Bolt Property Classes 4.8, 8.8, 10.9 and 12.9: How to Choose
Every metric bolt carries its strength in two numbers stamped on the head: 4.8, 8.8, 10.9, 12.9. These are property classes under ISO 898-1, and they are not marketing labels — the numbers encode the bolt's mechanical properties directly. Choosing between them is one of the most consequential decisions in a fastener specification: too low, and the joint is the weak point of the machine; too high, and you pay more for a bolt that is harder to make, harder to plate, and less forgiving to install.
This guide explains how to read the classes, compares the four we manufacture, and gives a practical selection order.
How to read the two numbers
The property class designation is a small formula:
- The first number is the nominal tensile strength in MPa, divided by 100. A class 8.8 bolt has a nominal tensile strength of 8 × 100 = 800 MPa.
- The second number is the ratio between yield strength and tensile strength, multiplied by 10. The ".8" in 8.8 means the yield strength is 0.8 × 800 = 640 MPa.
Applying the same math to all four classes:
| Property class | Nominal tensile strength | Nominal yield strength | Typical role |
|---|---|---|---|
| 4.8 | 400 MPa | 320 MPa | Light-duty, non-structural |
| 8.8 | 800 MPa | 640 MPa | General machinery standard |
| 10.9 | 1,000 MPa | 900 MPa | High-load joints |
| 12.9 | 1,200 MPa | 1,080 MPa | Maximum strength, critical joints |
These are the nominal values defined by the designation system. The exact minimum values in ISO 898-1 vary slightly with bolt diameter — class 8.8 above M16, for example, has a higher minimum tensile requirement — so for a certified design value, always work from the standard itself, not from a summary table.
The four classes, one by one
4.8 — light-duty and non-structural
Class 4.8 is a low-carbon steel bolt without the heat treatment that defines the higher classes. It is appropriate where the joint carries modest, mostly static loads and failure would be an inconvenience rather than a hazard: covers, guards, light brackets, furniture and fixture assembly, low-pressure flanges. Choosing 4.8 for a structural or dynamically loaded joint is the classic under-specification error.
8.8 — the general machinery standard
Class 8.8, quenched and tempered medium-carbon steel, is the default for machine frames, equipment assembly, agricultural machinery, and general structural connections. It offers roughly double the strength of 4.8 with good ductility and predictable tightening behaviour. When a drawing does not state a class and the joint is a real load path, 8.8 is the class engineers most often settle on — and the class we recommend confirming first rather than assuming.
10.9 — high-load joints
Class 10.9 is where bolt size can come down or clamp load goes up: automotive and powertrain applications, heavy equipment, high-pressure joints, and connections where an 8.8 bolt would need to be unacceptably large. Our high-strength production is based on 40Cr alloy steel, the standard material route to this class. The trade-offs start here: 10.9 is less forgiving of sloppy tightening, and the margin for installation error narrows.
12.9 — maximum strength, handled with respect
Class 12.9 is the strongest standard class — and the least forgiving. It is specified for critical, highly loaded joints where nothing weaker fits the design envelope. Three cautions belong in every 12.9 decision:
- Ductility is lower. A 12.9 bolt has less capacity to absorb overload by stretching; the joint design and tightening control must be right.
- Surface treatment needs care. High-strength fasteners are sensitive to hydrogen embrittlement in electroplating processes; if a 12.9 bolt needs corrosion protection, the treatment route must be chosen with this in mind and discussed with the manufacturer before production.
- Cost and lead time rise. Material, heat treatment, and inspection demands are all higher. Specifying 12.9 "to be safe" where 10.9 already meets the design usually buys risk, not safety.
A practical selection order
Work through these questions in order:
- Does a drawing, standard, or OEM specification name the class? Use it. The class on a certified design is not a suggestion, and substituting up or down changes the joint's behaviour.
- Are you replacing an existing bolt? Read the head marking and match it. If the head is unmarked or illegible, treat the class as unknown and have the application reviewed rather than guessing.
- How loaded is the joint, really? Light and static → 4.8 may be enough. A genuine load path in machinery → start from 8.8. High clamp load in a tight space → 10.9. Critical joint at the edge of the design envelope → 12.9, with tightening control to match.
- What environment and finish does the joint need? At the highest classes, the finish decision and the class decision interact (see the 12.9 cautions above) — settle them together, not separately.
What a higher class actually costs you
Moving up a class is not free, and the costs are not only the unit price:
- Material and processing — higher classes require alloy steels and controlled quenching and tempering; our production covers 4.8, 8.8, 10.9 and 12.9, cold-headed in M5–M24 and machined in M8–M64
- Verification — the class must be proven, not assumed; our in-house testing covers composition, hardness, metallography and dimensions, with inspection reports available on request
- Installation discipline — the higher the class, the more the joint depends on correct tightening; a 12.9 bolt installed like a 4.8 bolt is a failure waiting for a load
The cheapest bolt is the one whose class matches the design — no lower, and no higher than the joint can use.
Common mistakes
- Specifying "high strength" without a class. It is not a specification; two suppliers will read it two ways.
- Upgrading a replacement bolt on your own initiative. A stronger bolt changes how the joint shares load and can move the failure point somewhere more expensive.
- Choosing the finish before the class. At 10.9 and especially 12.9, treatment and class must be decided together.
- Assuming the class from appearance. Two bolts that look identical can be 4.8 and 10.9. Without a head marking or documentation, the class is unknown.
FAQ
Can I replace an 8.8 bolt with a 10.9 for extra safety?
Not automatically. The joint was designed around a clamp load and a failure mode; a stronger bolt torqued to the old value behaves similarly, but torqued to its own higher capacity it can overload the clamped parts. Match the specified class unless an engineer re-rates the joint.
How can I tell the class of a bolt I already have?
Read the head marking — the class is stamped there on metric hex bolts. If there is no marking, the bolt should be treated as low class or unknown.
Which class do most custom orders use?
In general machinery work, 8.8 is the most common specification we see, with 10.9 close behind for automotive and high-load applications. The right answer for your part is the one on your drawing — if you are preparing an inquiry, our checklist of what to send for a custom bolt quote covers how to state the class so it is quoted correctly the first time.
What classes can HyHaven manufacture?
We produce custom bolts and fasteners in property classes 4.8, 8.8, 10.9 and 12.9, in 40Cr as our standard high-strength material, with black oxide, Dacromet, colour zinc and zinc finishes. Send your drawing through the custom fasteners page and state the class you need — if the class is undecided, describe the application and the review will recommend one.
HyHaven manufactures custom fasteners to your drawing or sample, in property classes 4.8–12.9, from 2,000 pieces. Request a quote.