | The default clamp bore to design for |
| 35.0 mm | Stem clamp, newer MTB | Stiffer, allows thinner wall for the same strength |
| 22.2 mm | Grip and control area, flat bars | Sets shifter, brake lever and grip bores |
| 23.8 mm | Tops and drops, road bars | Sets lever clamp and bar tape area |
Checked against: Sheldon Brown, handlebar crib sheet, Wikipedia, Bicycle handlebar.
Seatposts, collars and saddle rails
Post diameter is set by the frame. The collar is sized to the frame tube, not to the post, so the two numbers are different.
| Item | Common sizes | Notes |
|---|---|---|
| Seatpost diameter | 27.2, 30.9, 31.6 mm | The three that cover most modern frames. 27.2 mm for comfort, 30.9 and 31.6 mm for dropper posts |
| Other post diameters | 25.4, 26.8, 28.6, 31.8, 34.9 mm | Older and speciality frames |
| Seat collar bore | 28.6, 30.0, 31.8, 34.9, 36.4 mm | Sized to the frame seat tube outside diameter, roughly post plus 2.8 to 3.3 mm |
| Saddle rails, round | 7 mm | Steel, titanium and alloy rails. The default clamp size |
| Saddle rails, oval | 7 x 9 mm | Carbon rails. Needs a clamp designed for the oval section |
Checked against: Sheldon Brown, seatpost sizes, Wikipedia, Seatpost.
Bottom bracket shells
Threaded shells have a thread spec, press fit shells have a bore diameter. Mixing the two up is the most common sourcing error in this area.
| Standard | Shell bore or thread | Shell widths |
|---|---|---|
| BSA / ISO threaded | 1.37 in x 24 tpi | 68 mm road, 73 mm MTB. Drive side cup is left hand thread |
| Italian threaded | 36 mm x 24 tpi | 70 mm. Both cups right hand thread |
| T47 | 47 mm x 1.0 mm thread | 68, 77, 85.5 and 86.5 mm |
| BB86 / BB92 | 41 mm bore, press fit | 86.5 mm road, 91.5 mm MTB |
| BB30 | 42 mm bore, press fit | 68 mm road, 73 mm MTB |
| PF30 | 46 mm bore, press fit | 68 mm road, 73 mm MTB |
| BB386EVO | 46 mm bore, press fit | 86.5 mm |
Checked against: Park Tool, bottom bracket standards and terminology, Sheldon Brown, bottom bracket sizes, Wikipedia, Bottom bracket.
Crank spindle interfaces
The spindle interface decides which bottom bracket the crank can run. These are defined geometries, so there is nothing here to reinterpret on a drawing.
| Interface | Spindle | Notes |
|---|---|---|
| Square taper | 2 degrees per side | JIS and ISO variants differ in taper length, not angle |
| Octalink V1 | 22 mm, 8 splines | Shimano. V1 and V2 differ in spline depth and are not interchangeable |
| ISIS Drive | 22 mm, 10 splines | Open standard, several makers |
| Hollowtech II | 24 mm | Shimano external bearing. Widely copied |
| SRAM GXP | 24 mm drive, 22 mm non-drive | Stepped spindle, non-drive bearing takes the smaller diameter |
| SRAM DUB | 28.99 mm | Single spindle diameter across shell standards |
Checked against: Sheldon Brown, bottom bracket tapers and splines, Park Tool, bottom bracket standards and terminology, Wikipedia, Bottom bracket.
