AGE HARDEN - To use modified heat treatments at various temperatures over a period of time to harden and strengthen a fastener.

AIRCRAFT QUALITY - Fasteners made with a particularly high level of attention in manufacture and inspection.

ALLOY STEEL - A mixture (or alloy) of ordinary steel added to other metals besides carbon with the specific purpose of attaining certain characteristics such as higher strength. A few exceptions to this definition exist, however, so that a chromium content above 4% is not considered alloy steel and above 12% is considered stainless steel.

ANCHOR BOLT – A fastener intended to be embedded in concrete with the threaded end protruding to anchor material to the concrete.  Usually bent, deformed or shaped in a way as to resist “pull out” from the concrete.

ANNEAL - To heat metal in order to lower its hardness. The term anneal refers to the heat treatment given all 300 series stainless and most 400 series stainless by the steel mill after the raw material has been completed but before fasteners are manufactured. Anneal also refers to the heat treatment given 400 series stainless fasteners after their manufacture (also called hardening and tempering) to lower hardness and increase toughness. For example, fasteners of 410 stainless may score over 200,000 psi after manufacture and be too brittle. By annealing at 1000 degrees F., tensile strength would reduce to 125,000-150,000 psi, while annealing the same material to 500 degrees F. would bring tensile to 160,000-190,000 psi.

AUSTENITIC - Refers to 300 series stainless, the most popular of the stainless alloys accounting for 85% - 90% of stainless fasteners sold. Named for Sir Robert Williams Austen, an English metallurgist, austenitic stainless is a crystal structure formed by heating steel, chromium, and nickel to a high temperature where it forms the characteristics of 300 series stainless steel. An “AUSTENITE” is a molecular structure where 8 atoms of iron surround one atom of carbon, thus limiting the corrosive effects of the carbon. Austenitic fasteners have the highest level of corrosion resistance in the stainless family, cannot be hardened by heat treatment, and are non-magnetic for practical purposes.

The most popular of austenitic grades is known generically as “18-8 stainless” and includes grades 302, 302HQ, 303, 304, 305, and XM-7. Typical industries using 18-8 fasteners include: food, dairy, wine chemical, pulp and paper, pharmaceutical, boating, swimming pool, pollution control, electronic, medical and hospital equipment, computer, textile.

Type 316 stainless has added nickel and especially molybdenum. The molybdenum (called moly) sharply increases corrosion resistance to chlorides and sulfates, including various sulfurous acids in the pulp industry. It has superior tensile strength at high temperatures compared to 18-8. Besides pulp and paper, typical industries using 316 are: photographic and other chemicals, ink, textile, bleach, rubber.

Exotic metals in the 300 series include 309, 310, 317, 321, and 347. With superior corrosion resistance at elevated temperatures, these metals are used for furnace parts, high temperature containers and processing equipment, aircraft parts such as collector rings, exhaust systems, and equipment for very corrosive compounds of sulfuric, nitric, citric, and lactic acids.

BEARING SURFACE - The part of a fastener such as the washer face of a nut or under the head of a machine screw that actually comes in contact with the part it fastens.

BEVEL- A small slant, usually describing a flat washer which is square and thicker on one side than the other

BINDER HEAD - Old term for pan head, “binder” has now come to mean “binding” head screws rather than pan.

BLANK - A fastener where one or two stages of manufacturing have been performed, but the fastener has not been finished.

BODY – The unthreaded portion of the shank.

BOLT – An externally threaded fastener which requires a nut to secure fastened joint.

BRASS - The most common alloy of copper, brass is basically two-thirds copper, one-third zinc. It is non-magnetic with good strength and toughness, high electrical conductivity, and an attractive lustrous finish. It has good corrosion resistance but not in salt water. Brass is commonly used by the electrical and communications industries, builders hardware, and some marine applications.

BROACH – Using sharp edges to cut material and push it away, broach usually refers to the socket drive on socket screws.

