Deciphering Bolt Head MarkingsPhoto by Ben Lodge

Originally Posted On: Deciphering Bolt Head Markings (


Have you ever looked at the head of a bolt and wondered what the markings mean? Maybe there’s a series of letters and numbers, maybe just a few lines.

The markings on the heads of bolts serve two different purposes:

  1. The markings can indicate who manufactured the bolt. This will vary from manufacturer to manufacturer, but is usually one, two or three characters.

  2. The markings can identify the standard to which the bolt was made. For instance, if a bolt head contains three radial lines, then it’s a Grade 5 bolt. If it contains five radial lines, then the bolt is Grade 8. If the bolt is manufactured to a particular grade based on standards set by the American Society for Testing and Materials (ASTM), then the marking will show a combination of letters and numbers.

Mudge Fasteners wants to help you decipher these head markings and when you’ll need to know them. Download our bolt head markings chart to reference any time you need to figure out the grade of a bolt you’re using or looking for. If you still have questions not addressed in the chart, don’t hesitate to contact us for help.