A large list of common fastener related terms. Use Ctrl+F to open a browser pop-up box that you can search words you are looking for in.
“A” Sheet Metal Screws
Sheet metal screws with a sharp-pointed ends. Type A is similar to Type AB, but Type A has fewer threads per inch than type AB screws and has deeper threads that result in better gripping power than type AB screws.
“AB” Sheet Metal Screws
Sheet metal screws with pointed ends like type-A screws and thread dimensions similar to type-B. See above.
A nut with a slightly pointed domed top that covers the end of a bolt. Acorn nuts are sometimes confused with cap nuts which are domed but lack a point.
Angle of Head
Used in reference to countersunk fasteners. This is the angle from one side of the cone to the. Standard US screws have a head angle of 82°where standard metric countersunk screws have a head angle of 90°.
American National Standards Institute.
American Society for Testing and Materials.
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
The most abundant metal in the earth, aluminum is blueish and silvery-white, very light, malleable, and ductile with high heat and electrical conductivity. It is non-magnetic and one-third the weight of steel with good corrosion resistance against certain chemicals and acids but weak resistance against other elements such as sea water.
to heat and then cool (a material, such as steel or glass) usually for softening and making less brittle
“B” Sheet Metal Screws
Referring to sheet metal screws, type-B indicates a blunt point with more threads per inch and smaller thread depth than type-A screws.
The part of a fastener such as the washer face of a nut or under the head of a machine screw that actually meets the part it fastens.
A small slant, usually describing a flat washer which is square and thicker on one side than the other. The slant of the beveled washer can offset a slanted surface, so that a bolt going into the beveled washer may be parallel to the floor or ground.
Old term for pan head, “binder” has now come to mean “binding” head screws rather than pan.
A fastener where one or two stages of manufacturing have been performed, but the fastener has not been finished.
An exterior finish commonly used on fasteners that has a basic protection from corrosion and abrasion resistance. Mostly found on steel fasteners but you will often find stainless steel fasteners offered with a black oxide option.
The smooth part of a fastener above the threads. Also called the shank. This is measured from the bottom of the head to the start of the transition to threads
The diameter of the body or smooth part of the bolt/screw above the threads. Also called shank diameter.
When a driver bit slips out of the drive of a fastener during installation once proper torque has been met. Repeated cam out can lead to stripping of the drive. Most commonly occurs on Phillips drive fasteners.
A nut with a domed top that covers the end of the bolt. Cap nuts are sometimes confused with acorn nuts which are also domed but come up to a point.
Cap Screw, Cap Bolt
Refers to a hex head cap screw. This is a hex bolt with a circular washer face under the head. This does not extend past the edge of the hex as a flange bolt would.
In most cases there is no operational difference between a hex head cap screw and a
Where the shoulder of a screw is perceptibly smaller in diameter than the threaded portion (technically the minor diameter or less).
Ordinary steel with no significant additions besides carbon.
A bolt with a smooth rounded head. Usually found with a square neck. This is a small square section under the head that prevents spinning during assembly. Carriage bolts are primarily used in wood.
A hex nut with a slightly reduced slotted cylindrical section on one end. Used with a cotter pin and drilled fastener to prevent loosening.
Certificate of Compliance
A certification that a fastener meets the description or standard to which it was sold.
A slight rounding on the end of a fastener or the edges of a hex nut for ease of assembly or smoother appearance.
Old term for fillister head in United States; like fillister in metric sizes.
Used in metric, class is a material designation equivalent to the US term grade
A pin with a head on one end and one or more drilled holes for a cotter pin.
Coarse Threads (UNC, Metric Coarse)
Coarse threads are those with larger pitch (fewer threads per axial distance), and fine threads are those with smaller pitch (more threads per axial distance). Coarse threads have a larger threadform relative to screw diameter, where fine threads have a smaller threadform relative to screw diameter.
Cold Forming / 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.
A fastener with a full cone shaped point on the end. Often used in automated assembly to guide the fastener into the hole.
A reddish metal that is an excellent conductor of heat and electricity. It is malleable, ductile, and non-magnetic with low to average strength and good corrosion resistance. Brass and silicon bronze, composed mainly of copper, gain their strength from the addition of other metals, such as silicon.
A folded pin with a loop at one end designed to have the other end bent to hold it in place.
A fastener head that sits at or below the surface of the material, such as a flat or oval head.
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.
A cone shaped end with a circular depression in the center. The contact area is thus a circular ridge. This is the most common point for set screws.
Forming threads on a fastener by cutting away and actually removing the unneeded metal.
To remove chips, burrs, or other imperfections through a secondary operation such as grinding.
A tool for cutting external threads into a rod.
