BOLT AND SCREW HEAD MARKINGS

hexagon hex bolts

BOLT AND SCREW HEAD MARKINGS

In the fastener industry, there are organizations that specify the strength and marking requirements for both metric and imperial size bolts and screws sold around the world. Here we explain what the markings are and what they mean for the most commonly used.

The term Grade is the word we use to identify the material, hardness, and strength level for fasteners. The purpose of the grade marking is to identify the strength level (often referred to as the “Tensile Strength”*) that the fastener has been manufactured to, and the manufacturer’s marking is to identify who is responsible for the quality and testing of the fastener. 

METRIC HEX BOLTS AND SETS 

The 3 main popular strength levels that are used in the industry for metric hexagon fasteners – bolts being part threaded and setscrews fully threaded – are Grade 8.8 (often referred to as high tensile), Grade 10.9 and Grade 12.9. You will see from the images that there are also letters which denotes the manufacturer.

METRIC SOCKET SCREWS

The 4 main types of socket screws (often referred to as Allen Bolts) are the socket cap screw, countersunk socket screw, button head socket screw and the socket set screw (grub screw) as shown in that order in the image. Mainly the socket cap screws are available in grade 12.9 but occasionally in grade 8.8, countersunk and button head socket screws will be grade 10.9 and the socket set screw grade 14.9. As with the hexagon fasteners the grade is stamped on the head along with the manufacturers marking, however the socket set screws are not stamped due to the lack of space available.

IMPERIAL HEX BOLTS AND SETS

The 2 main popular strength levels that are used in the industry for unified – UNF & UNC) hexagon fasteners – bolts being part threaded and setscrews fully threaded – are identified by either lines or a letter. 3 lines (image 1) is SAE grade 5 (similar to metric grade 8.8), this grade can also be referred to and stamped “S” (image 2). SAE grade 8 (similar to metric grade 10.9) will be marked with 6 lines (image 3).

British Standard (BSW & BSF) hexagon fasteners are usually marked grade “R” (image above) and is also similar to metric grade 8.8.

As with the metric fasteners, the imperial bolts are also marked with letters that denotes the manufacturer.

IMPERIAL SOCKET SCREWS

Unlike the previously mentioned fasteners, imperial socket screws, which include both unified and BS, quite often do not have head markings at all, the occasional exception may be a manufacturers identification marking. Grades are similar to equivalent metric socket screws.

It is always best to check the grade required for the job being undertaken. We will always advise what is available and we can obtain certification (where applicable) if it is requested by you at the time you are quoted.

*Tensile strengths can vary depending on diameter, length, head style and plating/finish

There are other grades available which are applicable to various industries, please call 0151 355 8045 or email; sales@leytonfasteners.co.uk if you have any questions or wish to discuss.