Fastener Grades Explained: 8.8, Grade 8 and A2-70

The short answer: a fastener grade only makes sense inside its governing standard. 8.8 is an ISO metric property class, Grade 8 is an SAE inch-series grade, A2-70 combines a stainless grade group with a strength class, and B7 is an ASTM pressure-bolting grade. The names may look comparable, but they do not form one universal strength ladder.^{[1] [2] [3] [4] [5]}

This article explains the grade field in a complete bolt specification. If you also need size, thread, dimensions and finish, start with How to Specify a Bolt: Size, Thread, Grade, and Finish.

Which grade system are you looking at?

Start with the marking or drawing callout. It usually identifies the standards family before it tells you anything useful about strength.

Marking or callout	System	What it tells you first	Detailed guide
`8.8`, `10.9`, `12.9`	ISO metric carbon/alloy steel	A coded mechanical property class	Bolt Grades 8.8, 10.9 and 12.9
`Grade 5`, `Grade 8`	SAE J429 inch steel	A table-defined mechanical and material grade	Grade 5 vs Grade 8 Bolts
`A2-70`, `A4-80`	ISO metric stainless steel	Stainless grade group plus tensile property class	A2-70 and A4-80 Markings
`A193 B7`, `A320 L7`, `F3125 A325`	ASTM application standards	A grade created for a particular product and service	Read the full ASTM specification and application requirements

The first three rows lead to focused articles. Use this overview when the system is unknown, when several systems appear on the same purchasing list, or when someone proposes an “equivalent” grade from another standard.

Why are there so many kinds of grade?

Because the word grade is doing several different jobs.

A finished-fastener property class, such as ISO 8.8, defines mechanical and physical requirements for an applicable finished bolt, screw or stud.
A material grade, such as Type 316, 42CrMo4 or titanium Grade 5, primarily identifies chemistry and material condition.
An application grade, such as ASTM A193 B7 or F3125 A325, belongs to a product standard written for a particular service.
An ISO product grade A, B or C is a dimensional-tolerance category—not a strength class.^{[6] [7]}

These requirements can appear together. M12 × 1.75, ISO 4014, 10.9, zinc flake identifies a thread, product geometry, mechanical property class and coating family. Adding 42CrMo4 would impose a separate material requirement. No single part of that callout silently supplies the others.

The useful rule is:

Keep the standard attached to the grade. Write ISO 898-1 class 10.9, SAE J429 Grade 8 or ASTM A193 Grade B7—not just “Grade 8” or “high tensile.”

How do 8.8, 10.9 and 12.9 work?

For the carbon- and alloy-steel metric fasteners covered by ISO 898-1, the two numbers form a code:^[1]

first number × 100 = nominal tensile-strength level in MPa;
second number ÷ 10 = nominal yield-to-tensile ratio; and
the two results multiplied together = nominal yield-strength level.

These are code-derived nominal values, not guaranteed acceptance minima or design allowables.

Class	Nominal tensile level	Nominal ratio	Derived nominal yield level
8.8	800 MPa	0.8	640 MPa
10.9	1,000 MPa	0.9	900 MPa
12.9	1,200 MPa	0.9	1,080 MPa

The calculation explains the marking; the standard tables control acceptance. For example, class 10.9 has a 1,040 MPa minimum tensile requirement rather than the 1,000 MPa nominal code value. Class 8.8 has a diameter breakpoint above 16 mm. Proof stress, yield or 0.2% proof strength, hardness, ductility, size and applicable test program still come from the standard tables.^[1]

ISO 898-1 also has clear limits. It evaluates the specified mechanical properties at 10–35 °C and does not assign corrosion resistance, fatigue resistance, shear resistance or a tightening torque.

Why does 08.8 have a leading zero? Some low-head and countersunk screws cannot carry the full tensile load associated with their thread stress area. A preceding zero identifies reduced loadability. ISO 14581 common-head countersunk screws are a current example; 08.8 is not a typographical variation of 8.8.^[8]

How do SAE Grade 5 and Grade 8 work?