CAP SCREW – Fastener formed by “cold forming” with tolerances normally tighter than hot forged bolts.  “Caps” usually pertaining to “Hex”, Socket” “Square” etc.

CAPTIVE SCREW - Where the shoulder of a screw is perceptibly smaller in diameter than the threaded portion (technically the minor diameter or less).

CARBIDE PRECIPITATION - Carbon that breaks loose from its bond within the stainless solution when material is heated between 800 - 1400 degrees F. Under severe corrosive conditions, it can result in extra oxidation and surface corrosion. See SOLUTION ANNEALED.

CARBON - Adds strength to stainless steel, but also lowers corrosion resistance. The more carbon there is, the more chromium must be added, because carbon offsets 17 times its own weight in chromium to form carbides, thus reducing the chromium available for resisting corrosion.

CARBON STEEL - Ordinary steel with no significant additions besides carbon.

CASE DEPTH – That area of a fastener, measured from the surface inward, which has a different hardness requirement that its core.

CASE HARDENED – Heat treated fastener in which the surface is harder than the core.

CERTIFICATE OF COMPLIANCE – A certification that a fastener meets the description or standard to which it was sold.

CHAMFER - A slight rounding on the end of a fastener or the edges of a hex nut for ease of assembly or smoother appearance.

CLAMP LOAD – The total load across the joint interface in service. This may vary during service life.

COLD FORMING or COLD HEADING or COLD WORKING - When fasteners are produced without heating or small heat below the recrystallization temperature (so the raw material bond of stainless remains unchanged) by pressing metal wire against various dies at high speed to form a fastener’s head or basic shape. Cold working causes an increase in tensile strength and hardness (known as work hardening) and a decrease in ductility.

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

CORROSION RESISTANCE – The ability of a fastener or material to resist corrosion under specific conditions.  Coatings such as Hot Dipped Galvanizing are usually used to increase the level of corrosion resistance of anchor bolts.

CRACKS – A clean, crystalline fracture which passes through or across the grain boundaries without inclusion of foreign elements.

CREEP STRENGTH - A measure of the resistance of fasteners to stress under elevated temperatures. At higher temperatures, a fastener can change in dimension under the same load, and that is called creep. Creep can cause the loosening of fasteners as temperature increases.

CREVICE CORROSION - Refers to joints and crevices in a fastener assembly where lack of oxygen caused by limited space or by surface grease prevents the passive film on stainless from forming.

CUT THREADING - Forming threads on a fastener by cutting away and actually removing the unneeded metal.

DEBURR- To remove chips, burrs, or other imperfections through a secondary operation such as grinding.

DESTRUCTIVE TESTING – Test used to determine the physical/mechanical properties of a material resulting in the destruction of the material sampled/tested.

DISCONTINUITIES - A variety of small or large disfigurations in a fastener such as pits (slight depressions on the surface), tool marks, voids (small cracks), laps, folds and seams (slightly bunched or folded material at the corners of a fastener), and inclusions (a slight non-metallic impurity in the metal). Minor discontinuities are permissible in both commercial fasteners and those made to various MS and other specs.

DRAWING - Where raw material shaped like wire is pulled through a die to reduce its diameter to that needed for the particular fastener being manufactured.

DRIVE TORQUE – A screw shall form a mating internal thread in a test plate, without damaging its own thread with the application of a rotational force not in excess of the drive torque.

DUCTILITY - The ability of a fastener to deform before breaking (for example, and elastic would be more ductile than a diamond). Ductility is a measurement similar to elongation.

ELONGATION - Stretching a fastener to the point that it breaks. The percent of elongation at rupture (same as measure of ductility) is determined by dividing the total length after stretching to the original length. Elongation decreases as strength and hardness increases.

