Acorn Nut –  This type of nut has a domed top that covers the end of the bolt.

A/F –  An abbreviation for Across Flats. Used in reference to the dimensions on hexagon head products.

Allowance – An intentional clearance between internal or externat thread. For metric threads the allowance is called the fundamental deviation.

Alloy Steel – Alloy steel is steel containing alloying elements, other than carbon, which have been added to achieve defined mechanical of physical properties and performance.

Anaerobic Adhesive – An adhesive which hardens in the absence of air, such adhesives are often used as a thread locking medium.

Angle of Head – Used in reference to countersunk fasteners. This is the angle from one side of the cone to the other. Standard angle for inch screws is 82 degrees & 90 degrees for metric screws.

Annealed –  A fastener is considered in the annealed state when it has been heat treated and cooled to make ~ malleable; that is, free of hardness caused by working or previous heat treatment.

ASME – American Society of Mechanical Engineers

ANSI – American National Standards Institute

Anti Friction coating – AF coatings are dry lubricants consisting of suspensions of solid lubricants, such as graphite, PTFE or molydbenum disulphide of small particle size in a binder.

Anti Seize compound – A liquid/gel compound thal is used to improve corrosion resistance to allow the parts to be subsequently dis-assembled .

ASTM – American Society of Testing and Materials



Basic Thread Profile – This is the theoretical profile of external and internal threads with no manufacturing tolerance applied.

Bearing surface of Bearing Area – The bearing surface is the supporting or locating surface of a fastener with respect to the surface contact wtth the mating materials that are in contact.

Black Bolts / Nuts – The word black refers to the comparatively wider tolerances and to some extent the colour of the surface finish of the fastener

Blank – A fastener in a non-specific intermediate stage of manufacture

Body – The body of a threaded fastener is the unthreaded portion of the shank diameter

Bolt – An externally threaded rod, headed at one end, designed to pass through an oversized hole and mate with a nut so as to hold two or more objects together.

Breakaway Torque –  The torque necessary to put into reverse rotation a bolt that has not been tightened.

Breakloose Torque – The torque required to effect reverse rotation when a pre-stressed threaded assembly is loosened.

Button Head – A button head as applied to threaded fasteners has a law rounded top surface with a large flat under head bearing surface.


Cap Screw – A term used to describe an externally threaded fastener with a protruding head, designed to be torqued by a spanner or wrench and always preceded by a head style such as ‘Hex cap screw’. ‘Socket
head cap screw’ etc.

Carbon Steel – A steel whose only significant alloying material is carbon.

Carriage Bolt –  A bolt with a smooth rounded head and a small square section under the head to prevent spinning during assembly.

Case Depth – That area of a fastener, measured from the surface inward, which has a different hardness requirement than its core.

Case Hardening – The process performed on quenched, tempered, ferrous fasteners which makes the surface of those parts harder than the inner core. Case hardness is measured in the threaded section of the fastener.

Castle Nut – A hex nut with a slightly reduced slotted cylindrical section on one end. Used with a coller pin and drilled fastener to prevent loosening.

Chamfer –  A beveled edge or corner.

Clamping Force – The compressive force which a fastener exerts on the joint.

Class of Fit – The Class of Fit is a measure of the degree of fit between mating internal and external threads.

Class –  Used in metric, class is a material designation equivalent to the US term Grade

Coefficient of Friction – A dimensionless number representing the ratio of the friction to normal force. Typically for threaded connections it is between 0.1 0 to 0.20 but can vary signtlicantly depending upon
the materials used and whether a lubricant has been used.

Cold Heading – Cold heading or forging is the process of forming ferrous and non-ferrous materials into specifie configurations, without the use of applied heat to assist the formation process.

Countersink –  A countersink is an internal chamfer or lead-in, into a preformed hole. The formation is carried out by a cutting or forming tool and provides a mating surface for a countersunk head type fastener.

Countersunk Head – The countersunk head has a flat top surface and a conical bearing surface with head angles of nominally 82° for inch fasteners and 90″ for Metric fasteners.

