Most bikes use metric hex socket bolts for most parts, usually m4, m5, or m6.
The m number represents the diameter of the shaft.
Here is a table of bolt specs:
| Diameter | Hex key size | Pitch |
|---|---|---|
| M4 | 3mm | 0.7 |
| M5 | 4mm | 0.8 |
| M6 | 5mm | 1 |
| M8 | 6mm | 1.25 |
| M10 | 8mm | 1.5 |
Thread pitch
The distance between threads.
Steel
Most common type, strong but heavy.
Titanium
Expensive, but strong and light.
Aluminum
Cheap, light, easy to break when over-tightened. Don't use on stems or handlebars.