Fastener Glossary of Terms

AISI – American Iron and Steel Institute

Alloy – Mixture of two or more metals or of a metal and/or other elements to improve certain qualities of the material being produced.

Allowance – The intentional space manufactured into mating threads.

American National Standards Institute (ANSI) – Established the “Unified Screw Thread Standards” which regulates the fastener thread industry.

Angle of a Thread – The angle between the flanks. Measured in an axial plane, or cross-section.

Anode – The material in a corrosive attack that becomes corroded.

Chamfer – Two surfaces that meet at an angle other than 90 degrees.

Coarse Thread – A wider thread usually having a higher stripping strength, faster assembly and better fatigue resistance.

Clamp Load / Compression Load – The resulting force that holds the two materials together by tightening the bolt to its mating part and clamping the two materials together.

Class 1A/1B Threads - These threads have a loose tolerance fit. Class “A” threads are external, while class “B” threads are internal.

Class 2A/2B Threads – These threads have a good thread tolerance that balances manufacturing costs with performance. Class “A” threads are external, while class “B” threads are internal.

Class 3A/3B Threads – These threads are made to an extremely close tolerance. Class “A” threads are external, while class “B” threads are internal.

Corrosion – The wearing away or alteration of a metal by an electrochemical reaction or a direct chemical attack.

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

Drive – The feature which allows a screw to be turned. It is where a tool is inserted to tighten or loosen the fastener into either a material or a mating fastener.

Drive, Allen –  Used in areas where secure holding of parts is required. The fasteners with the Allen type recess are usually hardened to offer the required performance.

Drive, Clutch – A slot-type system that is uncommon. The enclosed ends eliminate driver slide out. It is available in two variations: with or without the center opening.

Drive, Combination – Accepts screwdrivers with either Phillips or slotted drives.

Drive, Hexagon or Hex – Is the original of its type and is usually used on bolts or set screws. Sockets and open and closed wrenches are generally used to turn hex head fasteners.

Drive, One-Way – A tamper-resistant drive that ensures resistance to tampering by being able to be removed only by a special tool.

Drive, Phillips or Cross-Point – Offers better performance than the slotted head style because the number of contact points between the tool tip and the recess in the fastener head is doubled.

Drive, Posidriv or Supadriv – An improvement on the Phillips/cross-point head which offers better performance due to the contact points being effectively doubled again, further reducing stripping.

Drive, Security Tri-lobe – A high-torque drive that prevents normal Torx™ drives from being used on it.

Drive, Slotted or Straight-Headed – The oldest and by far the simplest drive available. The slot gives a clear indication of the position of the adjusting screws, but the drive is very prone to stripping and slipping.

Drive, Square – Also known as the Scrulox® or Robertson Drive® it is recognized by ANSI/ASME/IFI as Type III. It has poor off-angle capability and shallow head performance, but is widely used in Canada and is favored by the marine and furniture industries. It is sometimes used as part of a combination drive with a slotted or Phillips drive.

Drive, Torx™ – Offers the highest contact point area of all the fasteners mentioned here. It has very high torque capability and is widely used in automotive applications. Generally known as the Six-lobe or T-socket drive.

Effective Diameter – See Pitch Diameter

Fatigue Strength – The maximum stress a fastener can withstand for a specified number of repeated load cycles prior to its failure.

Fine Thread – A type of smaller thread that is stronger under tension, permits closer adjustment accuracy and is less likely to loosen than coarse-threaded screws.

Flanks – The straight sides of the thread that connect the crest and the root.

Fracture Point – The point at which failure occurs and a fastener under stress breaks.

Grade – A rating in a scale classifying fasteners according to strength.

Hardness – A measure of a material’s ability to resist abrasion and indentation.

Head – The head is usually the larger end of the fastener containing a drive of some sort.

Head, Binding Undercut – Has a rounded top surface and slightly tapered sides. The bearing surface is flat with an undercut cavity that allows for a firm electrical connection.

