Most car guys will remove, install and replace bolts many times and never look at bolt grades. After all, if it’s not broke why fix it? But if your project requires special bolts, or even if you’ve broken or damaged the bolt and need a replacement, you need to know how to read bolt grades.
For those looking for a custom application or custom use, bolt grades are a great way for you to choose the bolts to perform the job you need. No matter what the job is however, a huge part of putting your car back together is knowing how to read bolt grades, and knowing what those numbers mean.
The biggest obvious requirement for reading bolts is being able to tell the difference between metric and standard. Standard bolts (sometimes called U.S. or SAE) and metric hardware cannot be interchanged and are specific in the way they are rated and read.
All bolts, whether standard or metric are sized according to diameter, thread pitch and length. For example, a standard 1/2-16×2 bolt is 1/2 inch in diameter, has 16 threads per inch and is 2 inches long. An M12 – 1.5 x 50 metric bolt is 12 mm in diameter, has a thread pitch of 1.5 mm and is 25 mm long.
Looking at these two bolts from afar doesn’t tell you much, as they both look the same size and distance. In reality however they are very different and cannot be interchanged.
Although there are a few exceptions, like the 14mm metric and the 9/16″, standard wrenches should not be used on a metric bolts and vice versa.
Standard bolts have slashes radiating out from the center of the head to denote the grade or strength of the bolt, which is an indication of the amount of torque that can be applied to it. The greater the number of slashes, the greater the strength of the bolt. Grades 0 through 5 are commonly used on automobiles.
Metric bolts have a property class (grade) number, rather than a slash, molded into their heads to indicate bolt strength. In this case, the higher the number, the stronger the bolt. Property class numbers 8.8, 9.8 and 10.9 are commonly used on automobiles.
How to Buy Bolts
What do the numbers mean and knowing how to shop and replace bolts is extremely important. Reading the bolts and what the values are and represent go a long way in shopping for your bolt and cutting down on time wasted in research.
How to Read a Standard Bolt
SAE or Standard Bolts are comprised of a unique scaling system which allows you to know size, length, pitch thread and more.
- A – Grade marks (bolt strength)
- B – Length (in inches)
- C – Thread pitch (number of threads per inch)
- D – Nominal diameter (in inches)
Metric Bolts differ slightly in that the measurements are of course in mm, and that the distance between threads determines the pitch thread.
- A – Property class (bolt strength)
- B – Length (in millimeters)
- C – Thread pitch (distance between threads in millimeters)
- D – Diameter – Millimeters, what else?
Since fasteners of the same size (both standard and metric) may have different strength ratings, be sure to reinstall any bolts, studs or nuts removed from your vehicle in their original locations. Also, when replacing a fastener with a new one, make sure that the new one has a strength rating equal to or greater than the original.
That does it for our How To Read Bolt Grades article, for more information check out Part II of our Nuts and Bolts guide, How to Torque Fasteners.