Deutsches Institut für Normung, the German standards body. In reference to fasteners DIN indicates fasteners that conform to a specific metric standard.
A variety of small or large disfigurations in a fastener such as pits, tool marks, voids, laps, folds and inclusions. Minor discontinuities are permissible.
Dog / Dog Point
An unthreaded cylindrical tip that is smaller than the fastener diameter. This helps speed alignment during automated assembly.
Where raw material shaped like wire is pulled through a die to reduce its diameter to that needed for the fastener being manufactured.
The ability of a fastener to deform before breaking. Ductility is a measurement like 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 increase.
A chemical process that cleans and brightens aluminum after heat treatment.
A bolt with a circular ring on the head end. Commonly used for attaching rope or chain to the eye.
Threads on the outside of a fastener which differ from the counterpart, Internal Thread that is common with nuts and sex bolts.
Metal failure due to stresses that push first in one direction and then another.
Measures the endurance of a fastener by showing the load it can accept without breaking under repeated load cycles.
Less common but similar to the Pan Head style but with a smaller head diameter and higher vertical sides.
Fine Threads (UNF, Metric Fine)
Thread that are more tightly spaced than coarse threads. Advantages over coarse threads are being slightly stronger, less prone to vibrate loose and allows finer adjustment. Disadvantages generally fall in them being more prone to cross threading and as such take longer to assemble.
Normally referring to threads, fit is a measure of 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 (Thread Galling / Cold Welding / Seizing)
Occurs when female and male threads are placed under heavy pressure. There are differing degrees that this can happen, but in extreme cases this results in an extreme cold-welding process that is nearly impossible to undo as both screw and nut are fused or seized together. Stainless Steel fasteners are particularly susceptible to this issue but generally galling can be avoided by 1. use of a Thread Lubricant (lowers friction) 2. use coarse thread fasteners instead of fine (lowers friction) 3. using slower wrench driving speeds (reduce heat) 4. avoiding prevailing torque locknuts (reduce friction), and 5.use clamps instead of relying on fasteners to draw two materials together as this again puts heavy pressure on both nut and bolt adding to friction and heat.
An accelerated degree of corrosion occurring when two different metals are in contact with moisture, particularly sea water. 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.
The unthreaded part of a fastener.
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.
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.
Heating often combined with cooling at controlled temperatures to strengthen and harden a fastener.
Hex caps should not be confused with socket caps.
Hi Lo / High-Low screws
These screws have a double lead thread where one thread is high and the other is noticeably lower. This allows for easier penetration into the substrate or material. The high thread is sharper than a conventional thread. The sharp high thread causes less displacement of material when it is driven into the substrate. Consequently, it requires less effort to drive into material. In addition, the greater amount of material remaining in between the high threads, the smaller minor diameter thread increases contact with the substrate resulting in a stronger grip and increased resistance to pull out and vibrational loosening forces.
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 too large for cold forming, or the quantity required is too small to economically set up a cold-forming machine.
Hydrogen trapped under the surface of a fastener can later cause ruptures. It is generally associated with carbon and alloy steels, not stainless. There may be no external signs of corrosion before a break occurs.
Industrial Fasteners Institute.
Random samples of fasteners taken at different process points in the manufacture for testing quality and conformance.
Stands for International Organization for Standardization.
A thinner nut that is “jammed” against another nut to prevent loosening.
A rough or decorative surface on part of a fastener.
Knurled Cup Point
Serrated ridges on the point of a set screw that bite into the contact material to prevent unintentional loosening.
Opposite of commonly used fasteners. With left hand thread, a nut would be tightened on a bolt by turning it counterclockwise.
Lock Nut (locknut)
Any nut that is designed to help prevent backing off (loosening) of the nut after installation. Common types include Nylon Insert, Prevailing Torque and Serrated Flange Locknut types. All of these types are listed on this page in more detail.
A particular size of fastener processed from the same raw material heat and same production process.
Random samples taken from the same lot of fasteners for quality inspection.
Low Head Socket Cap - DIN 7984
Designed to be used in situations where socket head height is a problem. Because of their reduced head height and smaller internal socket size, they cannot be preloaded (e.g., tightened) as high as a standard socket head cap screws even though they are made from the same materials.
The outside, or largest diameter of the screw threads.
Milled from Bar (Machining)
Made on a screw machine or lathe by cutting material away from the original piece of metal. Generally 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.
Minor Diameter (see also – Major Diameter)
The inside or smallest diameter of the screw threads.
The chemical element for Iron is ‘Fe’ which comes from the Latin Ferrum. Non-Ferrous simply means without iron. Brass and silicon bronze are nonferrous fasteners.
