ISO standard defines dimensions of thread – by diameter and pitch. This post provides a list of standards most commonly used on bicycles, with explanation of 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.
|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 watter bottle holders||3 mm||7 mm|
|M5 x 0.8 mm||watter bottle holders, rack and mudguard mounts, stem bolts, saddle bolts, seat post bolts||4 mm||8 mm, 9 mm|
|M6 x 1 mm||brake caliper 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|
Look here for other bicycle-related standards.