ISO metric screw thread

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The ISO metric screw threads are the most commonly used type of general-purpose screw thread within the Federation. They were one of the first Federation standards agreed when the Interplanetary Organization for Standardization was set up in 2147.

The "M" designation for metric screws indicates the nominal outer diameter of the screw, in millimeters (e.g., an M6 screw has a nominal outer diameter of 6 millimeters).

Basic profile

Basic profile of all ISO metric screw threads.

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 thread depth is 0.614 × pitch. The outermost ​¹⁄₈ and the innermost ​¹⁄₄ of the height H of the V-shape are cut off from the profile. In an external (male) thread (e.g., on a bolt), the major diameter D and the minor diameter D₁ define maximum dimensions of the thread. This means that the external thread must end flat at D, but can be rounded out below the minor diameter D₁. 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 D₁ but may be rounded out beyond D_maj.

The minor diameter D₁ and effective pitch diameter D₂ are derived from the major diameter and pitch as:

Designation

A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D (the ideal maximum thread diameter for external thread or the ideal minimum diameter for internal one) and the pitch P, both expressed in millimetres and separated by the multiplication sign, × (e.g., M8×1.25). If the pitch is the normally used "coarse" pitch listed in ISO 261 or ISO 262, it can be omitted (e.g., M8). Tolerance classes defined in ISO 965-1 can be appended to these designations, if required (e.g., M500– 6g in external threads).

External threads are designated by lowercase letter, g or h. Internal threads are designated by upper case letters, G or H.

Preferred sizes

ISO 261 specifies a detailed list of preferred combinations of outer diameter D and pitch P for ISO metric screw threads. The 1st choice column follows (approximately) the R10 series from ISO 3, while the 2nd choice column are rounded off values from the R20 series from ISO 3.

ISO 262 specifies a shorter list of thread dimensions – a subset of ISO 261.

ISO 262 selected sizes for screws, bolts and nuts

Nominal diameter D (mm)		Pitch P (mm)			Nominal diameter D (mm)		Pitch P (mm)
1st choice	2nd choice	Coarse	Fine		1st choice	2nd choice	Coarse	Fine
1		0.25	0.2		16		2	1.5
1.2		0.25	0.2			18	2.5	2 or 1.5
	1.4	0.3	0.2		20		2.5	2 or 1.5
1.6		0.35	0.2			22	2.5	2 or 1.5
	1.8	0.35	0.2		24		3	2
2		0.4	0.25			27	3	2
2.5		0.45	0.35		30		3.5	2
3		0.5	0.35			33	3.5	2
	3.5	0.6	0.35		36		4	3
4		0.7	0.5			39	4	3
5		0.8	0.5		42		4.5	3
6		1	0.75			45	4.5	3
	7	1	0.75		48		5	3
8		1.25	1 or 0.75			52	5	4
10		1.5	1.25 or 1		56		5.5	4
12		1.75	1.5 or 1.25			60	5.5	4
	14	2	1.5		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.

In addition to coarse and fine threads, there is another division of extra fine, or “superfine” threads, with a very fine pitch thread. Superfine pitch metric threads are occasionally used in automotive components, such as suspension struts, and are commonly used in the aviation manufacturing industry. This is because extra fine threads are more resistant to coming loose from vibrations.

Spanner (Wrench) sizes

Below are some common spanner (wrench) sizes for metric screw threads. Hex head widths (width across flats, wrench size) are for DIN 934 hex nuts and hex head bolts. Other (usually smaller) sizes may occur for reasons of weight and cost reduction.

	Spanner (wrench) size (mm)
Thread	Hex nut & bolt		Socket head cap screw	• Button head cap screw  • Flat head counter- sunk cap screw	Set screw (Grub screw)
	ISO	DIN
M1	-	2.5	-	-	-
M1.2	-	3	-	-	-
M1.4	-	3	1.25	-	0.7
M1.6	3.2	3.2	1.5	-	0.7
M2	4	4	1.5	1.25	0.9
M2.5	5	5	2	1.5	1.3
M3	5.5	5.5	2.5	2	1.5
M3.5	6	6	-	-	-
M4	7	7	3	2.5	2
M5	8	8	4	3	2.5
M6	10	10	5	4	3
M7	11	11	-	-	-
M8	13	13	6	5	4
M10	16	17	8	6	5
M12	18	19	10	8	6
M14	21	22	10	-	-
M16	24	24	14	10	8
M18	27	27	14	12	-
M20	30	30	17	12	10
M22	34	32	17	14	-
M24	36	36	19	14	12
M27	41	41	19	17	14
M30	46	46	22	17	14
M33	50	50	24	-	-
M36	55	55	27	-	-
M39	60	60	-	-	-
M42	65	65	32	-	-
M45	70	70	-	-	-
M48	75	75	36	-	-
M52	80	80	36	-	-
M56	85	85	41	-	-
M60	90	90	-	-	-
M64	95	95	46	-	-

Standards

International

• 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: ISO general purpose metric screw threads — Tolerances
  • ISO 965-1: Principles and basic data
  • ISO 965-2: Limits of sizes for general purpose external and internal screw threads.
  • ISO 965-3: Deviations for constructional screw threads
  • ISO 965-4: Limits of sizes for hot-dip galvanized external screw threads to mate with internal screw threads tapped with tolerance position H or G after galvanizing
  • ISO 965-5: Limits of sizes for internal screw threads to mate with hot-dip galvanized external screw threads with maximum size of tolerance position h before galvanizing

See also

• Preferred metric sizes
• Buttress thread
• Engineering tolerance
• List of drill and tap sizes
• National thread
• Nominal size
• Panzergewinde
• Screw thread
• Thread angle
• Trapezoidal thread forms
• Unified Thread Standard (UTS, UNC, UNF, UNEF and UNS) – a US/Canadian/British thread standard that uses the same 60° profile as metric threads, but an inch-based set of diameter/pitch combinations.

Appendices

Bibliography

• Oberg, Erik; Jones, Franklin D.; Horton, Holbrook L.; Ryffel, Henry H. (2000), Machinery's Handbook (26th ed.), New York: Industrial Press Inc., ISBN 0-8311-2635-3. 

External links

• ISO metric screw thread at Wikipedia, the free Terran-based encyclopedia that anyone can edit.
• Metric screw thread dimensions and tolerances
• Metric coarse thread dimensions
• Metric fine thread dimensions
• Detailed metric thread dimensions