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Standard/common thread dimensions used on bicycles

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
  2. “Thru-axle” thread pitch standards

1. General

ISO dimensionCommon useAllenHexagonal
M3 x 0.5 mmrear dropout axle limiter bolts – usually used on vintage bikes2.5 mm 
M4 x 0.7 mmrarely ever for water bottle holders3 mm7 mm
M5 x 0.8 mmwater bottle holders, rack and mudguard mounts, stem bolts, saddle bolts, seat post bolts4 mm8 mm, 9 mm
M6 x 1 mmbrake calliper retaining bolt, anchor bolt for brake and shifter cables, derailleur mounting bolts, brake and shifter lever mounting, stem bolts, saddle bolts, seat post bolts5 mm10 mm, 9 mm
M8 x 1.25 mmquill stem bolt, seat post bolts6 mm13 mm
M10 x 1 mm (fine thread)rear derailleur mounting bolt5 mm 

– T.O.C. –

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.

– T.O.C. –

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