Metric bolts carry property class numbers on the head: 8.8, 10.9, 12.9. The first digit is roughly one tenth of the tensile strength in megapascals, so a 10.9 bolt has about 1,000 MPa tensile strength, and the second digit indicates how much of that strength is retained at yield. Higher classes mean harder steel and tighter torque specifications, and substituting downward turns a correctly torqued joint into an undertorqued one without anyone noticing. Torque-to-yield bolts, common on cylinder heads and some bearing caps, are stretched past their elastic limit by design and must never be reused. Match class, diameter, pitch, length and thread engagement, and read the service manual's torque table rather than a generic chart.
Common metric property classes and what they mean
| Head marking | What it tells you on the machine |
|---|---|
| 8.8 | Medium-strength class for general brackets, guards and low-load joints. Never a substitute for 10.9 or 12.9. |
| 10.9 | The workhorse class for most structural joints on excavators, including many boom and arm pins' retainers. |
| 12.9 | High-strength alloy class for final drives, pumps and high-clamp joints. Brittle under abuse; torque precisely. |
| Torque-to-yield mark | A line or dot indicating a stretch bolt: single use, torque plus angle method, replace every removal. |
| Manufacturer logo | Indicates the maker, not the class. A class number without a maker's mark on a critical joint is a warning sign. |
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Request a quoteFrequently asked
Can I reuse a torque-to-yield bolt one more time?
No. Its clamp load comes from being stretched past the elastic limit, and the manufacturer's specification assumes a new bolt every removal. Reusing one gives an unpredictable clamp load that can back out, relax, or snap, and on a head joint that means the gasket job is done again for the price of a set of bolts that were not worth saving.
Is a stronger bolt always a safe upgrade?
Usually not. Higher classes are harder and less ductile, so they tolerate shock loads and overtightening worse, and joint design may rely on the bolt yielding predictably. Fit what the manual specifies; the engineers chose the class for a reason that includes failure behavior, not just strength.
How do I torque a bolt when the markings have rusted away?
Match it from an identical fastener on the same joint, or identify the joint in the parts book and order to its number. Never estimate a class from head size, and never torque a critical joint with an unknown bolt in it; the cost of a correct bolt is trivial next to the joint it holds.