ELECTRICAL CONDUCTIVITY - Metals carry electric currents with varying capacities. As a relative guide to the conductivity of different metals, with electrolytic copper rated at 101 under the International Annealed Copper Standard at 68 degrees F., 18-8 stainless rates is rated at 5; silicon bronze 651 at 12; and brass at 27.

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.

ETCH – A chemical process that cleans and brightens aluminum after heat treatment.

EXTRUDING - When cold forming produces a fastener before threading with two different diameters. The portion with the larger diameter is the shoulder; the smaller portion will be roll threaded. In the extruding process, a manufacturer starts with raw material equal to the shoulder diameter and pushes part of it through a die, reducing the diameter of the portion which will later be roll threaded.

EYE BOLT – A bolt with a threaded shank and a single loop closed or open on the other end.

FASTENER – A device designed to hold or join two components in a defined position usually utilizing threaded connections between two “fastener” devices i.e. a bolt & nut., (normally bolts, nuts, screws, washers etc.)

FATIGUE - Metal failure due to stresses that push first in one direction and then another. FATIGUE CORROSION is caused by repeated stress in a corrosive atmosphere and is generally not associated with stainless.

FATIGUE STRENGTH - Measures the endurance of a fastener by showing the load it can accept without breaking under repeated load cycles.

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

FILLET – Concave junction at two intersecting surfaces of a fastener.

FIT - Normally referring to threads, fit is a measure for the tightness of mating parts.

FULL BODY DIAMETER - When the shoulder of a fastener equals the outside or major diameter of the threaded portion.

GALLING (also called SEIZING) - When two metals or fasteners stick together and cannot be easily loosened. In tightening fasteners, for example, pressure builds on threads as metals rub against each other, and the passive film preventing corrosion on stainless may not form due to lack of oxygen.

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.

GALVANIC CORROSION - An accelerated degree of corrosion occurring when two different metals are in contact with moisture, particularly sea water. All metals have what is termed a specific electric potential, so that low level electric current flows from one metal to another. A metal with a higher position in the galvanic series (see below) will corrode sacrificially rather than one with a lower position, meaning stainless, for example, will corrode before gold. The further apart the metals on the chart, the more electric current will flow and the more corrosion will occur. No serious galvanic action will occur by combining the same metals, only dissimilar ones. To prevent galvanic corrosion, use insulation, paint or coatings when separating dissimilar metals; or put the metal to be protected next to a metal which is not important in the assembly, so it can corrode sacrificially.

Metals listed first will corrode due to galvanic reaction before those at end of paragraph: magnesium, zinc, aluminum 1100, cadmium, aluminum 2024, steel and iron, lead, tin brass, copper, bronze, monel, 304 and 316 stainless (passive), silver, titanium, graphite gold.

GIMLET POINT - A threaded cone point usually having a point angle of 45-50 degrees.

GRADE ID SYMBOLS – Markings or Stampings indicating the specifications used to manufacture a specific fastener.  Usually indicating the material grade, mechanical/chemical and other specified criteria to produce the product.

GRIP - The unthreaded part of a fastener.

GRIP RANGE – The minimum and maximum thicknesses of materials a rivet can join together.

HARDNESS - Normally stated in terms of Rockwell or Brinell scale of measurement, hardness shows resistance of a fastener to rough marks and abrasions, can indicate yield strength and brittleness, and has a direct relationship to tensile strength in alloy steel fasteners. However, for stainless, brass, and silicon bronze, the correlation between hardness and tensile or yield is tenuous with no definite relationship.

Case-hardening uses surface heat treatment on ferrous material to cause a harder outside surface than the center. Through-hardening hardens the entire fastener. Bright hardening calls for heat treatment without oxygen, so no oxides are formed on the material surface.

HEAT TREATMENT - Heating often combined with cooling at controlled temperatures in order to strengthen and harden a fastener.

HEX BOLT – Bolt made to the ANSI B18.2.1 spec. having different tolerances than a standart Hex Cap Screw.