Core Hardness – The resistance a fastener material has to being permanently deformed, measured at a spot deeper than the case depth.

Cracks – Fractures .passing through or across material grain boundaries without the inclusions of foreign elements.

Crest –  The surface point, at which the thread joins the thread flanks.

Cut Thread –  A cut thread is a thread produced by removing material from the surface diameter with a form-cutting tool.


Dacromet – A high performance surface coating that can be applied to fasteners. The coating consists of passivated zinc flakes that are stoved onto the metal surface. The coating can be coloured and eliminates the risk of hydrogen embrittlement associated with electroplated metal.

Dacarburised – A fastener has a decarburised surface when the carbon content of the surface is lower than the carbon content of the fastener’s core.

DIN – An abbreviation for Deutsches Institut für Normung, the German standards body. ln reference to fasteners DIN indicates fasteners that conform to a specifie metric standard and will be followed by
the standard number.

Direct Tension Indicator – Direct Tension lndicators is a term used to describe load indicating washers. Projections on the face of the washer (usually
on the face abuting the bolt head or nut) that deform under loading as the bolt tensioned. An indication of the tension in the bolt can be made by measuring the gap between the washer face and the
nut or bolt head by using a feeler gage.

Ductility – The ability of material to be permanently bent or stretched without breaking.


Electroplating – Electroplating is carried out in an electrolyte containing a chemical compound of the metal to be deposited. The fasteners to be Electroplating plated are immersed in the plating bath and an electrical current is passed through the fasteners, acting as a cathode, attract the metal from the solution which builds up on the surface of the fasteners.

Elongation – The lengthening of fastener caused by a tensile force.

Engagement – A measurement of how much of the fastener is in the material being fastened.

Embedment, minimum – The minimum depth an anchor must be installed to meet the minimum pull out values. It is the distance measured from the concrete surface to the bottom of the anchor.

External Force/Load – Forces exerted on a fastener as a result of an applied loading to the joint.

External Thread –  Thread on the outside of a cylindrical piece of stock.


Fastener – A fastener is a mechanical device for holding two or more bodies in definite positions with respect to each other.

Fatigue Strenght – Under variations in applied stress, a fastener experiences internal stretching that can cause rupture after a specifie number of cycles. The number of cycles to failure for a specifie Join is the
fatigue life of the fastener

Ferrous – That which contains iron; usually refers to fasteners containing more iron than any other element.

Fillet – The concavely curved section at the angle formed at two intersecting surfaces of a fasteners.

Finish – The term finish is commonly applied on the condttion of the surface of fastener because of chemical or organic treatment, subsequent to the manufacture of the fastener.The term finish is also applied to some types of fasteners to indicate the condition of a materials surface texture because of mechanical operations and the degree of precision achieved or required

Fit – Fit is the general term used to signify the range of tolerance parameters.

Flange Bolt – A bolt with a buitt in washer-like flange just below the head.

Flat Head – A flat head with a conical bearing surface. Designed to be Countersunk

Forging – Forging is the process of forming a product by hammering or the displacement of material under force. When the material is forged below the re-crystallisation temperature it is said to be cold forged. When worked above the re-crystallisation temperature having been pre-heated, it said to have been hot forged.

Form of Thread – The profile of a thread in an axial plane for a length of one pitch.

Fundamental Deviation – An intentional clearance between internal or external thread and the design form of the thread when the thread form is on it’s maximum metal condition. For metric threads the fundamental deviation are designated by letters, capitals for internal threads and small letters for externat threads. For imperial threads the fundamental deviation is called the allowance.


Galling – A severe form of adhesive wear which occurs during sliding contact of one surface relative to another. Clumps of one part stick to the mating part and break away from the surface. Galling can frequently occur when both the nut and bolt are made from stainless or high alloy steels, aluminum, titanium, etc.

Galvanizing :

Hot-Dip Galvanizing – The process of coating iron or steel with zinc by means of hot dipping.

Electro-Galvanizing – The process of coating iron or steel with zinc through an electric current. This results in a somewhat smoother, shinier finish than hot-dipping.

