Bolt Grades: The Right Bolt for The Job

Introduction

Not every bolt is created equal.  As they say, the right bolt for the right job.  Well, maybe they don’t say that, but it is true in this scenario.  If you have ever worked on cars, you probably noticed that the hardware you use has numbers on the end, but you may not know exactly what they mean.  These numbers indicate the grade of the bolt, and depending on which grade, and where they are being installed on the car can make a huge difference.

In the automotive world, the three most common grades you will see are 8.8, 10.9, and 12.9, but what does this all mean?

In this article, we will cover a few different topics.

1. Mechanical Properties
2. Pros & Cons
3. Failure Characteristics
4. Automotive Applications
5. Visual Comparison

Mechanical Properties: Understanding Grades by Strength

Each of these 3 grades have a very different tensile strength.

• Grade 8.8: ~800 MPa tensile strength, 640 MPa yield. Standard structural bolts, ductile but lower strength.
• Grade 10.9: ~1,040 MPa tensile, 940 MPa yield. Higher strength, common in critical automotive parts.
• Grade 12.9: ~1,220 MPa tensile, 1,100 MPa yield. Very high strength, less ductile, used where maximum clamping force is required.

How the grades are derived is quite simple.  The first number refers to the tensile strength.  For example, with a grade 8.8 bolt, you would multiply 8 x 100, to give you a nominal tensile strength (MPa), of 800.  The second number is the percent of yield to tensile.  In the same example, 8 again, you would multiply 8 x 10%, to receive .8.  You then multiply the .8 x 800 (MPa), to receive a yield of 640.

Yield – This refers to the maximum stress that a bolt can withstand without being permanently deformed (stretched).

MPa – Stands for megapascals, which is a unit of pressure or stress.  This is the amount of force per unit the bolt can handle.

Pros and Cons

There are many different pros and cons to using each grade of bolt, and usually the manufacturer or BMW, in our case, chooses the correct bolt for the correct application.  BMW engineers are very smart, and run models for stress testing on all their components.  They choose the appropriate bolt where needed.

Failure Characteristics

8.8 – These bolts tend to stretch and deform before breaking.  They have very good energy absorption.

10.9 -  More resistant to deforming, but can snap under extreme shock

12.9 – These bolts are the hardest of the three to shear, but can fail more suddenly.

Note: Always use the recommended torque specification from the manufacturer to prevent premature failure or stretching.

Automotive Applications

As you can see, all three of these grades have unique characteristics and should be used for very specific applications.  You might be asking, if 12.9 is the best, why don’t we use it in all applications.  One of the biggest reasons is cost.  12.9 bolts are usually heat treated, which is a more expensive process.  You would not want to use 12.9 bolts in applications that do not require much clamping force, like interior pieces.

8.8. – These bolts are used mostly in interior parts, plastic brackets, lights, or other components on the car that are not structural.

10.9 – These bolts are the middle of the ground and are typically found in engine mounts, suspension arms, or drivetrain brackets.

12.9 – The highest grade, and can be used in brake systems, turbo/superchargers, head bolts, or other engine internals.

Conclusion

Using the right grade bolt for the correct application is vital in maintaining your car.  If you need to replace a bolt, make sure you choose the correct grade that the fancy BMW engineers recommend.  Going to your local hardware store and picking out the cheapest bolt for your suspension, may end up in a catastrophic failure.  After replacing the bolt, make sure you look up the torque value in the BMW maintenance manual.  Bentley manuals are a great resource, or you can pick up one of your Easy to read Torque Guide booklets found on our website.