HIGH STRENGTH BOLT – Bolt manufactured from high strength material for structural purposes usually referring to ASTM A325 or A490 bolts.

HOT FORGING - Heating metal to red-hot temperatures or temperatures above the recrystallization point to soften it before shaping a fastener. Hot forging is primarily used when the diameter of the metal is to large for cold forming or the quantity required is to small to economically set up a cold-forming machine.

HYDROGEN EMBRITTLEMENT – The condition of a fastener which has had hydrogen introduced into its steel, causing it to be substantially less ductile and prone to sudden and premature failure.

INDUCTION HARDENED – A heat-treated fastener that has undergone a selective hardening process, using induction coils, to further strengthen a part of the fastener.

IN-PROCESS SAMPLING – Random samples of fasteners taken at different process points in the manufacture for testing conformance.

JAM NUT - A thinner nut that is “jammed” against another nut to prevent loosening.

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

KNURL - A rough or decorative surface on part of a fastener.

LEAD - A heavy malleable ductile metal that increases machineability.

LEAD THREAD – The thread length from where it starts to where it becomes full-sized. This distance is usually one-half the fastener diameter.

LEFT HAND THREAD - Opposite of commonly used fasteners. With left hand thread, a nut would be tightened on a bolt by turning it counter-clockwise.

LENGTH OF ENGAGEMENT – The length of full-sized fastener threads that engage in the nut material. The length of the lead thread is not counted in the length of engagement, since its reduced size minimizes any performance benefits. The length of engagement is usually expressed in relationship to the nominal diameter of the screw (e.g. 2 to 2-1/2 diameters of engagement).

LIQUID PENETRANT TEST – Dipping fasteners into a dye and then viewing under ultraviolet light to look for cracks.

LOAD FLANK – The top portion of each thread which applies the preload or load pressure to the mating threads.

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 (i.e. a nylon ring) built into the nut. A free-spinning locknut achieves its locking action when tightened against another surface.

LOT – A particular size of fastener processed from the same raw material heat and same production process.

LOT SAMPLING – Random samples taken from the same lot of fasteners for quality inspection. Maximum lot size for inspection purposes should not exceed 250,000 pieces; thus, lots over 250,000 would require additional samples.

MS - Stands for Military Standards. The overriding characteristic of MS fasteners compared to commercial products is the extensive inspection and lot traceability for MS, guaranteeing the chemical, physical and dimensional qualities. While commercial fasteners may look similar and happen to pass many tests given MS products, the commercial fasteners lack the pedigree of guaranteed quality for chemical, physical and dimensional aspects that users who order MS fasteners rely on.

MACHINEABILITY - Same as free machining. Refers to the malleable characteristics of metal when cutting or forming on screw machines.

MAGNETISM - As related to stainless fasteners, 300 series stainless is non-magnetic in its raw material condition. Cold working can sometimes induce traces of magnetism in 300 series, depending on the severity of cold working and chemical composition of the stainless. A rise in magnetism is related to an increase in tensile strength and work hardening caused by the heat and friction of cold forming and does not reduce corrosion resistance or cause any molecular change in austentic raw material. A higher portion of nickel can increase stability in stainless, thus decreasing work hardening and any possibilities of magnetism. Brass and silicon bronze are non-magnetic.

MAGNETIC PERMEABILITY test simply determines the level of magnetism.

MAJOR DIAMETER - Largest or outside diameter of the screw threads.

MANDREL BREAK LOAD – The axially applied load required to break the mandrel while a rivet is being set.

MATERIAL TEST REPORT – Report verifying specific requirements of a material including chemical analysis, Mechanical testing and smelting country origin.

MATERIAL TRACABILITY – The ability to verify material properties through the entire manufacturing process including original country of origin, heat number assigned to original material, locations and following process documentation.