Chainring bolt circles and mounts
Dimension a bolt circle as a pattern with a positional tolerance from the center bore, never as separate hole positions. Chained dimensions stack their tolerances.
| Pattern | BCD | Notes |
|---|---|---|
| Road 5-bolt standard | 130 mm | Older Shimano and compatible |
| Road 5-bolt compact | 110 mm | The long running compact road pattern |
| Campagnolo 5-bolt | 135 mm | Campagnolo specific |
| Road 4-bolt | 110 mm asymmetric | Shimano 11 and 12 speed. Bolt spacing is not equal |
| MTB 4-bolt outer | 104 mm | The long standing MTB pattern |
| MTB 4-bolt inner | 64 mm | Granny ring position on triples |
| Shimano MTB 4-bolt | 96 mm | Symmetric and asymmetric versions exist |
| SRAM 4-bolt | 94 mm | SRAM specific MTB pattern |
| Direct mount | No bolt circle | SRAM 3-bolt and Shimano direct mount. Proprietary interface geometry, take it from the crank maker |
Checked against: Sheldon Brown, chainring bolt circle crib sheet, Wikipedia, Crankset.
Chainline
The distance from the frame centerline to the center of the chainring teeth. This is the dimension that puts the chain where it needs to be, and half a millimeter of error is a shifting complaint.
| Setup | Typical chainline | Notes |
|---|---|---|
| Road double | About 43.5 mm | Measured to the midpoint between the two rings |
| MTB 1x, non-Boost | About 49 mm | Matches 142 mm rear spacing |
| MTB 1x, Boost | About 52 mm | Matches 148 mm rear spacing. Boost specific rings exist for this reason |
| MTB 1x, Super Boost | About 56 mm | Matches 157 mm rear spacing |
Chainline figures vary a little by drivetrain maker. Confirm against the groupset specification for the build.
Checked against: Sheldon Brown, chainline, Wikipedia, Chainline.
Hub spacing and axle standards
Over locknut dimension and axle diameter. This pair decides the hub shell length and the end cap design.
| Position | Standard | Notes |
|---|---|---|
| Front, quick release | 100 mm QR | Rim brake and older disc |
| Front, road disc | 100 x 12 mm | Current road and gravel disc |
| Front, MTB | 100 x 15 mm | Pre-Boost MTB thru axle |
| Front, Boost | 110 x 15 mm | Current MTB standard |
| Rear, road rim brake | 130 mm QR | Legacy road |
| Rear, MTB quick release | 135 mm QR | Legacy MTB |
| Rear, road disc | 142 x 12 mm | Current road, gravel and older MTB |
| Rear, Boost | 148 x 12 mm | Current MTB standard |
| Rear, Super Boost | 157 x 12 mm | Some enduro and plus tyre frames |
Checked against: Sheldon Brown, hub spacing crib sheet, Wikipedia, Thru-axle, Wikipedia, Bicycle wheel.
Freehub body standards
Defined spline geometries. The cassette has to slide on and off cleanly, so this is one place to take the dimensions from the standard rather than from a measured sample.
| Standard | Used by | Notes |
|---|---|---|
| Shimano HG | Shimano and compatible | The long standing spline. Road 11 speed uses a longer body than MTB |
| SRAM XD | SRAM MTB | Threaded cassette interface, allows a 10 tooth smallest cog |
| SRAM XDR | SRAM road | XD geometry, body 1.85 mm longer |
| Shimano Micro Spline | Shimano 12 speed MTB | Also allows a 10 tooth cog |
| Campagnolo N3W | Campagnolo | Adapter allows older Campagnolo cassettes |
Checked against: Sheldon Brown, cassettes and freehubs, Wikipedia, Freehub.
Disc brake mounts and rotors
Two separate interfaces: how the rotor attaches to the hub, and how the caliper attaches to the frame or fork.
| Interface | Specification | Notes |
|---|---|---|
| Rotor, 6-bolt ISO | 44 mm BCD, six M5 | The common rotor mount. Bolts usually torque to 4 to 6 Nm |
| Rotor, Center Lock | Splined, lockring | Shimano interface, also used by others |
| Caliper, IS | Two M6, perpendicular to the mount | Older frames and forks, needs an adapter for most calipers |
| Caliper, Post Mount | Two M6, 74 mm spacing | The common MTB mount. Adapters set the rotor size |
| Caliper, Flat Mount | Two M5 from below | Road and gravel disc |
| Rotor diameters | 140, 160, 180, 203 mm | The adapter converts between the frame mount size and the rotor size |
Checked against: Wikipedia, Bicycle brake, Wikipedia, Disc brake.
