Thread Notations & Profile Terms

Cut a thread along its axis and the outline is a row of teeth. These are the words used for it:

• Crest: the top of a tooth, the surface farthest from the axis on an external thread.
• Root: the bottom of the groove between two teeth.
• Flank: the sloping side that joins a crest to a root. The two flanks of a groove meet at the thread angle.
• Pitch: the distance from one tooth to the next, measured along the axis. Inch threads give it as threads per inch (TPI) instead, which is its reciprocal.
• Lead: how far the thread advances in one full turn. For a single-start thread the lead equals the pitch; for a multi-start thread it is the pitch times the number of starts.
• Thread angle: the angle between the two flanks, seen in the axial section.
• External and internal thread: the bolt or screw is external; the nut or tapped hole is internal.

A thread has three diameters that matter, and the standards name each of them for the external and the internal thread:

• Major diameter is the largest: across the crests of an external thread (d) and across the roots of an internal thread (D).
• Minor diameter is the smallest: across the roots of an external thread (d1) and across the crests of an internal thread (D1).
• Pitch diameter lies between them, on the imaginary cylinder where the tooth and the groove have the same width (d2 external, D2 internal). It is the diameter that controls how two threads fit, which is why tolerance classes are written mainly around it.

Convention to remember: a lower-case letter belongs to the external thread, a capital letter to the internal thread.

On the thread pages each value is drawn on the profile with its symbol, so a symbol you do not know can be looked up on the picture.

Metric: M10 × 1.5 – 6g

M says it is an ISO metric thread, 10 is the nominal diameter in millimetres, 1.5 is the pitch in millimetres, and 6g is the tolerance class: the number is the tolerance grade (the size of the zone) and the letter is its position relative to the basic size. External threads use lower-case letters, internal threads capitals (for example 6H). A class is often given for both pitch and crest diameter; a single class means both are the same.

Inch: 1/4-20 UNC-2A

1/4 is the nominal diameter in inches (small sizes use a gauge number, for example #10), 20 is the threads per inch, UNC is the series, and 2A is the class. The number is the class of fit (1 loosest, 3 tightest). A means external and B internal.

Series and forms in the Unified standard

• UNC, UNF, UNEF: coarse, fine and extra-fine pitch series for each diameter. UN: a constant pitch used across many diameters.
• UN, UNR, UNJ: the thread form. UN is the standard form, UNR adds a rounded root on the external thread, UNJ is a form with a controlled root radius, used where fatigue strength is critical.

• V threads (60°): the Unified inch threads (B1.1) and the ISO and ASME metric M threads. Made for fastening: many turns, held in place by friction.
• MJ and UNJ: V threads with a rounded, controlled root, for aerospace and other fatigue-loaded parts.
• Acme and Stub Acme: trapezoidal profiles for lead screws and other power transmission, where the thread moves a load rather than only holding it. Stub Acme has shorter teeth.
• Buttress: an asymmetric profile with one nearly square flank, for loads in one direction.
• Pipe threads (NPT, NPTF and the straight forms): tapered or straight threads for pipes and fittings, where the job is to seal as well as to hold. Dryseal forms are made so the threads themselves seal.
• Hose coupling and microscope objective threads: special-purpose forms with their own standards.