Thread & Fastener Glossary

Thread Geometry

Allowance

Intentional clearance between mating threads ensuring assembly under worst-case tolerance conditions.

Crest

The outermost surface of a thread ridge (top of the V).

Depth of Thread

The distance from crest to root, measured perpendicular to the thread axis.

Flank

The side surface of a thread connecting crest to root.

Hand of Thread

Direction of helix. Right-hand (RH) tightens clockwise; left-hand (LH) tightens counter-clockwise.

Helix Angle

The angle between the thread helix and a plane perpendicular to the thread axis. Affects self-locking behavior.

Lead

Axial distance a thread advances in one full rotation. For single-start threads, lead = pitch. For double-start, lead = 2 × pitch.

Major Diameter

The largest diameter of a screw thread, measured from crest to crest. For external threads, this is the nominal size (e.g., M10 bolt has 10 mm major diameter).

Minor Diameter

The smallest diameter of a screw thread, measured from root to root. Also called the core or root diameter. Used to calculate tensile stress area.

Multi-Start Thread

Two or more interleaved helices providing faster linear travel per rotation. Used in quick-release mechanisms.

Pitch

Distance between adjacent thread crests, measured in mm (metric) or as TPI (inch-based). Determines how fast a thread advances per rotation.

Pitch Diameter

The theoretical diameter at which thread ridge width equals groove width. Critical for thread fit and engagement calculations.

Root

The innermost surface of the groove between thread ridges (bottom of the V).

Single-Start Thread

A thread with one continuous helix; lead = pitch. The most common configuration.

Thread Angle

The included angle between the two flanks. ISO metric and Unified threads use 60°; BSP/Whitworth use 55°.

Thread Form

The cross-sectional profile of a thread defined by angle, crest, and root geometry (e.g., 60° V, trapezoidal, buttress).

Thread Height (h)

Radial distance between major and minor diameters. For 60° V-threads: h = 0.5√3 × pitch ≈ 0.866 × pitch.

TPI Threads Per Inch

Number of thread crests within one inch of axial length. Used in UNC, UNF, UNEF, NPT, and BSP standards. Inverse of pitch in inches.

Thread Standards

ACME Thread

Trapezoidal thread form (29° angle) for power transmission, lead screws, and linear actuators.

BSP British Standard Pipe

British pipe thread standard (ISO 228/7). G = parallel (BSPP); R = tapered (BSPT). 55° thread angle.

BSPP

Parallel BSP thread (prefix G). Requires O-ring or gasket for sealing.

BSPT

Tapered BSP thread (prefix R). Self-sealing by interference, similar to NPT.

ISO Metric

International thread standard (ISO 68-1), 60° thread angle, mm dimensions. Designation: M[dia]×[pitch], e.g., M10×1.5.

NPT National Pipe Taper

American taper pipe thread (ASME B1.20.1). 1°47′24″ taper; seals by thread interference/deformation.

Trapezoidal Thread (Tr)

ISO metric power thread (30° angle); metric equivalent of ACME.

UNC Unified National Coarse

American/Canadian coarse pitch standard (ASME B1.1). Most common for general fasteners.

UNEF Unified National Extra Fine

Extra fine pitch Unified thread. Very high TPI; used in precision instruments and thin-walled parts.

UNF Unified National Fine

Fine pitch variant of Unified standard. Higher TPI for same diameter; better in vibration environments.

Whitworth

Original British thread standard (55° angle), developed by Joseph Whitworth circa 1841. Largely superseded by ISO metric.

Taps, Dies & Drilling

A hole that does not pass through the workpiece. Requires bottoming tap for full-depth threading.

Bottoming Tap

A tap with only 1–2 chamfered threads; cuts to the bottom of blind holes. Used after plug tap.

Clearance Hole

A hole sized to allow a bolt to pass through without engaging threads; typically 5–10% larger than bolt OD.

Counterbore

Flat-bottomed cylindrical recess for socket-head bolt to sit flush with surface.

Countersink

Conical seat at hole opening matching flat-head screw angle (typically 82° or 90°).

Die

Hardened cutting tool used to cut external threads on a rod or bolt blank.

Plug Tap

Most common tap type; 3–5 tapered lead threads. Good for through-holes.

Tap

Hardened cutting tool used to cut internal threads in a pre-drilled hole.

Tap Drill

Drill bit size used before tapping to leave ~75% thread depth engagement. Calculated as: Tap drill = Major diameter − Pitch.

Thread Engagement

Depth (length) over which mating threads contact. Typically 1.0× to 1.5× bolt diameter for steel.