Head, Fillister – Has a rounded top surface, cylindrical sides and a flat bearing surface. The greater side height is what distinguishes it from a Pan Head. It is the preferred head style for use in counter-bored holes.

Head, Flanged – Combines a washer and a pan head into one unit. The resulting fastener head is lower in height and provides a greater bearing surface.

Head, Flat – Has a countersunk flat head with a cone-shaped bearing surface of 82° (90° for metric), 100° or 120°. Used in applications where the protrusion of the fastener above the mating surface is unacceptable.

Head, Hexagonal or Hex – Has a flat head, six flat sides and a flat bearing surface.

Head, Oval or Raised Countersunk Head – Is functionally the same as the flat countersunk variety and offers deeper and therefore stronger recess that is neater in appearance.

Head, Pan – Has a flat or rounded top surface, cylindrical sides and a flat bearing surface. Has a general-purpose bearing area and can be substituted in most applications for round, truss or binding heads.

Head, Round – Has a semi-elliptical top surface and a flat bearing surface. It is sometimes preferred over pan heads for its smooth surface and appearance.

Head, Truss – Has a low, rounded top surface with a flat bearing surface greater in area than a round head fastener of the same nominal size. Weaker than a pan or round head fastener, but preferred in applications where minimal clearance exists above the head. The profile provides a trim, finished assembly.

Hot Dip Galvanizing – A process to coat fasteners that involves dipping the fastener in a prepared item bath of molten zinc.

Metal Spray (Metalizing) – The various processes of applying zinc, as well as other plating materials, to the fastener surface by means of a spray or blast of molten or semi-molten metallic particles.

Pitch – See Thread Pitch

Pitch Diameter – The diameter that lies halfway between the major and the minor diameters, or the root of an internal thread.

Plating – The process of depositing an adherent metal onto the surface of a base metal.

Point – The point of the fastener is the end designed to thread into a mating fastener part, drill itself into a material or tighten against another material.

PSI, Pounds Per Square Inch –  A measurement often used as a common standard in comparing the strength of metals and applied pressures.

Robertson Drive® - See Drive, Square

Rolled Thread – A thread produced by the action of a form tool which presses the material and radially displaces it to form threads.

Scrulox® - See Drive, Square

Shank – The un-threaded portion of the fastener between the threads and the head.

Shear Strength – The maximum load that can be supported prior to fracture when applied at a right angle to the fastener’s axis.

Shoulder – An enlarged portion of the body of a threaded fastener, or the shank of an un-threaded fastener.

Stripping – To damage or break the threads of a screw, nut, bolt, or the teeth of a gear.

Tensile Strength – The ultimate strength of a given material, alloy or product that determines how much load it can withstand before breaking.

Thread Forming Screw – A screw with a type of thread that allows it to create mating threads into the surface that it is being screwed into.

Thread Pitch – The distance between corresponding points on adjacent threads. Used to designate threads in the metric system.

Thread Tolerance – The slight variation between the theoretically perfect threads and the unit of actual production that is allowed and expected. The total amount by which a specific dimension is permitted to vary.

Torque – A twisting force that causes rotation of a shaft. Generally expressed in foot pounds or inch pounds.

Torx™ - See Drive, Torx™

TPI, Threads Per Inch – The number of threads contained in one inch of a threaded fastener’s length. Used to designate threads in the unified system.

Ultimate Tensile Strength – The ultimate point at which a fastener can withstand load before breaking.

Fastener Components

Point - The point of the fastener is the end designed to thread into a mating fastener part or to drill itself into a material that is opposite the drive end.

Thread - The thread is the inclined plane that wraps around the cylinder of the fastener.

Runout - The portion at the end of a threaded shank that is not cut or rolled to full depth, but produces a transition between full depth threads and the fastener shank or head.

Standard Bolt Grade Markings

The rule of thumb for bolt grade 2 through 8 is to the number of lines on the head plus two.  Here are some common bolt grade markings-

Grade 2

Grade 5

Grade 8

Grade 9