ISO standard defines the dimensions of thread – by diameter and pitch. This post provides a list of standards most commonly used on bicycles, with an explanation of their most common use. Tools most commonly used for (un)screwing the bolts and nuts of the given standards will also be noted (also by ISO standard), for both Allen (hex) and hexagonal (“standard”) wrenches.
Before mounting, always prepare the threads with a proper mounting paste.
Look here for other bicycle-related standards.
Table Of Contents (T.O.C.):
1. General
| ISO dimension | Common use | Allen | Hexagonal |
| M3 x 0.5 mm | rear dropout axle limiter bolts – usually used on vintage bikes | 2.5 mm | |
| M4 x 0.7 mm | rarely ever for water bottle holders | 3 mm | 7 mm |
| M5 x 0.8 mm | water bottle holders, rack and mudguard mounts, stem bolts, saddle bolts, seat post bolts | 4 mm | 8 mm, 9 mm |
| M6 x 1 mm | brake calliper retaining bolt, anchor bolt for brake and shifter cables, derailleur mounting bolts, brake and shifter lever mounting, stem bolts, saddle bolts, seat post bolts | 5 mm | 10 mm, 9 mm |
| M8 x 1.25 mm | quill stem bolt, seat post bolts | 6 mm | 13 mm |
| M10 x 1 mm (fine thread) | rear derailleur mounting bolt | 5 mm |
2. “Thru-axle” thread pitch standards
Diameter and thread pitch (ISO, in mm) of threads for screwing “thru-axles” into forks and frames:
- 12 mm x 1 mm
- 12 mm x 1.5 mm
- 12 mm x 1.75 mm
- 15 mm x 1 mm
- 15 mm x 1.5 mm
- 20 mm x 1 mm
- 20 mm x 1.5 mm
- 20 mm x 2.0 mm
Note: these are not expressed in threads-per-inch (TPI), so, for example, 2 mm is coarser than 1 mm thread.
It is important to know this to avoid mistakes when buying new “thru-axles.” For example, the 12 x 1.5 thread looks very similar to the 12 x 1.75.