Mechanical Galvanizing – The process of coating iron or steel with zinc at room temperature where the zinc powder becomes cold welded to the metal parts. It results in a more uniform finish than hot-dipping and greatly reduces the chance of hydrogen embrittlement which can occur in electro-galvanizing.

Gimlet Point – A conically-shaped, threaded point having an angle of 45-50˚. Lag screws, wood screws, Type-A & AB tapping screws all have gimlet points.

Grip Lenght – Total distance between the underside of the nut to the bearing face of the bolt head; includes washer, gasket thickness etc.



Hardened Washers – A washer that is made of a suitable material that is hardened and designed to distribute the force over a wider are a into the clamped material. A more modern alternative is to use a flange headed
nuts and bolts.

Hardened – A fastener that has been heat treated to improve its mechanical properties

Head Diameter – It is the diameter at the largest extremity of the head

Header point – A blunt point with chamfered edges. Machine screws typically have header points.

Head Style – The type of head or top that is on the fastener. Such as a Round head or Hex head

Head – The head of a fastener is the enlarged configuration, preformed on one end of a headed fastener, to provide a bearing surface.

Headless Fastener – A headless threaded fastener is a fastener normally having a slot, recess, or socket in one end to drive the fastener into the assembly.

Heat Treatment – Surface treatment is any treatment which changes the chemical physical or mechanical properties of a surface.

Hex Bolt –  A bolt with a six sided head.

Hexagon (hex) head – The hexagon head has a flat or indented top surface, six flat sides Head and a flat bearing surface.

High Strenght Fastener – A high strength fastener is a fastener having high tensile and shear strengths attained through combination of material types,  work hardening and heat treatment.

Hydraulic Tensioner – A hydraulic tool used to tighten a fastener by stretching it rather than applying a large torque to the nut. After the fastener has been stretched the nut is run dawn the thread to snug up with the Joint, the hydraulically applied load is then removed resulting in tension being induced into the fastener.

Hydrogen Embrittlement – Steel fasteners exposed to hydrogen can fail prematurely at a stress level well below the materials yield strength. Hydrogen embrittlement occurs in fasteners usually as a result of the part
being exposed to hydrogen at some time during its manufacturing process but it can also occur through in-service corrosion. Electroplating is generally considered to be a major cause of  hydrogen absorption in steel fasteners due to the release of hydrogen during this process. Higher strength steels are more susceptible to hydrogen embrittlement than lower strength steels, however it is considered that there is no lower strength limit. As a rule of thumb, steels below Rockwell C 35 are considered to be far less susceptible.


Installation Force – A term expressed in pounds, tons or newtons applied axially to a self-clinching fastener to achieve proper installation.

Interference Fit – The insertion of one member into another whose diameter is slightly smaller than the part being inserted.

Internal Thread –  Thread on the inside of a cylindrical hole.

ISO –  International Organisation for Standardisation. A worldwide federation of national standardisation bodies.


Key Engagement – The distance from the head surface of a socket to that depth to which the hex wrench will penetrate.

K Factor – The factor in the torque tightening equation: T- KDP where T is the fastener tightening torque in Newton metres, D is the  fastener diameter in inches/mm and P is the fasteners preload in kilonewtons and K is the coefficient of friction. The formula gives the approximate tightening torque for standard fasteners used under normal conditions.


Left Hand Thread – A screw thread that is screwed in by rotating counterclockwise.

Lenght of engagement –  The axial distance over which an external thread is in contact with internal thread.

Lenght – The length of a headed fastener is the distance from the intersection of the largest diameter of the head with the bearing surface to the extreme point, measured in a line parallel to the axis of the fastener.

Locknut – A nut constructed to resist loosening when subjected to vibration or axial load. A prevailing torque type locknut achieves its locking action without being against another nut or a bearing surface, but by a controlled distortion in its threads or by means of another locking element (ie. a nylon ring) built into the nut. A free-spinning locknut achieves its locking action when tightened against another surface.


Major Diameter – This is the diameter of an imaginary cylinder parallel with the crests of the thread; in other words it is the distance from crest to crest for an external thread, or root to root for an internal thread.

