This abbreviated format omits the pitch definition which means that the bolt has a coarse thread. Whenever the pitch dimension is omitted from a metric bolt specification then the bolt is always coarse threaded. This example (M12-50) has a diameter of 12mm and a length of 50mm. Metric Thread Pitch Tables of the derived dimensions for screw diameters and pitches defined in ISO 261 are given in ISO 724. a b c ISO 965-1:2013 ISO general purpose metric screw threads — Tolerances — Part 1: Principles and basic data. International Organization for Standardization. 15 September 2013.

The ISO metric screw thread is the most commonly used type of general-purpose screw thread worldwide. [1] They were one of the first international standards agreed when the International Organization for Standardization (ISO) was set up in 1947. [ citation needed]

DEFINITIONS

Metric bolts and fasteners are manufactured in conformance with standards set by the International Organisation for Standardisation (ISO) and the German Institute for Standardisation (DIN). Here’s a list of the most common DIN and ISO numbers and what they describe. Descriptions of DIN and ISO fastener standards. Standard No. Oberg, Erik; Jones, Franklin D.; Horton, Holbrook L.; Ryffel, Henry H. (2000), Machinery's Handbook (26thed.), New York: Industrial Press Inc., ISBN 0-8311-2635-3.

Basic profile [ edit ] Basic profile of all ISO metric screw threads, where the male part has the external thread P = 2 tan ⁡ θ ⋅ H = 2 3 ⋅ H ≈ 1.154701 ⋅ H {\displaystyle P=2\tan \theta \cdot H={\frac {2}{\sqrt {3}}}\cdot H\approx 1.154701\cdot H} In engineering circles a Spanner is often called a wrench and is generally used to grip a nut or bolt while applying torque or put more simply to help you fasten and loosen nuts and bolts. BS3643 defines the limits and tolerances for ISO screw thread gauges. The standard is divided into two parts. Part 1 is based on ISO 965/1 and ISO 965/3 providing the principles and basic data for specifications of ISO metric screw threads.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. In practice this means that one can measure the diameter over the threads of a bolt to find the nominal diameter D maj, and the inner diameter of a nut is D min. Here’s a short list of standard spanner and allen key sizes that apply to the most commonly used metric nuts and bolts. Standard metric spanner and allen key sizes. Size D min = D maj − 2 ⋅ 5 8 ⋅ H = D maj − 5 3 8 ⋅ P ≈ D maj − 1.082532 ⋅ P D p = D maj − 2 ⋅ 3 8 ⋅ H = D maj − 3 3 8 ⋅ P ≈ D maj − 0.649519 ⋅ P {\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\\[3pt]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}}}