Guide · Fasteners · Reference
Bolt grades explained.
Three separate grading systems appear on bolt heads in a British workshop: the metric property class, the imperial SAE grade, and the stainless designation. They use different units, different markings and different reference standards, and none of the three converts directly into another. This guide sets out what each one encodes and where the systems are most often confused.
The metric property class: 8.8, 10.9, 12.9
Carbon and alloy steel bolts are graded under ISO 898-1. The class is stamped on the head as two numbers separated by a point, and both halves carry meaning.
The first number is the nominal tensile strength in hundreds of megapascals. The second is the ratio of yield strength to tensile strength, expressed in tenths. A class 8.8 bolt therefore has a nominal tensile strength of 800 MPa and yields at approximately 80 per cent of that figure, around 640 MPa.
Yield strength is the more useful of the two in practice. Tensile strength is the load at which the bolt finally breaks; yield is the load beyond which it stretches and does not return. A joint that has yielded has already lost its clamp force, whether or not anything has visibly failed.
| Class | Tensile (nominal) | Yield (approx) | Typical application |
|---|---|---|---|
| 4.6 / 4.8 | 400 MPa | 240 / 320 MPa | Mild steel bolting. Brackets and non-structural fixing. |
| 8.8 | 800 MPa | 640 MPa | General engineering and structural work. The everyday high-tensile bolt. |
| 10.9 | 1000 MPa | 900 MPa | Higher-load joints: plant, machinery, vehicle chassis work. |
| 12.9 | 1200 MPa | 1080 MPa | Socket cap screws, tooling and jigs. The strongest common class. |
The figures above are the nominal values the class name encodes. The specified minima in ISO 898-1 sit marginally higher for the upper classes, and vary with diameter at 8.8. For design work, take the figures from the standard itself rather than from any summary table, including this one.
Grade 8 is not class 8.8
This is the single most common misreading of a bolt head, and the consequence runs in the dangerous direction.
Imperial fasteners are graded under SAE J429, which uses a whole number and identifies the grade by raised radial lines on the head rather than by a stamped figure. The systems share no units: SAE quotes pounds per square inch, ISO quotes megapascals.
| Grade | Head marking | Tensile (min) | Nearest metric class |
|---|---|---|---|
| Grade 2 | No lines | 74,000 psi | Below 8.8 |
| Grade 5 | Three radial lines | 120,000 psi | Approximately 8.8 |
| Grade 8 | Six radial lines | 150,000 psi | Approximately 10.9 |
An SAE Grade 8 bolt is closer to a metric 10.9 than to an 8.8. At 150,000 psi it is equivalent to roughly 1,030 MPa. A class 8.8 bolt, at 800 MPa, converts to about 116,000 psi — which places it alongside Grade 5, not Grade 8.
Reading "Grade 8" as "8.8" therefore understates the imperial bolt. Reading "8.8" as "Grade 8" overstates the metric one by a full class, and that is the error that puts an under-specified bolt into a loaded joint.
The two systems also differ in thread form and dimensional tolerance, so the equivalence above is a guide to relative strength and nothing more. Substituting across the systems is an engineering decision, not a stores decision.
Stainless: A2 and A4, then 70 or 80
Stainless fasteners are graded under ISO 3506 and do not use the property-class system at all. The designation joins two independent pieces of information.
The letter and number — A2 or A4 — identify the material. A2 corresponds to 304 stainless. A4 corresponds to 316, which contains molybdenum and consequently resists chlorides: salt, marine air and pool chemistry, all of which will eventually pit and streak A2.
The number following the dash is the property class, and it states tensile strength directly: 70 denotes 700 MPa, 80 denotes 800 MPa. An A4-80 fastener is therefore 316 stainless with a tensile strength of 800 MPa.
| Marking | Material | Tensile (min) | Where it belongs |
|---|---|---|---|
| A2-70 | 304 stainless | 700 MPa | General indoor and sheltered outdoor use. The common stainless bolt. |
| A4-70 | 316 stainless | 700 MPa | Coastal, pool, food and chemical environments. |
| A4-80 | 316 stainless | 800 MPa | Corrosive environments that also carry load. |
Stainless is not the stronger option. A2-70 is rated at 700 MPa. A plain zinc-plated 8.8 is rated at 800 MPa and a 10.9 at 1,000 MPa. Substituting stainless for a high-tensile bolt because stainless is the better-regarded material reduces the rated strength of the joint by between a quarter and a third.
Corrosion resistance and mechanical strength are two separate specifications. Where a joint requires both, the answer is A4-80, a proprietary high-strength stainless, or a plated high-tensile bolt with a different approach to protecting it.
