Rick's Free Auto Repair Advice

Bolt Grades

What are bolt grades for U.S. and Metric bolts

Understanding bolt “strength” terms

You may think steel bolts are solid, stiff, or rigid. They’re not. It has a certain amount of elasticity. In other words, a bolt stretches when torqued. Think of a bolt as if it’s a spring or bungee cord that clamps pieces together. As it’s tightened to the proper torque, it stretches slightly and can handle a certain amount of additional stretch, yet keeps the pieces clamped together.

The concept of bolt stretch is important to understand because bolts need to stretch once they’re torque in order to accommodate normal expansion and contraction, especially in automotive use.

What is a bolt’s proof load?

Proof load is the maximum stretching force (torque) that can be applied to a bolt that will allow it to return to its original length when the force is removed.

Proof load is typically between 85-95% of the yield strength

What is Yield Strength?

Yield strength is the point at which a bolt is tightened or loaded beyond its proof load, causing it to exceed its maximum allowable stretch. Once a bolt it tightened to yield strength, it will no longer return to its original length if the torque or load is removed. At that point, the bolt is permanently damaged.

What is Tensile Strength?

Tensile strength is the maximum amount of stretch the bolt can withstand without actually breaking. In other words, the bolt has been tightened or loaded beyond the point where it will no longer return to its original and is continuing to stretch right before it actually breaks apart.

US Bolt Grade Marking and strength

US bolt markings
Metric Bolt Grade Marking and strength

metric bolt markings and strength
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