The ISO metric screw threads are the world-wide most commonly used type of general-purpose screw thread. They were one of the first international standards agreed when the International Organization for Standardization was set up in 1947.

Basic profile

The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1. Each thread is characterized by its major diameter D and its pitch P. ISO metric threads consist of a symmetric V-shaped thread. In the plane of the thread axis, the flanks of the V have an angle of 60° to each other. The outermost 1/8 and the innermost 1/4 of the height H of the V-shape are cut off from the profile.

The height H and the pitch P are related through the 60° angle via

$H\; =\; sin(60^circ)cdot\; P\; =\; frac\{sqrt\; 3\}2cdot\; P\; =\; 0.866025404\; times\; P$

In an external (male) thread (e.g., on a bolt), the major diameter D_maj and the minor diameter D_min define maximum dimensions of the thread. This means that the external thread must end flat at D_maj, but can be rounded out below the minor diameter D_min. Conversely, in an internal (female) thread (e.g., in a nut), the major and minor diameters are minimum dimensions, therefore the thread profile must end flat at D_min but may be rounded out beyond D_maj.

The minor diameter D_min and effective pitch diameter D_p are derived from the major diameter and pitch as

$D\_mathrm\{min\}\; =\; D\_mathrm\{maj\}\; -\; 2cdotfrac58cdot\; H\; =\; D\_mathrm\{maj\}\; -\; frac\{\; 5\; \{sqrt\; 3\}\}\{8\}cdot\; P\; =\; D\_mathrm\{maj\}\; -\; 1.082532\; times\; P$

$D\_mathrm\{p\}\; =\; D\_mathrm\{maj\}\; -\; 2cdotfrac38cdot\; H\; =\; D\_mathrm\{maj\}\; -\; frac\{\; 3\; \{sqrt\; 3\}\}\{8\}cdot\; P\; =\; D\_mathrm\{maj\}\; -\; 0.649519\; times\; P$

Designation

A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D (D_maj in the diagram above) and the pitch P, both expressed in millimetres and separated by the multiplication sign “×”.

Example: M8×1.25

If the pitch is the normally used "coarse" pitch listed in ISO 261 or ISO 262, it is omitted.

Example: M8

Tolerance classes defined in ISO 965-1 can be appended to these designations, if required.

Example: M10 – 6g in external threads.

Preferred sizes

International standard ISO 261 defines a detailed list of preferred combinations of outer diameter D and pitch P for ISO metric screw threads. ISO 262 defines the following shorter list of thread dimensions – a subset of the combinations listed in ISO 261 – which are selected sizes for screws, bolts and nuts:

Nominal diameter D (mm)		Pitch P (mm)
1st choice	2nd choice	coarse	fine
1		0.25
1.2		0.25
	1.4	0.3
1.6		0.35
	1.8	0.35
2		0.4
2.5		0.45
3		0.5
	3.5	0.6
4		0.7
5		0.8
6		1
	7	1
8		1.25	1
10		1.5	1.25 or 1
12		1.75	1.5 or 1.25
	14	2	1.5
16		2	1.5
	18	2.5	2 or 1.5
20		2.5	2 or 1.5
	22	2.5	2 or 1.5
24		3	2
	27	3	2
30		3.5	2
	33	3.5	2
36		4	3
	39	4	3
42		4.5	3
	45	4.5	3
48		5	3
	52	5	4
56		5.5	4
	60	5.5	4
64		6	4

The "coarse" pitch is the commonly-used default pitch for a given diameter. In addition, one or two smaller "fine" pitches are defined, for use in applications where the height of the normal "coarse" pitch would be unsuitable (e.g., threads in thin-walled pipes). The terms "coarse" and "fine" have in this context no relation to the manufacturing quality of the thread.

References

International standards:

• ISO 68-1: ISO general purpose screw threads — Basic profile — Metric screw threads.
• ISO 261: ISO general purpose metric screw threads — General plan.
• ISO 262: ISO general purpose metric screw threads — Selected sizes for screws, bolts and nuts.
• ISO 965-1: ISO general purpose metric screw threads — Tolerances — Part 1: Principles and basic data.

Equivalent national standards:

• BS 3643: ISO metric screw threads
• ANSI/ASME B1.13M: Metric Screw Threads: M Profile

See also

• Unified Thread Standard – a US/Canadian thread standard that uses the same 60° profile, but an inch-based set of diameter/pitch combinations.
• BSW - a (now obsolete) British thread standard (Whitworth) with 55° profile.
• British Association screw threads
• National pipe thread
• British standard pipe thread

External links

• Spanner Jaw Sizes Additional background information and spanner jaw size table.