Bolt grades 4.8, 8.8, 10.9 and 12.9 explained: what HT means
The two numbers on a metric bolt head give its strength. The first times 100 is the tensile strength in megapascals (MPa), and that figure times the second number divided by ten is the yield strength. An 8.8 bolt breaks at about 800 MPa and starts to stretch for good at about 640 MPa. "HT" (high tensile) on a quote or a box usually means grade 8.8 or above.
| Grade | Tensile (approx.) | Yield (approx.) | Typical use |
|---|---|---|---|
| 4.8 | 400 MPa | 320 MPa | Light fixing, timber, non-structural |
| 8.8 | 800 MPa | 640 MPa | General structural steel and machinery: the everyday HT bolt |
| 10.9 | 1,000 MPa | 900 MPa | Heavy machinery and high-load joints, where the design calls for it |
| 12.9 | 1,200 MPa | 1,080 MPa | Socket cap screws, tooling, high-stress assemblies |
Pair the nut with the bolt
A nut has its own class, marked with a single number. Use at least class 8 with an 8.8 bolt and class 10 with a 10.9 bolt: a weaker nut strips before the bolt reaches its strength. High-strength joints also want hardened washers.
DIN 931 or DIN 933: half or full thread
DIN 931 bolts are threaded only part of the way, leaving a plain shank where the joint takes shear. DIN 933 bolts are threaded right up to the head, for clamping thin parts or when the nut must run the full length. In our range the product name says which: Half Thread or Full Thread.
Black (self-colour) HT bolts carry a light oil and suit painted steelwork or indoor use; zinc-plated ones resist rust a little better indoors. For outdoor and coastal work, see our guide to stainless and galvanised fasteners.
Ordering HT bolts without back-and-forth
- Diameter and length: M12 x 100, the length measured under the head.
- Grade: 8.8 or 10.9.
- Thread: half (DIN 931) or full (DIN 933). Add the pitch only when it is fine, for example M12 x 1.25.
- Finish: black, zinc-plated, galvanised or stainless.
- Nuts and washers to match, and the quantity in pieces.
Browse HT hex head bolts and HT nuts.
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HT bolts and nuts
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Questions buyers ask
Is an HT bolt the same as an 8.8 bolt?
Usually. HT (high tensile) is the trade name for grade 8.8 and above; check the head marking, or ask for the grade on the quote.
Can I replace an 8.8 bolt with a 10.9?
Often, but not always. A 10.9 bolt is stronger but less ductile, and the nut and washers must be rated to match. Where an engineer specified the grade, use what they specified.
What does M12 x 1.25 mean?
A 12 mm bolt with a fine 1.25 mm thread pitch. Without a pitch, M12 means the standard coarse thread, 1.75 mm.