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Common Fastener Related Terminology

Term Definition
Acorn Nut Also referred to as a cap nut. This type of nut has a domed top that covers the end of the bolt.
Angle of Head Used in reference to countersunk fasteners. This is the angle from one side of the cone to the other. Standard measurement is 82 degrees. Special 100 degree heads are also made.
Backing Off Loosening of a nut over time by spinning back up the thread.
Bearing Surface The area of a fastener that caries a load. Washers are commonly used to increase the bearing surface.
Body The smooth part of the bolt above the threads. Also called the shank.
Body Diameter The diameter of the body or smooth part of the bolt above the threads. Also shank diameter or major diameter.
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 Also referred to as an acorn nut. This type of nut has a domed top that covers the end of the bolt.
Cap Screw A term used to describe a machine bolt or machine screw. Usually preceded by a head style as in 'Hex cap screw'.
Carriage Bolt A bolt with a smooth rounded head and a small square section under the head to prevent spinning during assembly. Used in wood.
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). Also in US fasteners, a specification describing how tightly the fastener fits.
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 A folded pin with a loop at one end designed to have the other end bent to hold it in place.
Countersunk 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).
Dog / Dog Point An unthreaded cylindrical tip that is smaller than the fastener diameter. Used to help speed alignment during assembly. Also called a Pilot Point
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 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 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 A washer designed for use with countersunk screws. Used to enhance appearance in some applications.
Flange Bolt A bolt with a built in washer-like flange just below the head.
Flat Head A flat head with a conical bearing surface. 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 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 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 A thinner pattern of nut, sometimes used as a second nut to 'jam' another nut in place preventing loosening.
Knurl A pattern of ridges on the surface to provide grip.
Left Hand Thread Thread that is formed opposite to normal thread so it tightens counter clockwise. Used on spinning equipment to prevent loosening. Generally custom made.
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.
Major Diameter The diameter from the top of the thread on one side, to the top of the thread on the other side.
Nylock Nut A nut with a nylon insert to help prevent backing off.
One way screw A fastener designed so it can be installed with a slotted screw driver but can not be removed. Used to prevent tampering.
Oval Head A countersunk screw with a slightly rounded top surface for a more finished look.
Pan Head 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 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.
Roll Thread Threads formed by tightly pressing or rolling the piece of stock between serrated dies. This is the most common method of creating threaded fasteners today.
Set Screw A machine screw with no head usually used to secure parts on a shaft.
Shank The smooth part of the bolt above the threads. Also called the body.
Shank Diameter The diameter of the shank or smooth part of the bolt above the threads.
Slotted 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. Must be followed with a regular tap.
Stove Bolt Machine screws, generally slotted, packaged with nuts.
Stud A double ended bolt or piece of threaded rod. Often used in machine and automotive applications.
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 The 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 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 describe the number of threads per inch for a given diameter.
Threaded Fastener Any fastener with threads such as a screw or bolt.
Through Hardened A bolt that has been heat treated all the way through. More brittle but stronger than a Case Hardened bolt. Also called Fully Hardened.
Truss Head An extra wide low profile head with a slightly rounded top surface.
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 frequently in extremely short screws to provide more 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 British thread standard that is no longer in use.
Wing Nut A nut with 'wings' for easy manual assembly.
Yield Strength The maximum load at which a material exhibits a specific permanent deformation.

 

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