Thread Terminology

Terms

Major diameter

The largest diameter of the thread: over the crests of a bolt, or across the roots of a nut. It is the nominal size, so an M8 bolt has a basic major diameter of 8 mm. Symbols d (external) and D (internal).

Minor diameter

The smallest diameter: at the roots of a bolt, or across the crests of a nut. For a nut it is the bore left by the tap drill. Symbols D₁ for the internal thread and d₃ for the rounded root of an external thread.

Pitch diameter

The diameter of an imaginary cylinder that cuts the thread where the ridge and the groove are equally wide. It controls the fit between nut and bolt and is what thread gauges check. Symbols d₂ and D₂; also called effective diameter.

Pitch

The distance from a point on one thread to the same point on the next, measured along the axis. Metric threads state it in millimetres (M8×1.25).

Threads per inch (TPI)

The number of threads in one inch of length, used for inch threads. It is the reciprocal of the pitch: 20 threads per inch is a pitch of 0.050 in, or 1.27 mm.

Lead

The distance a nut advances in one turn. For a single-start thread it equals the pitch; for a two-start thread it is twice the pitch.

Lead angle

The angle of the thread helix at the pitch diameter, arctan(lead ÷ (π × pitch diameter)). Fine threads have a smaller lead angle.

Crest

The top surface of the thread ridge, furthest from the body the thread is cut on.

Root

The bottom of the groove between two ridges.

Flank

The sloping surface joining crest and root. The flanks are the surfaces that carry the load.

Flank angle and thread angle

The thread angle is the angle between the two flanks: 60° for metric, Unified and NPT threads, 55° for Whitworth and BSP. The flank angle is half of it.

Fundamental triangle height (H)

The height of the sharp V that the flanks would form if they met at a point. For a 60° thread H = 0.866 × pitch. Real threads are truncated to a fraction of H.

Thread height

The radial distance from crest to root. In the basic 60° profile it is 5H/8, or 0.541 × pitch.

Percentage of thread

How much of the thread height is left in a tapped hole, set by the tap drill. Standard tap drills give about 75%.

Length of engagement

The axial length over which nut and bolt threads are in contact. Tolerances are given for a normal length of engagement.

Tolerance class

A code for how far a thread may deviate from its basic size. Metric classes pair a grade number with a position letter, capital for nuts: 6H for a nut, 6g for a bolt. Unified classes are 1A, 2A, 3A for bolts and 1B, 2B, 3B for nuts, with 3 the tightest.

Allowance

An intentional gap between the largest bolt and the smallest nut. A 6g or 2A bolt has an allowance; a 6h or 3A bolt has none. It leaves room for plating and eases assembly.

Tensile stress area

The cross-section used to calculate the strength of a bolt in tension. It is based on a diameter between the pitch and minor diameters and is larger than the area at the root.

Proof load

The highest tensile load a bolt must withstand without permanent stretch. It is roughly 90% of the load at yield.

Taper

The change of diameter along a taper thread. Pipe threads use 1 in 16: the diameter grows by one unit for every sixteen along the axis.

Hand-tight engagement

How far a taper pipe thread screws in by hand before the flanks meet, written L₁ in ASME B1.20.1.

Right-hand and left-hand

A right-hand thread tightens clockwise and is the default. Left-hand threads are marked LH in the designation.