MILLED FROM BAR(also called MACHINING) - Made on a screw machine or lathe by cutting material away from the original piece of metal. It is used for manufacturing very large diameters which cannot be cold formed and for small quantities where it would not be economical to set up cold forming equipment. However, machining can interrupt the grain of metal causing a lessening in tensile and fatigue strength.

MINOR DIAMETER - The inside or smallest diameter of the screw threads.

MINIMUM TORSIONAL STRENGTH – The amount of rotational force a fastener must endure before failure occurs. This presumes the screw is driven into a proper size hole.

NOMINAL DIAMETER – The major diameter of a screw, or, in tri-round fasteners, the “c” dimension.

NON-FERROUS - Metals without iron. Brass and silicon bronze are non-ferrous; stainless is often characterized as non-ferrous, but it is not non-ferrous.

NUT - An internally threaded fastener manufactured to be used on external(male) threads of a stud or bolt to assemble two components.

NYLON - Light and low in strength compared to metal fasteners, nylon in non-magnetic, good for insulation, and corrosion resistant against many chemicals.

PASSIVATING - A very confusing term, since the common usage has taken on a different meaning than the technical definition. In past experience, users (including engineers) of commercial fasteners seldom mean the old technical terminology. Technically, passivating is not cleaning but is a process of dipping fasteners into a nitric acid solution to rapidly form a chromium oxide on the surface of the material, creating a passive film that protects stainless from further oxidation (see PASSIVE FILM). The purpose of passivating is to remove both grease left from manufacturing and traces of steel particles which may have rubbed off manufacturing tools onto the fastener. In common commercial parlance (meaning non-military and aerospace), passivating means cleaning to users, and the terms “passivating” and “cleaning” are used interchangeably. A wide range of cleaning methods using different mixtures containing nitric, phosphoric and other acids or simply exposing cleaned stainless fasteners to air for a period of time will result in a “passivated” condition. For fasteners that have been properly cleaned, it is impossible to determine the method of cleaning or passivation that was used.

PASSIVE FILM - The major characteristic of stainless is its ability to form a thin layer of protection, called a “passive film”, on its outside surface. This film results from a continual process of low-level oxidation, so oxygen from the atmosphere is needed for the passive film to exist. Once formed, it prevents further oxidation or corrosion from occurring. Even if chipped or scratched, a new passive film on stainless will form.

PENETRATION GAUGE DEPTH – A range of measurements 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.

PICKLING - Removing surface impurities by using chemicals.

PILOT HOLE SIZE – An opening of sufficient size for a specific fastener to be properly installed.

PILOT POINT - Similar to a “B” point, a pilot point is a small (perhaps 1/8” - 1/4”) unthreaded blunt portion at the end of a sheet metal or drive screw.

PITCH - The distance between two adjacent threads measured at the outside diameter of the threads.

PITCH CYLINDER – A volume parallel to the fastener axis whose diameter is equidistant between the major and minor diameters.

PITCH DIAMETER - Approximately in-between the major and minor diameters.

PITTING CORROSION - Pitting indicates deep corrosion in localized spots on a fastener. Dirt or grease on certain portions of a fastener may block oxygen from that surface, thus impeding the passive film which protects stainless from corrosion.

PLATING – The application of a metallic deposit on the surface of a fastener for protective and/or decorative purposes (see plating and coatings chart link).

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.

PRELOAD – The initial load put on a fastener once assembly is complete.

PROOF LOAD - A test load that a fastener must undergo without showing significant deformation. It is usually 90% of yield strength.

QUENCH - To cool suddenly and rapidly after heating.

RADIAL STRESS – Forces that propagate from the fastener towards the outside diameter of the joint.

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.

REDUCTION OF AREA - A measurement like elongation which is related to the tensile strength of a fastener. While elongation measures the length of a fastener stretched to its breaking point compared to its original length, reduction of area measures the diameter of a fastener just before breaking compared to its original diameter.

ROCKWELL HARDNESS TEST – Test designed to measure the hardness of the fastener, based on an alphabetical-numerical scale. The higher the number, the harder the fastner.

