Button head, grade 5, pan head, 8 x 1.25 – what’s it all mean? If you’re buying nuts and bolts, this is all vital information.
If you don’t know how to describe what you want, chances are that you won’t be able to get it! Threaded fasteners are identified by these attributes: type, material, diameter, length, and thread pitch or count.
Fasteners are divided into categories based on their function or design eg wood screw, sheet metal screw, hex bolt, lag bolt, etc. 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.
Fastener material describes the material from which the fastener was made as well as any material grade eg stainless steel, zinc plated steel, silicon bronze, etc. Fastener material is covered in more detail later in this feature.
Fastener diameter is measured either as a size number or as a direct measurement. There are several different locations on a fastener where one can measure the diameter. The most commonly used diameters are:
In some cases the head size of a hex bolt (diameter across the flats) is also used. However, due to the fact that different head sizes may be used with the same diameter bolt, this is not a reliable way to determine bolt diameter.
The diameter of a hex bolts, machine screws and socket head screws is the shank diameter, expressed in inches for US bolts and millimetres for metric bolts. Because this is approximately the same as the major or thread diameter, the thread diameter measurement can be used for fully threaded fasteners.
Both nuts and washers are sized by the fastener they fit. For example a 1/2 inch nut fits a 1/2 inch bolt and a 1/2 inch washer fits the same bolt.
For fasteners where the head usually sits above the surface (such as hex bolts and pan head screws) the length 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.
TPI stands for Threads Per Inch. This is simply a count of the number of threads per inch measured along the length of a fastener. TPI is used only with American fasteners. Metric fasteners are specified with a thread pitch instead of a thread count. The thread pitch is the distance between threads expressed in millimetres (measured along the length of the fastener). For example a thread pitch of 1.5 means that the distance between one thread and the next is 1.5mm. In general, smaller fasteners have finer thread so they have lower thread pitch.
Downloadable Thread Identification Tools
If you have a bolt, machine screw or washer and you’re not sure what it is, use these downloadable pdf files to print out full-size templates for easy identification.
US Bolt Diameter and Thread Chart
US Machine Screw Diameter and Thread Chart"
Metric Bolt Diameter and Thread Chart
USS and SAE Washer Chart
US Thread Gauge
Metric Thread Gauge
US fasteners are described by material, head style, and type followed by or preceded by the diameter dash thread count (machine threads only) X length. The diameter and length are in inches with the exception of small screws whose diameter may be a size number (ex. 12). In this case the number will often have a # sign before it (ex. #12)
Example: Brass Philips Flat Head Machine Screw 1/4 – 20 x 6. This is a brass counter sunk (flat head) machine screw with a Phillips drive. It has a diameter of 1/4 inch, 20 threads per inch (coarse thread), a length of 6 inches.
There are some variations on this using abbreviations and sometimes moving the material and or head style and type to after the size.
Sometimes a machine thread is specified as coarse thread or fine thread, rather than giving the actual thread count. This is usually done with an abbreviation at the end or occasionally in front of the diameter and length. Example: Brass Philips Flat Head Machine Screw 1/4 x 6 CT
Metric fasteners material, head style, and type are described in a similar manner to US fasteners. However, instead of threads per inch, the metric system uses thread pitch (the length of each thread in millimetres measured along the shaft of the bolt). The thread pitch is written between the diameter and length (both measured in millimetres) and is separated on each side by an X. Typically the diameter of a metric bolt is preceded by a capital M to designate metric. Example: Class 8.8 Hex Bolt M10 x 1.5 x 20 This is a hex head bolt with a diameter of 10mm, a 1.5 thread pitch, measuring 20mm long and made of Class (grade) 8.8 steel.
Note: Metric fasteners may also be specified by DIN number. The DIN number specifies a particular set of standards that the bolt adheres to.
Fasteners are manufactured in a wide range of materials from common steel to titanium, plastic and other exotic materials. Many materials are further separated into different grades to describe specific alloy mixtures, hardening processes, etc. In addition, some materials are available with a variety of coatings or plating to enhance the corrosion resistance, or appearance of the fastener.
Fastener material can be important when choosing a fastener due to differences between materials in strength, brittleness, corrosion resistance, galvanic corrosion properties, and of course cost.
When replacing fasteners, it is generally best to match what you are replacing. Replacing a bolt with a stronger one is not always safe. Harder bolts tend to be more brittle and may fail in specific applications. Also some equipment is designed so that the bolts will fail before more expensive or critical items are damaged. In some environments such as salt water galvanic corrosion must also be considered if changing fastener materials.
Stainless steel is an alloy of low carbon steel and chromium for enhanced corrosion characteristics. Stainless steel is highly corrosion resistant for the price and because the anti-corrosive properties are inherent to the metal, it will not lose this resistance if scratched during installation or use.
It is a common misconception that stainless steel is stronger than regular steel. In fact, due to the low carbon content, stainless steel cannot be hardened. Therefore when compared with regular steel, it is slightly stronger than an un-hardened (grade 2) steel fastener but significantly weaker than hardened steel fasteners.
Stainless steel is also much less magnetic than regular steel fasteners though some grades will be slightly magnetic. 18-8 is a specific grade of stainless steel and is the most common grade used in fasteners. For those familiar with the 300 series of stainless steel, 18-8 is similar to 304 stainless.
Steel is the most common fastener material. Steel fasteners are available plain as well as with various surface treatments such as zinc plating, galvanization and chrome plating.
Steel fasteners are commonly available in four grades. Many other grades exist but are used far less often. The most common grades are Grade 2, Grade 5, Grade 8, and Alloy Steel. Grade 2, 5, and 8 are usually plated with a silver or yellow zinc coating or galvanized to resist corrosion.
Alloy steel bolts are made from a high strength steel alloy and are further heat treated. Alloy steel bolts are typically not plated resulting in a dull black finish. Alloy steel bolts are extremely strong but very brittle.
Silicon bronze, often referred to simply as bronze, is an alloy made mostly of copper and tin with a small amount of silicon. Bronze is used primarily in marine environments. It is preferred over stainless in wooden boat construction and re-fastening due to its superior corrosion resistance, and over brass due to its higher strength. Bronze is similar to copper in colour and is also sometimes seen in fine woodworking where it is used for its appearance. The main drawback of bronze is its high cost.
Brass is an alloy of primarily copper and zinc. Brass is highly corrosion resistant and electrically conductive. However, its use as a fastener is somewhat limited due to its relative softness. It is used primarily for its appearance.
Determining Bolt Grade
Bolts of different grades are marked on the head to show what grade bolt they are.