Mandrel Break Load – The axially applied load required to break the mandrel while a rivet is being set.

Milled from bar – A description of fasteners made from stock on a screw machine or a lathe.

Minor Diameter –  This is the diameter of an imaginary cylinder which just touches the roots of an external thread, or the crests of an internal thread.

Molybdenum Disulphine – A solid lubricant that acts as a high pressure resistant film. Can be used by itself as a dry lubricant as well as in with other solid lubricants and in oils and greases.


Neck – Neck is used to define a reduced diameter for a portion of the shank of a fastener, which is required for design or manufacturing reasons.

Non-Ferrous Metal – Metals of alloys without an appreciable amount of iron. Examples are aluminum, brass, copper, etc.

Non-Standard fastener – A non-standard fastener is a fastener which differs in size, length,material or finish from established international standards.

Nut Thickness – Nut thickness is the overall distance from the top of the nut to the bearing surface, measured parallel to the axis of the nut.


Oval Head  – A countersunk screw with a slightly rounded top surtace for a more finished look.

Overtapping – Tapping of a thread following a plating or galvanizing operation so that the thread tolerances comply within specification allowing theiInternal and external threads to assemble. It is normal practice to
overtap the internal thread after galvanizing.


Pan Head – A head with a slightly rounded top surface and short vertical sides.

Passivating – The process performed on stainless fasteners of removing surface imperfections and producing a slight film on the surface which enhances the part’s resistance to corrosion.

Penetration Gauge Depth – A range of measurement which determine the acceptability of a recess in the head of a screw. It is measured from a plane where the edge of the recess wings meet the top of the head’s surface, downward into the recess.

Physical Properties – Physical Properties are the strength grade properties defining the basic characteristics of the material or fastener.

Pilot Hole Size – An opening of sufficient size for a specific fastener to be properly installed.

Pitch :         

  • Thread Pitch – The distance between corresponding points on adjacent threads in the same plane parallel to the part’s axis and on the same side of the axis.          
  • Pitch Cylinder – An imaginary cylinder which, sharing the same axis as a fastener, would cut through the threaded portion of that fastener in a way that made the widths of the thread ridge and groove equal, or half way between the major and minor diameters.          
  • Pitch Diameter– The diameter of the pitch cylinder. Plating – The application of a metallic deposit on the surface of a  fastener for protective and/or decorative purposes.

Plain Finish – Plain, as applied to the finish of fasteners, is used to indicate that the fastener has had no supplementary surface treatment, such as plating, coating etc, other than being oiled. Sometimes referred to as self-colour or black.

Plating build up –  Plating build up is the term used to describe the condition of excessive plating on threads, edged or corners that on the other surfaces of the fastener.

Plating – Plating is the process of a metallic deposit on the surtace of the fastener by electrolysis.

Plow Bolt – A bolt with a smooth flat countersunk head that has a small square section underneath. Used on plows.

Point Angle – The point angle is the included angle of the point.

Point Taper Length – The length of the pointed portion of the fastener measured parallel to the axis, from the end of the point to the first full form thread.

Pointing – Painting is a secondary machining operation consisting of cutting p0ints on fastener blanks, which have not forge-pointed during the heading operation.

Preload – The tension created in a fastener when first tightened. Reduces alter a period of time due to embedding and other factors.

Prevailing Torque Nut – A  type of lock nut which has a prevailing torque to assist in preventing self loosening. There are two main categories of prevailing torque nuts, all metal and nylon insert. All metal torque prevailing nuts generally gain a prevailing torque by distorting the threads at the top of the nut by some means. Nylon insert torque prevailing nuts ultilise a nylon (or other polymer) in sert to achieve a prevailing torque.

Proof Load – The tension applied force a fastener must support without any permanent change in size or shape.

Proof Test – A proof test is any specilied test required for a fastener to indicate that it is suitable for the purpose intended.