Threads and fasteners
The threads that appear most often on machined bike parts. Thread engagement in aluminum wants roughly 1.5 to 2 times the bolt diameter.
| Location | Thread | Notes |
|---|---|---|
| Pedal to crank | 9/16 in x 20 tpi | Right hand thread drive side, left hand thread non-drive side. One-piece cranks use 1/2 in x 20 tpi |
| Derailleur to hanger | M10 x 1.0 | Standard on modern hangers including UDH |
| Bottle cage | Two M5 at 64 mm | The universal accessory mount pattern |
| Disc rotor bolts | M5 | Six per rotor on the ISO pattern |
| Post mount caliper | M6 | Two bolts at 74 mm |
| Stem and faceplate | M5 | M6 on some single bolt steerer clamps |
Checked against: Wikipedia, Bicycle pedal, Wikipedia, Bottle cage, Wikipedia, Derailleur hanger.
Common cartridge bearings
Sizes that turn up in hubs, pulleys, pivots and headsets. Always state the bearing part number on the drawing and work to the bearing maker's recommended fit rather than picking a number.
| Bearing | Bore x OD x width | Typically used in |
|---|---|---|
| 688 | 8 x 16 x 5 mm | Derailleur pulleys, small pivots |
| 6901 | 12 x 24 x 6 mm | Pivots, small hubs |
| 6001 | 12 x 28 x 8 mm | Pivots, suspension links |
| 6802 | 15 x 24 x 5 mm | Light hub shells, pulleys |
| 6902 | 15 x 28 x 7 mm | The most common hub bearing |
| 6903 | 17 x 30 x 7 mm | Rear hubs, freehub bodies |
| 6805 | 25 x 37 x 7 mm | Bottom brackets, large bores |
| 6806 | 30 x 42 x 7 mm | BB30 and large hub shells |
Checked against: SKF product data, deep groove ball bearing 6902, SKF product data, deep groove ball bearing 6805, ISO 15, boundary dimensions for radial rolling bearings.
Spokes and spoke holes
Sets the spoke hole in a hub flange. Every hole is a stress concentration and a burr trap, so chamfer both sides.
| Item | Size | Notes |
|---|---|---|
| Spoke, 14 gauge | 2.0 mm | The common straight gauge spoke |
| Spoke, 15 gauge | 1.8 mm | Butted section of a double butted spoke |
| Spoke, 13 gauge | 2.34 mm | Heavy duty, cargo and tandem |
| Spoke hole in flange | About 2.5 mm | For a 2.0 mm spoke. Angled or counterbored so the elbow seats without bending |
| Straight pull | No elbow | Different flange geometry entirely, hole is a slot or a stepped bore |
Checked against: Sheldon Brown, glossary Sp to Ss (spoke gauge), Wikipedia, Spoke.
Changes to this page
Every correction and addition, dated. A reference you cannot see the history of is a reference you cannot trust.
- Sources listed under every interface table. Tables published as CSV and JSON under CC BY 4.0.
How to use this without getting burned
A reference table is a starting point. Three rules keep it from costing you a batch of parts.
- Verify before you cutCheck the dimension against the component maker's own drawing. Makers deviate from standards, and a standard that says one thing does not stop a crank from doing another.
- Measure the real partIf you have the mating component in your hand, measure it. A caliper beats every table on this page.
- Say the standard by name on the drawing"Shimano Micro Spline" on a drawing is unambiguous. A set of measured dimensions taken from a worn sample is not.
Found a mistake?
Tell us and we will fix it, not soften it. If you machine bike components for a living and something here does not match what you see on the shop floor, you know something we do not. Email .
Next: the BCD calculator, alloy and finish data, or the RFQ checklist for what a shop needs before it can quote.