Rockwell tests are utilized to test for decarburization and carburization and to determine the amount of resistance to permanent deformation during the testing procedure. They also assure that heat treating was performed to specification.

ASTM standards require socket screw products meet specific Rockwell Hardness Standards. Socket screws are typically in the “C” scale, which is the hardest Rockwell designation, but the scale designation is dependent on the size of the socket screw.

ROLL THREADING - Forming threads on a fastener by pushing or rolling dies against the fastener without any removal of metal. Roll threading, as opposed to cut threading, hardens the material making the threads stronger.

ROOT – The base of the V thread. This is the weakest point on a fastener because it has the smallest cross sectional area.

ROOT DIAMETER - Refers to the minor diameter on screws or the major diameter on nuts.

RUNOUT THREADS – The thread section that is between the last scratch of thread and the fillet or body.

SCALE - A discoloring or oxidation on the surface of hot forged fasteners.

SCREW – An externally threaded fastener which does not require a nut to secure the fastened joint.

SCREW MACHINE - Cutting and removing material in order to form a fastener.

SEAMS – A narrow, non-crystalline discontinuity, which is usually inherent in the raw material. Seams are usually straight or smooth-curved line discontinuities running parallel to the product axis.

SHANK – That portion of a headed fastener that lies between the head and the point.

SHEAR – Force that tends to divide an object along a plane parallel to the opposing stresses.

SHEAR STRENGTH - Measured by the push or pull against the side of a fastener until the fastener breaks (for example, moving an object continually against the side of a screw that is protruding from a wall). As a rule of thumb, shear strength is two-thirds of tensile strength. DOUBLE SHEAR STRENGTH is applying a load against the fastener in two places causing the fastener to break into three pieces.

SOCKET DEPTH – Distance measured parallel to fastener axis from the top of the head to the extreme end of the recess.

SOLUTION ANNEALED - (same as CARBIDE SOLUTION ANNEALED) - A process of heating and removing carbide precipitants (carbon that has broken loose from its stainless steel solution) by heating a finished fastener to over 1,850 degrees F. and cooling it quickly, usually in water, so carbon content goes back into the stainless solution.

STAINLESS STEEL - With the addition of 12% chromium to iron, stainless steel is formed. The chromium protects the iron against most corrosion or red-colored rust; thus the term “stainless” steel. The ability of stainless to form a thin layer of protection on its outside surface, called a “passive film”, is its most important characteristic in preventing corrosion (see PASSIVE FILM).
The overriding purpose of stainless steel is to provide corrosion resistance against: (a) atmospheric conditions such as carbon dioxide, moisture, electrical fields, sulfur, salt, and chloride compounds; (b) natural and artificially produced chemical, (c) extremes of weather where cold temperatures cause brittleness and hot temperatures reduce strength and increase corrosion. For more information, see AUSTENITIC, MARTENSITIC, FERRITIC, and PRECIPITATION HARDENING.

STAMPING - Punching out parts with dies, usually referring to flat washers.

STRAIN HARDENED - To increase hardness and strength by (a) cold working of raw material by a steel mill or (b) cold forming by a fastener manufacturer can sharply increase tensile strength and hardness, so that ordinary material from a steel mill may often be used. However, fasteners that are milled from bar will decrease in strength and hardness, so that raw material would need to be strain hardened by a steel mill before milling fasteners.

STRESS CORROSION - Occurs when corrosion causes a highly stressed part (one that is pushed to its maximum tensile strength) to crack. Except for heat treated 400 series stainless, stress corrosion does not normally apply to austenitic stainless, brass, or bronze, since these metals are relatively ductile and not normally used for high tensile operations.

STUD – A headless fastener with threads at both ends of the shank.

STRUCTURAL BOLT -  Hex Bolt manufactured for structural applications usually to the ASTM A325 or A490 specifications.

