Common Fastener Related Terminology

Term Definition
Acorn Nut Acorn Nut 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 Head Angle Used in reference to countersunk fasteners. This is the angle from one side of the cone to the other (A in the illustration). Standard US screws have a head angle of 82°. Special 100° heads are also made. Standard metric countersunk screws have a head angle of 90°.
Backing Off Loosening of a nut over time by spinning back up the thread.
Bearing Surface The area of a fastener that carries a load. Washers are commonly used to increase the bearing surface.
Body Hex Bolt Sections 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.
Body Diameter Hex Bolt Diameter The diameter of the body or smooth part of the bolt/screw above the threads. Also called shank diameter (S in the illustration).
Bottom Tap A special tap for finishing the threads to the very bottom of a hole that does not pass through the material. A regular tap must be used first.
Cap Nut Cap Nut 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 Hex Cap Screw vs Hex 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 (left) and a hex bolt (right).
Hex caps should not be confused with socket caps.
Carriage Bolt Carriage Bolt A bolt with a smooth rounded head. The vast majority of carriage bolts have a square neck. This is a small square section under the head that prevents spinning during assembly. Carriage bolts are primarily used in wood.
Castle Nut Castle Nut A hex nut with a slightly reduced slotted cylindrical section on one end. Used with a cotter pin and drilled fastener to prevent loosening.
Class Used in metric, class is a material designation equivalent to the US term Grade (ex. Class 10.9).
In US fasteners this refers to how closely the threads fit on machine threaded fasteners (ex. Class 2A threads). This usage is rare as few applications call for non-standard thread fit.
Clevis Pin A pin with a head on one end and one or more drilled holes for a cotter pin.
Cone Point A fastener with a full cone shaped point on the end. Often used in automated assembly to guide the fastener into the hole.
Cotter Pin Cotter Pin A folded pin with a loop at one end designed to have the other end bent to hold it in place.
Countersunk Example Countersunk Head Example Countersunk Head A fastener head that sits at or below the surface of the material, such as a flat or oval head.
Cup Point 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.
Cut Thread Threads formed by cutting into a piece of stock. See also Roll Thread.
Die A tool for cutting external threads into a rod.
DIN An abbreviation for Deutsches Institut für Normung, the German standards body. In reference to fasteners DIN indicates fasteners that conform to a specific metric standard (will be followed by the standard number for example DIN 933).
Dog / Dog Point An unthreaded cylindrical tip that is smaller than the fastener diameter. This helps speed alignment during automated assembly. Also called a Pilot Point
When replacing a screw, the dog point is usually not important (with the exception of set screws).
Dowel Screw A double ended screw with wood threads and points on both ends.
Drilled Head A fastener head with a hole drilled through it for a safety wire.
Drilled Shank A fastener shank with a hole drilled through it for a pin or safety wire.
Elevator Bolt Elevator Bolt A bolt with a large flat disk on the top and a square section underneath, resulting in a flush finish.
Engagement A measurement of how much of the fastener is in the material being fastened. See Length of Engagement and Thread Engagement.
External thread Thread on the outside of a cylindrical piece of stock.
Eye Bolt Eye Bolt A bolt with a circular ring on the head end. Used for attaching rope or chain.
Fillister Head Similar to a pan head but with a smaller head diameter and a taller head, with higher vertical sides.
Finishing Washer Finishing Washer A washer designed for use with countersunk screws. Used to enhance appearance in some applications.
Flange Bolt Hex Flange Bolt A bolt with a built in washer-like flange just below the head.
Flat Head Flat Head Profile A head with flat top and a conical bearing surface (bottom). Designed to be Countersunk.
Flex Lock A type of lock nut where the top of the nut has been slotted and bent inward. When assembled the pressure on the threads prevents the nut from backing off.
Hanger Bolt Hanger Bolt A double ended bolt with machine threads on one end (takes a nut) and wood threads on the other end.
Hardened A fastener that has been heat treated to increase strength. Grade 5, Grade 8 and alloy steel bolts are hardened.
Head Style The type of head or top that is on the fastener. Such as a Round head or Hex head.
Heli-Coil A system for re-threading stripped internal threads. A proprietary tap is used to re-thread the hole and a spring like insert is placed inside to make up the difference in width, allowing the original size bolt to be used.
Hex Bolt Hex Bolt A bolt with a six sided head.
