Bolt Grade Strengths
Bolts are made from different grades of steel and these grades indicate the strength of the bolt and we’re often asked about the different bolt grade strengths available and when they should be used.
The greater the tensile strength of the bolts, the more torque the bolts can take and tighter the joint can be fixed.
No matter the system, the higher the number the greater the strength of the bolt.
Most metric bolts will be graded in one particular way, and most imperial using another system, but it is not uncommon to see the two systems used interchangeably, so this is something to be aware of when selecting the right bolt for your needs.
Here's our run down of bolt grades, where to look to find which grade, and any immediate differences or common applications to note when bolt shopping!
Metric Based Bolt Grades
The following grades can all be determined by looking at the head of the bolt where the number will be etched. They are generally used for metric bolts but can sometimes be seen on imperial versions, although this is less common, and are to be used as a guide only:
- Grade 4.6 & 4.8 – Mild Steel.
- Grade 5.6 &6.6 – Mild Steel/High Tensile.
- Grade 8.8 – High tensile, commonly used on hex head fasteners.
- Grade 10.9 & 12.9 – commonly used in the automotive industry.
- Grade 14.9 – High grade steel, used in pressure critical applications.
Imperial Based Bolt Grades
The following grades are generally only used for imperial bolts, but can sometimes be seen on metric too, although again this is less likely. These are markings you can expect to see on some steel bolts, but are to be used as a guide only:
- Imperial Grade B – identified by a ‘B’ on the head of the bolt. This grade is classified as mild steel.
- Imperial Grade P – identified by a ‘P’ on the head of the bolt.
- Imperial Grade T, V and X can all be identified by a ‘T’, ‘V’ and ‘X’ marking on the head of the bolt respectively.
There are also multiple different types of bolts which will share a grade type, such as the following, which are all classified as high tensile grades:
- British Standard Fine (BSF), British Standard Whitworth (BSW) and British Association Grade (BA) – identified by ‘R’ on the bolt head.
- Unified National Fine (UNF) and Unified National Coarse (UNC) – identified by ‘S’ on the bolt head.
Metric and Imperial – Other Bolt Grade Types
All the following are generally usually seen on metric and imperial bolts when necessary:
- Grade 307A – indicates low carbon steel. Grade to be found on the head of the bolt marked ‘307A’.
- Grade 2 – indicates low or medium carbon steel. This grade usually has no markings, making it the hardest to reliably identify.
- Grade 5 – indicates medium carbon steel. Can be identified by searching for 3 radial lines on the head of a bolt.
- Grade 8 – indicates medium carbon alloy steel. Can be identified by searching for 6 radial lines on the head of a bolt.
- Grade A325 - indicates a carbon alloy steel with or without boron, and you would see ‘A325’ on the head of these bolts.
It’s important to remember that some bolts may also contain a combination of these elements. However, if you keep in mind the higher the number the higher the strength, you’ll be able to decipher most grade types in an instant.
Alternatively, if you’re struggling with any fixings jargon, you can always get in contact with our experts here at BOLTS.co.uk. Trading since 1980, we really do know our stuff! Get in touch here.
