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How Fasteners are Identified

General

Fasteners are identified by four or five attributes; Type, Material, Diameter, Length, and Thread Pitch or Count (machine thread fasteners only). This page describes what these attributes represent. Information on how they are written can be found on our How Fastener Measurements Are Notated page. Also for information on fastener abbreviations please see our Common Fastener Abbreviations page.

Note: With some fasteners, the terms bolt and screw are frequently used interchangeably.

Type

Fastener type can be divided into information on the Category or kind of fastener, Drive Type, and Head Style.

Category
Fasteners are divided into categories based on their function or design. For help identifying different fastener categories see our Fastener Type Chart.
Ex. Wood Screw, Sheet Metal Screw, Hex Bolt, Lag Bolt, etc.

Drive Type
Fasteners in some categories are available with different drive types such as Philips or Slotted. In other categories such as lag bolts there is either only one drive type or the drive type is implied to be of a standard type. For help identifying different fastener drive types see our Fastener Type Chart.
Ex. Philips, Slotted, Allen (a.k.a. Socket), Frearson (similar to Philips), etc.

Fastener Type Standard or Implied Drive
Bolt Hex Head
Lag Bolt/Screw Hex Head
Carriage Bolt/Screw No Drive
Socket Bolt/Screw Allen Drive
Button Head Allen Drive

 

Head Style
Many categories are also available with different head shapes or styles. For help identifying different fastener head styles see our Fastener Type Chart.
Ex. Flat head, Pan head, Truss head, etc.

Material

Fastener material describes the material from which the fastener was made as well as any material grade. For help identifying different fastener materials please see our About Fastener Materials and Grades page.
Ex. Stainless Steel, Zinc Plated Steel, Silicon Bronze, etc

Diameter

Fasteners diameter is measured either as a size number or as a direct measurement. For information on how to measure fastener diameter please see our Measuring Fastener Diameters page.

Length

How fastener length is measured varies based on the type of head. As a general rule, the length of fasteners is measured from the surface of the material, to the end of the fastener.

For fasteners where the head usually sits above the surface such as hex bolts and pan head screws, the measurement is from directly under the head to the end of the fastener.

For fasteners that are designed to be counter sunk such as flat head screws, the fastener is measured from the point on the head where the surface of the material will be, to the end of the fastener.

For more information please see our Measuring Fastener Length page.

Thread Pitch or Count

Thread pitch or thread count is used only on machine thread fasteners (those that take a nut or thread into a tapped hole). The thread pitch or count describes how fine the threads are. For more information please see our About Thread Pitch and TPI page.

 

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