What Is an 8.8 Bolt? Grade Meaning, Material, and Strength Explained

An 8.8 bolt is a metric steel property class rated for 800 MPa minimum tensile strength and 640 MPa minimum yield strength under ISO 898-1 — it is a strength grade, not a size.

Two bolts can measure identical in diameter and thread pitch and still be completely different fasteners, because the “8.8” stamped on the head says nothing about how big the bolt is. It describes how strong the steel is. That single misunderstanding causes more bad substitutions than almost anything else in a hardware aisle, and it’s the reason a mechanic will reject a bolt that threads in perfectly.

Here’s what the numbers actually mean, what the bolt is made of, and where the grade belongs.

What Do the Two Numbers in 8.8 Actually Mean?

The first 8 sets nominal tensile strength at 800 MPa, and the second 8 says yield strength is 80% of that, or 640 MPa. Multiply the first digit by 100 and you have tensile strength in megapascals; the second digit is the decimal fraction that applies to it.

That convention comes from ISO 898-1, the international standard that assigns property classes to carbon steel and alloy steel fasteners. SpecVise’s breakdown of ISO 898-1 lists those two figures as the defining values for the class.

One detail worth knowing: the numbers move slightly at the large end. SpecVise notes that ISO 898-1 raises the requirement for certain diameters above 16 mm to roughly 830 MPa tensile and 660 MPa yield. For most sizes you’ll handle, 800 and 640 are the numbers that matter.

To put 800 MPa in perspective, that’s about 116,000 psi of tensile strength — well above the roughly 60,000–70,000 psi you’d expect from a common Grade 2 or Grade 5 inch-series bolt, though the two systems aren’t graded the same way.

What Is an 8.8 Bolt Made Of?

Class 8.8 bolts are made from medium carbon steel or alloy steel, then quenched and tempered to reach the specified mechanical properties. That heat treatment is the whole point of the grade — the alloy alone doesn’t get you there.

Quenching locks in hardness by cooling the steel fast; tempering then reheats it to trade a little of that hardness back for toughness, so the bolt bends before it shatters. Skip either step and you get a bolt that’s strong on paper and dangerous in a joint.

Two things this grade is not:

• Not stainless. Class 8.8 is a steel property class. Stainless fasteners are graded on an entirely different scale (A2, A4, and similar), and mixing the two systems is a common error.
• Not a shear rating. The 800 and 640 figures describe tensile and yield strength in tension, not how the bolt behaves in shear. Treating 8.8 as a shear spec is a mistake worth avoiding.

Where Do 8.8 Bolts Get Used?

These bolts turn up in general engineering, automotive work, machinery, and structural applications — anywhere a joint needs more load capacity than a low-carbon fastener can give. Atlas Copco’s guide to bolt grades and joint types groups them among the standard higher-strength choices for exactly that reason.

That strength is also the reason substitution is risky. A higher-grade bolt changes how much clamp load the joint carries and how it fails when it does. Swap an 8.8 for something softer and the joint may loosen or yield early; swap it for something harder and you can crush the clamped material or snap the bolt with less warning. Atlas Copco’s material makes the point plainly: grades are selected for applications that need higher load capacity, and the grade is part of the design, not a detail.

If you need replacements and want to compare what’s actually available before you buy, this roundup of tested 8.8 bolt options covers the common sizes and finishes.

How Does 8.8 Compare to Other Common Grades?

Class 8.8 sits in the middle of the metric steel range — above 4.6 and 4.8, below 10.9 and 12.9. Here’s how the classes line up using tensile and yield figures for the standard sizes.

Bolt Depot’s grade chart is a reliable reference for how the tensile and yield figures scale across classes, and it’s a useful cross-check when a supplier’s listing is vague about what you’re actually buying. The pattern is consistent: each step up the class ladder buys more clamp load, and usually more brittleness to go with it.

The practical takeaway is that 8.8 is the workhorse grade, not the strongest one. If a manufacturer specified 8.8, replacing it with 12.9 isn’t an upgrade — it’s a change to how the joint behaves.