Property Class – A designation system which defines the strength of a bolt or nut. For metric fasteners, property classes are designated by numbers where increasing numbers generally represent increasing tensile


Recess Depth – The distance measured, axially, from the plane where the edge of the recess wings meet the top of the head’s surface to the bottom of the recess.

Reduce Shank Bolt – A bolt whose shank diameter is smaller than the nominal diameter of the bolt.

Reduction of Area – one of the tests performed on a cap screw or bolt when determining its tensile and yield strengths. After a test specimen has been broken in a tensile tester, the original diameter of the specimen (d) is compared to its smallest diameter after fracture (t). The formula for determining reduction of area is : [1.000 – (.5xt)2/(.5xd)2] x 100 = % area reduction.

Reverse Thread -Thread thal is formed opposite to normal thread so it tightens counter clockwise. Used on spinning equipment to prevent loosening. Also called a Left Hand Thread.

Rockwell Hardness – A test performed on fasteners to determine the resistance a fastener material has to being permanently deformed by a specifically shaped object under a specific amount of weight. The test measures how deep an indentation is made in the fastener. That measurement is converted to a reading on the Rockwell hardness scale.

Right Hand Thread – A screw thread that is screwed in by rotating clockwise. The majority of screw threads are right handed.

Rockwell Hardness – A relative measure of hardness. Rockwell C Scale is used for hard materials. Rockwell B for softer materials, such as sheet metal.

Rockwell Hardness Test – A measurement of hardness by determining the depth of penetration of a hardened calibrated steel ball into the hardened specimen under specified test conditions. The hardness
reference number is a comparison scaling, relative to the depth of indentation. The higher the number the harder the material.

Rolled thread –  Threads formed by plastically deforming a blank between 2 serrated dies to form a thread. This is the most common method  of creating threaded fasteners today.

Root Diameter – Identical to Minor Diameter

Runout of Thread – A threaded portion with incomplete profile generated by chamfering part or leading part of tool. The incomplete thread are generated at the boundary between body and complete thread and also at thread-end portion as for an external threaded fastener with body. The same are generated at under-head portion and thread end portion as for an external threaded fastener with continuous thread.


SAE – Society of Automotive Engineers

Self Locking – A locking element, formed as an integral part of a fastener, which provides force to restrict the rotational movement of a threaded member.

Set Screw – A set screw is a threaded fastener that is typically used to hold a sleeve, collar or gear on a shaft to prevent relative motion. It is a threaded screw that normally does not have a head.

Screw – The term used for a threaded fastener, with or without a head (headless – as in set screw) so designed as to permit  to be properly assembled in a pre formed internal threaded hole (or capable of forming its own thread) and secured by means of tightening the head.

Screw thread – A ridge of constant section which is manufactured so that a helix is developed on the internal or external surface of a cylinder.

Shank Diameter – The diameter of the shank or smooth part of the bolt above the threads.

Shank – The smooth part of the bolt which lies below the head and above the threads. Also called the body.

Shear Load, Ultimate – A minimum amount of force, applied transversely, which a blind rivet must withstand without failing.

Shoulder Screw – A threaded fastener with a plain, precision machined, shank that is used for location purposes. They are typically used for pulleys.

Shoulder – A shoulder is an enlarged portion of a threaded fastener or the shank of an unthreaded fastener.

Skidmore Bolt Tension Calibrator – The Skidmore-Wilhelm bolt tension calibrator is a hydraulic load cell used to determine the tension in a bolt or other threaded fastener. The tension in the bolt compresses fluid in a hydraulic cylinder, a pressure gauge connected to the cylinder is then calibrated to read in terms of force rather than pressure.

Slotted Nut – A nut with slots cut into it for the insertion of a coller pin. Used with a drilled shank fastener. Similar to a castle nut.

Socket Depth –  The socket depth is the distance measured parallel to the fastener axis from the socket head face to the extreme end of the socket. ln socket head cap screws, the effective socket depth is most often specified as « Key Engagement » as related to the hexagon wrench used for insertion.

Socket Head Cap Screw – A screw with a round head, usually with a hexagon indentation in the head for tightening purposes. Used on machine parts and is typically made from high strength steel.

