International measuring system based on the meter (approximately 39.37 inches) as the unit of length. Metric bicycle parts and dimensions are usually given in millimeters (mm, 1/1000 meter) or centimeters (cm, 1/100 meter). Strictly speaking, the system of measurement now in use is called "standard international units". It is different in subtle ways that are unimportant to a bicycle mechanic.
Most wrenches used on bicycles have sizes in millimeters.
The most common sizes are 8 mm, 9 mm, 10 mm, 13 mm, 14 mm, 15 mm, 17 mm, 32mm and 36 mm; also, Allen wrenches in 4 mm, 5 mm, 6 mm and 8 mm.
Less common are 7 mm, 11mm, 12mm 16 mm and 19 mm, 30 mm, 40 mm and Allen sizes 2mm, 2.5 mm, 3 mm, 7 mm and 10 mm.
Metric threads are specified by diameter followed by the thread pitch (distance between threads)
For example, the common "M5" thread used for water bottle mounts, cable anchor bolts, fender/rack eyelets, shifter mounts etc. on bicycles is more specifically described as "5.0 x 0.8", which specifies a 5 mm diameter, with threads 0.8 mm apart.
Similarly, the common "M6" thread used for brake mounting bolts, threadless stems, many seatpost bolts and so forth is actually "M6.0 x 1.0" That's 6 mm diameter, threads 1 mm apart.
Normal coarse metric threads are commonly designated with the letter "M" followed by the diameter, with the thread pitch understood. For example:
To give another example, the common size for chainring stack bolts is 10 x 1.0 mm. This is a fine thread, not a standard coarse thread. It would be incorrect to refer to this simply as "M10" since the standard pitch for M10 is 1.5 mm. Metric threading is used for small parts on most bicycles. Larger diameter threads, however, such as headsets , bottom brackets and freewheels , are usually inch-based threads, most often 24 TPI .
- M2 = 2.0 x 0.40 mm
- M2.5 = 2.5 x 0.45 mm
- M3 = 3.0 x 0.50 mm
- M3.5 = 3.5 x 0.60 mm
- M4 = 4.0 x 0.70 mm
- M5 = 5.0 x 0.80 mm
- M6 = 6.0 x 1.00 mm
- M8 = 8.0 x 1.25 mm
- M10 = 10.0 x 1.5 mm
- M12 = 12.0 x 1.75 mm
- M14 = 14.0 x 2.00 mm
I have developed a color-coding system for quickly determining the size of a Metric wrench. This is explained in my article on Color Coding Your Tools.