Torque figures do not transfer between grades
A torque value is derived from the bolt's yield strength and from the friction in the thread and under the head. Change either and the figure is no longer valid.
The practical consequence concerns stainless. Applying an 8.8 torque figure to an A2-70 bolt of the same diameter overstresses it, because the stainless bolt yields at a lower load. Stainless also has a markedly different friction coefficient, so more of the applied torque is consumed by friction and less becomes clamp force.
Galling
Austenitic stainless is prone to galling: running a stainless nut down a stainless thread quickly generates enough local heat and pressure for the two surfaces to cold-weld. The fastener seizes part-way on and will then move in neither direction. Assemble slowly, and use an anti-seize compound on any stainless joint intended to come apart again.
Why 12.9 is not simply the safe choice
Specifying the highest available class as a precaution is a common instinct and a mistaken one. Strength in a bolt is obtained through hardness, and hardness carries costs.
High-hardness fasteners are considerably more susceptible to hydrogen embrittlement, in which absorbed atomic hydrogen — introduced during acid pickling or electroplating — causes a delayed brittle fracture at a load the bolt would otherwise carry indefinitely. The risk rises sharply from 10.9 to 12.9. Harder material is also more notch-sensitive, and a bolt is full of notches: every thread root and the underside of the head are stress concentrations.
This is reflected in design practice rather than merely in theory. Eurocode 3 and AISC-360 permit structural bolting up to class 10.9; the Australian standard AS4100 stops at 8.8. Class 12.9 is a machine and tooling fastener, used where the joint is understood in detail, and it is not a general-purpose upgrade.
Matching nuts, and unmarked heads
A nut carries its own strength class, and a high-tensile bolt assembled into an under-rated nut fails at the nut. The convention pairs a class 8 nut with an 8.8 bolt and a class 10 nut with a 10.9 bolt. The nut is the component most often taken from a different box, and therefore the one most often mismatched.
A head bearing no marking should be treated as being of unknown grade rather than assumed to be low grade. Property-class marking is required for classes 8.8 and above under ISO 898-1, so an unmarked head is normally either a low-class fastener or one whose provenance cannot be established. Neither belongs in a joint carrying load.
Where DIN and ISO disagree
DIN 933 and DIN 931 were superseded by ISO 4017 and ISO 4014 respectively. At four sizes the two standards specify a different width across flats. This is not a rounding difference; it is a different spanner.
| Size | DIN | ISO |
|---|---|---|
| M10 | 17 mm | 16 mm |
| M12 | 19 mm | 18 mm |
| M14 | 22 mm | 21 mm |
| M22 | 32 mm | 34 mm |
M22 reverses the pattern. At M10, M12 and M14 the ISO head is one millimetre smaller than the DIN head. At M22 it is two millimetres larger, so the rule of thumb that ISO is one size down fails precisely where someone has come to rely on it.
A number of published charts, including some issued by suppliers, record this incorrectly. Where a spanner almost fits, this is the usual explanation. Danetre works to DIN, and the setscrew and hex bolt size charts give DIN figures throughout.
The standard's length ranges and the trade's differ
ISO 4014 begins its part-threaded ranges comparatively high — M12 at 50 mm, M16 at 65 mm, M20 at 80 mm — and below those lengths directs the specifier to a fully threaded setscrew. The trade never adopted this in full. Wholesalers list M12×40, M16×45 and M20×55 part-threaded as stock items, carried over from the earlier DIN ranges. The hex bolt size chart follows what is actually available rather than the standard's preferred series.
Identifying a fastener in hand
Where a bolt cannot be identified from its markings, it can usually be identified from the part. Danetre's counter at Baird Close in Daventry holds metric and imperial stock in the common grades and finishes, and a sample can generally be matched against it directly.
More size charts
Every size, drawn to scale.
Hex setscrews
DIN 933 · full thread
Hex bolts
DIN 931 · part thread
Socket cap screws
DIN 912
Low head cap screws
DIN 7984
Button head screws
ISO 7380-1
Flanged button heads
ISO 7380-2
Countersunk socket screws
DIN 7991
Grub screws
DIN 913–916 · five points
Nuts
DIN 934 · nyloc · thin · Stover · flange
Washers
ISO 7089 · BS 4320 · penny
Not sure what the markings on the head mean? Bolt grades explained covers 8.8, 10.9, 12.9, stainless A2 and A4, and the imperial SAE grades.
All of it comes off the shelf at Baird Close — see the full fasteners & fixings range, or every size guide.