Square Bolt – A bolt with a four sided head.

Standard Fastener – A standard fastener is a fastener, which conforms in all respects to recognised international standards.

Stress Area – The effective cross sectional area of a thread when subjected to a tensile force. It is based upon a diameter which is the mean of the pitch (or effective) and the minor (or root) diameters of  the thread.

Stud – A stud is a fastener with no head, which has threads at both ends of the shank. Like a screw, one end is inserted into an internally tapped hole and tightening a nut on the other end induces tension. If a stud is threaded its entire length and a nut is used on both ends to create tension, it serves the function of a bolt and is then classified as a stud bolt.

Surface Treatment – Surface treatment is any treatment which changes the chemical, physical or mechanical properties of the surface of a fastener. eg.phosphating, zinc plating, anodizing etc.

Swaging – An operation whereby a reduced diameter of a fastener is deformed to secure it to a panel. Note: the antonym of swaging is self-clinching where the panel material is caused to deform.


Tamper Proof – A fastener designed so it can be installed wilh a screw driver or other special tool and cannot be removed without destroying the screw. Used to prevent tampering.

Tap Bolt –  A fully threaded bolt.

Taper – Taper may refer to the geometry of the head, shank or some other feature of a fastener.

Temper – Temper is a process that is carried out after hardening whereby the fastener is reheated for a controlled period in order to refine the grain structure and acquire certain mechanical properties.

Tensile Strength – Tensile strength is the maximum tension (pull or tautness) applied as a loading, that a fastener can support prior to, or coincidental with, its fracture.

Thread Engagement – The percentage of the thread height that is in the material being fastened. For full thread engagement the pilot hole should be equal to or smaller than the root diameter of the fastener.

Thread Series – A term used to describe the number of threads per inch for a given diameter.

Thread – A thread is a triangle profile of uniform section in the form of a helix in the external or internal surface of a cylinder. This is known as a straight or parallel thread to distinguish it from a taper thread which is formed on a cone or frustum.

Through Hardened – A bolt that has been heat treated all the way through. More brittte but stronger than a case hardened bolt. Also called fully hardened.

Through Hole – A hole, threaded or not threaded, which transverses the entire length of a part and is usable from either end.

Tolerance – A tolerance is the total permissable variation around a nominal size. The tolerance is the difference between the limits of size.

Torque-out – The amount of torque necessary to spin the fastener out of the sheet. This is torque applied to the fastener. No axial load is applied.

Torque-through – The amount of torque necessary to fail the fastener in axial load.

Torsion –  Twisting moment of force applied to a fastener during tightening.

Toughness – Toughness is the ability of a material to absorb considerable energy without fracturing.


UNC – Unified National Coarse Threads

UNF – Unified National Fine Threads

Under Head Fillet – An under head fillet (radius) is the fillet at the juncture of the head and shank of a headed fastener.

Undercut Head – A countersunk head that has been cut off at 70% of the normal height.


Washer face – A circular boss on the bearing surface of a screw, bolt or nut.

Washer Head – A washer head is a fastener head having a flanged collar under the head of the fastener to provide a bearing surface. Commonly known as a flange head, relative to the generic product type.

Washer –  A washer is a part having a centrally located hole. The washer performs various functions when assembled between the bearing surface of a fastener and the part being attached for example lnsulation, lubrication, increasing the bearing area over large clearance holes to improved stress distribution etc.

Width Across Corners – The width across corners of a hexagon, square or rectangular geometry measured from corner to corner.

Width Across Flat – The width across flats of hexagon or square heads of fasteners is the distance measured across the flats of the fastener head.

Wing nut – A nut with ‘wings’ for easy manual assembly.

Work Hardening – Work Hardening is the increase in hardness and hence strength, resulting from plastic deformation at a temperature below the re-crystallisation range.


Yield Point – The stress necessary to produce elongation under load of 0.5% of the specimen’s original iength. Expressed in N.mm2.

Yield Strength – The maximum load at which a material exhibits a specific permanent deformation. It is usually at a point of 0.2% permanent strain.

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