Thread Percentage

Ratio of actual to full theoretical thread depth. Standard: 75%. Higher = stronger but harder to tap.

Through Hole

A hole that passes completely through the workpiece.

Taper Tap

7–10 tapered threads at tip; easiest to start but cannot thread to bottom of blind holes.

Bolt Grades & Materials

A2 Stainless

Austenitic 304/18-8 stainless steel fasteners per ISO 3506-1. Good general corrosion resistance; non-magnetic.

A4 Stainless

Marine-grade 316 stainless steel fasteners. Superior corrosion resistance especially in chloride/marine environments.

ASTM Grade

American bolt specification from ASTM International. E.g., A325, A490 for structural applications.

Head Marking

Numbers, lines, or symbols on a bolt head identifying grade and manufacturer.

Proof Load

Maximum load a fastener can withstand without permanent deformation. Typically 85–92% of yield strength.

Property Class

Metric bolt grade per ISO 898-1. Format X.Y: X×10 = nominal UTS (kgf/mm²); X×Y×10 = yield strength. Common: 8.8, 10.9, 12.9.

SAE Grade

American bolt grade system (SAE J429). Grade 2, 5, 8 are most common; higher number means higher strength.

Tensile Stress Area (As)

Effective cross-sectional area for thread stress calculations, based on mean of pitch and minor diameters. As = π/4 × [(d2+d3)/2]².

Ultimate Tensile Strength

Maximum stress before fracture. Also called tensile strength.

Yield Strength

Stress at which permanent deformation begins. Key parameter for fastener selection.

Torque & Assembly

Breakaway Torque

Torque required to initiate loosening of a tightened fastener.

Clamp Load

Compressive force generated in a joint when a fastener is tightened. Core purpose of preloading.

Friction Coefficient (μ)

Ratio of friction to normal force at thread and bearing surfaces. Affects torque-tension relationship significantly.

Locking Insert

Nylon patch, deformed thread section, or insert preventing vibration loosening.

Preload

Initial tensile force in a fastener after tightening, before any external load. Typically 70–90% of proof load.

Prevailing Torque

Additional torque needed to overcome a built-in locking feature (nylon insert, distorted threads, etc.).

Seating

Process of mating surfaces fully conforming under initial load before final torque is applied.

Torque

Rotational force applied to tighten a fastener. Measured in N·m or ft·lb.

Torque Coefficient (K Factor)

Dimensionless constant relating torque to clamp load: T = K × d × F. Typically K = 0.15–0.20.

Coatings & Finishes

Black Oxide

Conversion coating providing mild corrosion resistance and low reflectance; requires oil topcoat.

Galvanizing

Hot-dip zinc coating providing thick, durable, sacrificial corrosion protection.

Galling

Cold-welding adhesion between metal surfaces under high contact pressure. Common issue with unlubricated stainless threads.

Passivation

Chemical treatment of stainless steel removing free iron and strengthening native oxide layer.

Phosphate Coating

Iron or zinc phosphate conversion coating; improves paint adhesion and mild corrosion resistance.

PTFE Coating

Low-friction polytetrafluoroethylene coating reducing galling and torque requirements.

Zinc Plating

Electroplated zinc coating; thinner than hot-dip but good corrosion resistance for indoor/mild use.

Fit & Tolerance

Class of Fit

Unified system designating tolerance combination. 1A/1B = loose, 2A/2B = standard, 3A/3B = precision.

g Tolerance

ISO designation for external thread (bolt); lowercase. 6g is standard for metric bolts.

Go/No-Go Gauge

Pair of gauges verifying threads are within tolerance limits without measuring dimensions directly.

H Tolerance

ISO designation for internal thread (nut); capital letter. 6H is standard for internal metric threads.

Tolerance Class

Designation specifying allowed size deviation. ISO metric format: 6g (external) or 6H (internal).

Pipe Threads

DN Diamètre Nominal

ISO/European nominal pipe diameter in approximate millimeters.

Nominal Pipe Size (NPS)

North American pipe dimension standard. NPS number does NOT directly equal pipe OD.

Pipe Dope

Thread sealant compound providing lubrication and sealing for pipe joints.

PTFE Tape

Polytetrafluoroethylene tape wrapped on tapered pipe threads to improve seal.

Taper

Gradual diameter reduction along thread length. NPT and BSPT taper is 1:16 (0.75″ per foot).

Wrenching Engagement

Number of thread turns engaged in a pipe joint. Typically 3–5 turns for NPT/BSPT.

Engineering Glossary