Hex Head A head with six vertical sides and a flat bearing surface.
Internal Thread Thread on the inside of a cylindrical hole.
Jam Nut Hex Jam Nut A thinner pattern of nut, sometimes used as a second nut to 'jam' another nut in place preventing loosening.
Knurl Knurled Head Socket Cap Screw A pattern of ridges on the surface to provide grip, such as on the head of many socket cap screws (see illustration).
Left Hand Thread Thread that is formed opposite to normal thread so it tightens counter clockwise. Left hand thread is used on spinning equipment to prevent loosening. Left hand thread is extremely uncommon.
Length of Engagement The length of the part of the fastener where the threads are engaged (in) the material being fastened.
Lock Nut Any nut with a special design to help prevent backing off (loosening).
Major Diameter Hex Bolt Diameter The diameter from the top of the thread on one side, to the top of the thread on the other side (T in the illustration).
Nylock Nut Hex Nylon Lock Nut A nut with a nylon insert to help prevent backing off. Short for Nylon Insert Lock Nut.
One way screw One Way Screw Head A fastener designed so it can be installed with a slotted screw driver but can not be removed. Used to resist tampering.
Oval Head Oval Head Profile A countersunk screw with a slightly rounded top surface, used for their looks.
Pan Head Pan Head Profile A head with a slightly rounded top surface and short vertical sides.
Pilot Point An unthreaded cylindrical tip that is smaller than the fastener diameter. Used to help speed alignment during automated assembly. Also called a Dog Point.
Pitch The distance from one thread to the next along the length of the fastener. Used mostly in metric, where it is expressed in millimeters.
Plow Bolt A bolt with a smooth flat countersunk head that has a small square section underneath. Used on plows.
Reverse Thread Thread that is formed opposite to normal thread so it tightens counter clockwise. Used on spinning equipment to prevent loosening.
Also called left hand thread.
Roll Thread Threads formed by tightly pressing and rolling a piece of stock between serrated dies. This is the most common method of creating threaded fasteners today.
Root Diameter Hex Bolt Diameter The diameter from the bottom of the thread on one side, to the bottom of the thread on the other side (R in the illustration). Also known as Minor Diameter.
Set Screw Set Screw A machine screw with no head, usually used to secure parts on a shaft.
Shank The smooth part of a fastener above the threads. Also called the body.
Shank Diameter Hex Bolt Diameter The diameter of the shank or smooth part of a fastener above the threads (S in the illustration).
Slotted nut Slotted Hex Nut A nut with slots cut into it for the insertion of a cotter pin. Used with a drilled shank fastener. Similar to a castle nut.
Square Bolt A bolt with a four sided head
Starter Tap A special tap for starting the creation of internal threads in very hard metals. Must be followed with a regular tap.
Stove Bolt An older term for machine screws, generally slotted, packaged with nuts.
Stud A double ended bolt or piece of threaded rod. Often used in machine and automotive applications. Studs may have different threads on each end.
T nut T Nut A thin nut with a flange on one end. Intended to be inserted in a hole, in wood, with the flange on the surface. Used to allow a bolt to be fastened in wood without a protruding nut.
Tap A tool used to create internal threads in a hole, or the act of creating such threads. See also Bottom Tap, and Starter Tap.
Tap Bolt Tap Bolt A fully threaded bolt.
Tensile Strength The maximum load in tension (pulling apart) that a bolt can withstand before failure.
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 refer to a standard set of thread pitches and diameters. For example the UNC thread series specifies course thread for US standard US bolts.
Threaded Fastener Any fastener with threads, such as a screw or bolt.
Truss Head Truss Head Profile An extra wide low profile head with a slightly rounded top surface.
U Bolt U Bolt A bolt in the shape of a U, threaded on both ends.
Undercut Head A countersunk head that has been cut off at 70% of the normal height. Screws in this style will have a second flat surface parallel to the top just before the threads. Used in very short countersunk screws to provide enough thread.
Unified Coarse Thread (UNC) The standard US thread pattern.
Unified Fine Thread (UNF) The standard US thread pattern for fine thread.
Whitworth / Whitworth Thread A now obsolete British thread standard, sometimes found in old cars or machinery.
Wing Nut Wing Nut A nut with 'wings' for easy manual assembly.
Yield Strength The maximum load at which a material exhibits a specific permanent deformation.