Fastener Grades 5 vs 8 vs 8.8 vs 10.9

SAE grades 5 and 8 are equivalent to metric classes 8.8 and 10.9, respectively. Grade 5 and metric class 8.8 have approximately equivalent tensile strengths, and the same is true between Grade 8 and class 10.9.

It may be tempting to think of metric class 8.8 as the equivalent to the SAE grade 8. That is not the case! The metric naming system is derived from the performance of the bolts. The naming system of SAE grades is arbitrary, counting up from 1. Higher means stronger in both systems, but the similarities end there. A fun (and useful) fact about the metric convention is in what follows the decimal in the class. That is, the .8 in class 8.8 and the .9 in class 10.9. The number before the decimal corresponds to the tensile strength, and the decimal value after corresponds to the yield strength, expressed as a percentage of tensile strength. To illustrate:

Tensile strength is the point at which a bolt breaks. It’s yield comes before that. The yield is the point at which the bolt is deformed past the point that it can return to shape. Keep in mind that bolts will “stretch” while in service, but they will “bounce” back to size after each stretch, like a spring. However, at a certain point, they will get stretched too far. They will get deformed past the point of return. That is the yield point. Between the yield point and the tensile point, the bolt with elongate. It will behave like taffy or mozzarella cheese until it breaks away from its other half. Higher grades are typically harder and more brittle than lower grades. They will not experience very much “stretch” before breaking. We might think of lower grades as being weaker but also “stretchier” – after beginning to yield, they experience greater elongation than the higher strength grades before breaking. 

Is there any benefit to getting a weaker grade bolt for its “stretchiness”? After all, the ability of a bolt to stretch and reform is part of what makes it an effective fastener. In general, no. A properly manufactured high strength bolt maybe be more brittle than its weaker counterpart but the weaker bolt (at least in the case of grade 5 and 8) will both yield AND break before the stronger bolt has begun to yield.

Both grade 5 and grade 8 are available in plain steel and zinc plated steel. The zinc plating for grade 5 is almost always clear and the plating for grade 8 is almost always yellow. They are not functionally different – the color just helps distinguish them from each other. They can also be distinguished by the head markings (three radial lines for grade 5 and six radial dashes for grade 8).