The ISO metric screw threads are the world-wide most commonly used type of general-purpose screw thread.^[1] They were one of the first international standards agreed when the International Organization for Standardization was set up in 1947.^{[citation needed]}

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

The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1.^[2] Each thread is characterized by its major diameter, D (D_maj in the diagram), 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.

The relationship between the height H and the pitch P is found using the following equation where ${\displaystyle \theta }$ is half the included angle of the thread, in this case 30 degrees:^[3]

${\displaystyle H={\frac {1}{2\tan \theta }}\cdot P={\frac {\sqrt {3}}{2}}\cdot P\approx 0.866\cdot P}$

or

${\displaystyle P=2\tan \theta \cdot H={\frac {2}{\sqrt {3}}}\cdot H\approx 1.155\cdot H}$

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

${\displaystyle {\begin{aligned}D_{\text{min}}&=D_{\text{maj}}-2\cdot {\frac {5}{8}}\cdot H=D_{\text{maj}}-{\frac {5{\sqrt {3}}}{8}}\cdot P\approx D_{\text{maj}}-1.082532\cdot P\\D_{\text{p}}&=D_{\text{maj}}-2\cdot {\frac {3}{8}}\cdot H=D_{\text{maj}}-{\frac {3{\sqrt {3}}}{8}}\cdot P\approx D_{\text{maj}}-0.649519\cdot P\end{aligned}}}$

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, × (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). If, for instance, only M20 is given then it is coarse pitch thread.

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.^[4]

ISO 262 specifies a shorter list of thread dimensions – a subset of ISO 261.^[5]

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.^[6]

Wrench sizes

Below are some common 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.

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^[7]
  • 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

National

• BS 3643: ISO metric screw threads
• ANSI/ASME B1.13M: Metric Screw Threads: M Profile
• DIN 13-1

See also