TAP - To put internal threads in a hole or in a nut.

TAP BOLT - Fully threaded bolt.

TEMPER - To heat material after hardening to a temperature of perhaps 1000 degrees F. and allow to cool naturally in order to soften material and make it less brittle. Or to heat to a lower temperature of possibly 500 degrees F. to relieve stress in metal without affecting the hardness.

TENSILE STRENGTH - A common measure to compare the strength of a fastener. It is the load needed to pull the fastener apart.

TENSILE STRESS AREA – Selected area or areas used to calculate the tensile strength of an externally threaded fastener, so the fastener strength is consistent with the material strength. IT corrects for the notch and helix effect of the threads and is a function of the pitch and minor diameters.

THREADS - Class 1 threads are a loose tolerance. Class 2 threads comprise 90% of stainless fasteners and are normal commercial tolerance. Class 3 threads have a stricter tolerance and tighter fit such as socket cap and set screws. No definite relationship exists between tensile strength and tightness or looseness of fit. The symbol “A” added to threads, such as 2A, means external threads (screws), and “B” means internal (nuts).
With the exception of 10/32 diameter, which is extremely popular, coarse thread comprises 90-95% of hex head cap screws and hex nuts sold in 18-8 stainless, and perhaps 98% of other stainless items including machine screws and socket products. Coarse threads are deeper than fine threads with fewer threads per inch, so coarse threads may have greater protection against thread stripping, better tap in brittle materials, and better fatigue resistance, while fine threads may have better fit in thin-walled materials, higher torque strength, and increased tightness during vibration.

THREAD ENGAGEMENT – The amount of thread tooth that is filled by the application material. This measurement is usually expressed as a percentage and is used to determine optimal hole size.

THREAD PROFILE – The angle between the flank of the thread and a line perpendicular to the axis.

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.

THROUGH HARDENED – Heat treated fastener with uniform hardness from the surface to the core.

TIGHTENING TORQUE – The amount of rotational force which is approximately midway between a fastener’s “drive torque” and “ultimate torque”.

TIE ROD – Rod manufactured with threads on both ends usually used in pre-cast bridge decks or in assemblies with Turnbuckles or Clevises.

TOLERANCE – The difference between the lower and upper limits between which a fastener’s size(s) must be measured.

TORQUE or TORSION STRENGTH - Torque is the force used in twisting, such as tightening a fastener. Torsion strength is the amount of force needed to twist a fastener apart. Both measures consider the amount of pressure applied to the fastener and the length of the wrench used in the application.

TORSION – Twisting force applied to fastener.

TOUGHNESS - A fastener’s capacity to accept various impacts and shocks.

TUMBLING - To flip fasteners around like clothes in a dryer in order to clean fasteners and increase the shininess of stainless. Soap or a cleansing solution are often added.

UNC – Unified National Coarse Threads

UNF – Unified National Fine Threads

UN, UNR - Indicates “unified” screw threads to “inch” dimensions used in the U.S. as distinguished from metric dimensions.

UNDERSIZE BODY OR REDUCED BODY DIAMETER - Where the shoulder of a fastener equals the pitch diameter or less, which means the shoulder is smaller than the outside diameter of the threads. It would indicate that a fastener was not extruded during its manufacture.

WASHER FACE - A circular rim on the underside of the head of a bolt or on one side of a nut with the purpose of providing a flat bearing surface for the bolt or nut to sit on. A smooth washer face takes away any burrs or imperfections caused by the manufacturing process.

WORKK HARDENED - An increased level of hardness caused by cold forming fasteners.

YIELD - The resistance to a load pulling on the middle of a fastener until the fastener shows permanent deformation.

YIELD STRENGTH - The amount of pressure required to cause permanent deformity. When a fastener is stretched, yield strength is the point where the fastener will not return to its original length following testing. It is measured in terms of pounds per square inch (psi) or megapascals (MPa). Yield strength is often determined by the offset method.