Fastener Basics

Common Fastener Types

Hex bolts, or hex cap screws, are used in machinery and construction. Can be used with a nut, or in a tapped hole. Fully threaded hex bolts are also known as tap bolts
Wood screws have large threads and a smooth shank f or pulling two pieces of material together. They can be used in wood and other soft materials.
Sheet metal screws have sharp points and threads, and are designed to be driven directly into sheet metal. They can also be used in softer materials like plastic, fiberglass, or wood.
Machine screws are fully threaded for use with a nut or in a tapped hole. Certain types are some- times referred to as stove bolts.
Socket screws are machine screws with an internal hex socket (Allen) drive. Longer lengths may have a smooth shank.
Lag bolts, or lag screws, are large wood screws with hex heads. Typically used for wood construction and landscaping.
Carriage bolts have smooth, domed heads with a square section underneath that pulls into the material to prevent spinning during installation.
Nuts are used to fasten machine threaded fasteners in through-hole applications. Lock nuts help prevent loosening.
Washers spread the load over a greater surface area when tightening a bolt, screw or nut. Lock washers help preventing loosening.

Tip: See our full Fastener Type Chart for a more comprehensive list of fastener types.

Grade/Class & Fastener Strength

Fastener Grade (US) or Class (metric) refers to the mechanical proper- ties of the fastener material. Generally, a higher number indicates a stronger, more hardened (but also more brittle) fastener.

Tip: See Bolt Depot's Grade Markings and Strength Chart for a more comprehensive chart of fastener grades, head markings and mechanical properties.

US Bolt Head Markings

Low carbon

Grade 5

Grade 8

Metric Bolt Head Markings

Class 8.8

Class 10.9

Class 12.9

Fastener Materials

Note: Do not rely on this guide for color-matching. The appearance of these materials sometimes differs significantly from the photos below.

Zinc plated fasteners Yellow zinc plated fasteners Zinc-plated steel is a low carbon steel for general use. Relatively inexpensive, with the zinc plating providing moderate corrosion resistance suitable for indoors or otherwise dry conditions. Color is either a blue-ish tint or yellow depending on the exact process.

Galvanized fasteners Hot-dipped galvanized steel has a thicker zinc coating for better corrosion resistance, making it suitable for outdoor use. Because of the thick plating, only galvanized nuts and washers will fit galvanized bolts. The coating typically has a rough, dull grey finish.

Stainless fasteners Stainless steel offers good corrosion resistance, making it suitable for outdoor use and marine applications, but is more expensive than zinc plated.

Chrome plated fasteners Chrome and nickel plated steel are smooth and polished for appearance. The plating offers moderate corrosion resistance.

Brass fasteners Bronze fasteners Brass and bronze are copper alloys with good corrosion resistance. More expensive than steel, these materials are typically used for decorative applications. Colors can vary significantly.

Alloy steel fasteners Alloy steel is highly hardened and usually black oxide and/or oil coated, offering little corrosion resistance.

How Fasteners are Notated: An Example

Machine screws,

Fastener
type

Phillips

Drive
type

pan head,

Head
style

Stainless steel 18-8,

Material

#12

Diameter

-24

Thead
count
(TPI)

x 1"

Length

Drive Types

Phillips and Slotted drives are common in screws, but prone to cam-out (stripping).
Combo drives, that can be used with either driver, are available for many fastener types
Frearson and Pozidriv are similar to Phillips, but less prone to cam-out.
Hex socket (Allen) drives are compact and easy to drive, but prone to cam-out.
Star (Torx™) and Square drive (Robertson) are resistant to cam-out and can be installed single- handed.

Note: Most drive types (Frearson and Slotted being notable exceptions) require the correct driver size for proper installation.

Head Styles

Hex heads are typically used with larger bolts and screws, and tightened with a wrench.
Pan heads have a slightly domed head that sits above the surface.
Flat heads are installed in a countersunk hole for a flat surface.
Round heads are tall domed heads, used primarily for decorative purposes.
Oval heads are a low domed and countersunk heads, used primarily for decorative purposes.
Truss heads are slightly domed, with a wide head for an extra large surface area.
Socket heads are narrow with a socket drive, and knurled or smooth sides.
Button heads feature a medium dome. Typically used with a hex socket drive.

Measuring Diameter

For most types of fasteners, the diameter is measured on the outside of the threads.

Note: US diameters under 1/4” are given as numbers (e.g. #12) instead of inches, in order of increasing size. If you need to find the actual diameter, use a table corresponding to your fastener type at https://boltdepot.com/info

Thread Count & Thread Pitch

Machine threaded fasteners specify a thread density in Threads Per Inch (US) or as a Thread Pitch in mm (Metric).

For a given diameter, a fastener may be available in coarse (standard), fine and sometimes super fine thread.

Measuring Length

Fastener length is usually measured from where the material is assumed to be to the end of the fastener.

Thus, countersunk fasteners are measured overall and non-counter- sunk fasteners are measured from under the head.