Light and low in strength compared to metal fasteners, nylon is non-magnetic, good for insulation, and corrosion resistant against many chemicals.
Nylon Insert Locknut (Nyloc / Nylock)
A prevailing torque type of locking nut that uses a a nylon collar to increase friction on the screw thread. The nylon collar insert is placed at the end of the nut, with an inner diameter slightly smaller than the major diameter of the screw. The screw thread does not cut into the nylon insert, however, the insert deforms elastically over the threads as tightening pressure is applied. Especially useful for situations that involve vibrations.
A countersunk screw with a slight rounded top surface. Oval heads are one of the more troublesome screws to measure length on as they follow rules that are unique to them. Oval head screws are measured from the widest part of the head to the tip of the screw. In practice, the part of the head that extends beyond the countersunk screw is not measured for length.
Old term for truss head.
A head with a slightly rounded top surface and short vertical sides.
Passivation / Passivating
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 greases left from manufacturing and traces of steel particles which may have rubbed off manufacturing tools onto the fastener.
A non-metallic substance that lowers the rate of oxidation, thereby helping resist corrosion.
Removing surface impurities by using chemicals.
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.
The distance between two adjacent threads measured at the outside diameter of the threads.
Approximately in-between the major and minor diameters.
Prevailing Torque Lock Nut
Locking nuts that are designed to deform or provide friction against the bolt to stay in place once property tightened or not self-loosen. The two main types of Prevailing Torque Lock nuts are Stover (all metal) and Nylon Insert lock nuts.
A test load that a fastener must undergo without showing significant deformation. It is usually 90% of yield strength.
To cool suddenly and rapidly after heating. Often used as part of the hardening process for fasteners.
Forming threads on a fastener by pushing or rolling dies against the fastener without any removal of metal. Roll threading differs from cut threading as thread rolling hardens the material making the threads stronger.
Refers to the minor diameter on screws or the major diameter on nuts.
Society of Automotive Engineers.
A discoloring or oxidation on the surface of hot forged fasteners.
A machine screw with no external head, usually using a hex or flat head drive. Set screws can have multiple point types, but the cup point is generally the standard set screw point. Other points include, Cone, Knurled, Dog, Oval and Flat points. Set screws are usually used to secure parts on a shaft, but you are probably more used to seeing set screws used installing bathroom towel racks and the like.
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.
Nut with slots cut into it. Generally used with a cotter pin and a drilled shank fastener to act as a locking mechanism.
The smooth, or unthreaded part of a screw or bolt.
Diameter of the shank, or unthreaded part of a bolt. Generally the Shank Diameter is slightly larger than the major thread diameter of the screw or bolt.
Square Nut / Square Bolt
Bolt (or Nut) with four sides.
Punching out parts with dies, usually referring to flat washers.
Usually used for starting internal threads on very hard metals. Starter taps must be followed by traditional taps after to create functional threads.
Old term for truss head.
one piece, all-metal prevailing torque hex nut with a conical top that is engineered to slightly deform when installed correctly. The cone shape makes top threads of the nut apply more inward pressure than the bottom threads of the nut causing them to 'bite' into the threads on the mating part. Not recommended for installations where there are long thread assemblies as the extended thread travel can damage the natural locking feature.
Fully threaded Rod, or double ended bolt (middle unthreaded). Studs may have different threads on each end.
To put internal threads in a hole or in a nut.
Fully threaded bolt.
To heat material after hardening to a temperature of perhaps 1000 degrees F. and allow to cool naturally 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.
A common measure to compare the strength of a fastener. It is the load needed to pull the fastener apart.
The percentage of the thread height that is in the material being fastened
Any fastener with threads such as bolts and screws.
A strong, lightweight silvery gray metal with high corrosion resistance against salt waters, chlorides, and many acids.
The force used in twisting, such as tightening a fastener.
An extra wide head with a rounded top surface and a low profile.
To flip fasteners around like clothes in a dryer to clean fasteners and increase the shininess of stainless. Soap or a cleansing solution are often added.
A bolt in the shape of a U that is threaded on both ends.
UN (UNC, UNF)
Indicates “unified” screw threads to “inch” dimensions used in the U.S. as distinguished from metric dimensions. Common uses are UNC (Coarse) and UNF (Fine) thread patterns.
A countersunk head that has been cut off below their normal height. Used in very short countersunk screws to provide enough thread.
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.
nut with 'wings' for easy manual assembly.
An increased level of hardness caused by cold forming fasteners. Too much work hardening may cause a slight degree of magnetism in austenitic stainless.
The resistance to a load pulling on the middle of a fastener until the fastener shows permanent deformation.
The maximum load at which a material exhibits a specific permanent deformation.