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Standard thread dimensions used with bicycles

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 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 watter bottle holders3 mm7 mm
M5 x 0.8 mmwatter bottle holders, rack and mudguard mounts, stem bolts, saddle bolts, seat post bolts4 mm8 mm, 9 mm
M6 x 1 mmbrake caliper 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 

Look here for other bicycle-related standards.

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