SAE J429 covers inch-series steel bolts, screws, studs, sems and U-bolts for automotive and related industries through 1 1/2 in. Its grade number is a label. There is no calculation that turns 8 into tensile strength.^[2]

SAE J429 grade and diameter	Proof stress	Minimum yield strength	Minimum tensile strength
Grade 5, 1/4–1 in	85 ksi	92 ksi	120 ksi
Grade 5, over 1–1 1/2 in	74 ksi	81 ksi	105 ksi
Grade 8, 1/4–1 1/2 in	120 ksi	130 ksi	150 ksi

Grade 5 commonly has three radial head lines and Grade 8 six, together with manufacturer identification where required. The familiar “count the lines and add two” shortcut applies to these grades. It is not a universal rule for studs, stainless fasteners, ASTM products or special grades.

SAE Grade 5 is often placed near ISO 8.8, and SAE Grade 8 near ISO 10.9, because their common tensile values are similar. That comparison helps identify a strength neighborhood. It does not make an inch bolt and a metric bolt interchangeable.

Put plainly, ISO class 8.8 and SAE Grade 8 are not the same grade. One belongs to the ISO metric system; the other belongs to SAE J429 and is normally used with Unified inch products.

What do A2-70 and A4-80 mean?

Stainless markings under ISO 3506-1 contain two different facts:^[3]

A2 or A4 identifies an austenitic stainless fastener grade group;
70 or 80 identifies the finished fastener's minimum tensile-strength class in tens of MPa.

Therefore A2-70 means an A2 stainless fastener with a 700 MPa minimum tensile requirement, while A4-80 combines the A4 group with an 800 MPa class. A2 is commonly associated with the 304 family and A4 with the molybdenum-bearing 316 family, but they are fastener grade groups rather than guarantees of one exact UNS composition.

The two blocks answer different questions. A2 versus A4 is mainly a material and corrosion decision. Class 70 versus 80 is a mechanical-property and manufacturing-condition decision. A4 is not automatically stronger than A2, and A2-70 is not a stainless version of carbon-steel class 8.8.

For Unified inch stainless products, ASTM F593 provides a separate route for general corrosion-resistant bolts, hex cap screws and studs. ASTM A193 B8 and B8M belong to pressure, high-temperature and special-purpose bolting. Neither converts to an ISO A2/A4 designation by matching alloy family alone.^{[4] [9]}

Why does ASTM use grades such as B7, L7 and A325?

ASTM is best read as an application map, not an ASTM-wide strength scale.

For example, ASTM A193 Grade B7 is quenched-and-tempered chromium-molybdenum alloy-steel bolting for pressure, high-temperature and other special-purpose service. The B7 mark only has that meaning when it remains attached to ASTM A193/A193M.^[4]

Standard and grade	Intended domain	What changes the decision
ASTM A307 Grade A or B	General carbon-steel bolts/studs or specified cast-iron flange joints	Intended use, tensile range, size and companion nut
ASTM A574	Alloy-steel socket-head cap screws	Product geometry and high-strength socket-screw requirements
ASTM A193 B7, B8 or B8M	Pressure, high-temperature and special-purpose bolting	Material, processing, size, class/condition and service
ASTM A320 L7	Low-temperature pressure and special-purpose bolting	Impact qualification at the specified temperature
ASTM F3125 A325 or A490	High-strength structural bolts and assemblies	Grade, type, style, assembly components and structural installation rules
ASTM F1554 Grade 36, 55 or 105	Anchor bolts for structural supports to concrete	Minimum yield strength in ksi plus anchor-specific requirements

The letter or number only has meaning with the specification. A193 B7 and A320 L7 can have similar room-temperature properties, but L7 carries low-temperature impact requirements. They are not interchangeable without approval of the application, geometry, assembly and documentation. A574 is tied to socket-head cap screws. F1554 Grade 105 is an anchor-bolt grade, not a generic replacement for another 105 ksi material.^{[4] [5] [10] [11] [12] [13]}

Current editions matter too. ASTM F3125/F3125M-26 covers 120, 144 and 150 ksi inch tensile levels. Older summaries that show only two levels are incomplete. ASTM F568M, still used by some online metric comparison charts, was withdrawn in 2012 with no replacement.^{[5] [14]}

Compare the property that controls the failure

“Stronger” is too vague for design or substitution. The main mechanical terms answer different questions.

Property	What it means in practice
Proof load / proof stress	The specified axial load or stress sustained in the proof test without prohibited permanent set; not the installation target
Yield strength / 0.2% proof strength	Yield strength is the standard-defined yield value where distinct yielding exists; 0.2% proof strength is the specified offset-stress measure used when it does not
Tensile strength	The maximum engineering stress reached in the specified tensile test
Hardness	A production check on material and heat-treatment condition; not a complete tensile or toughness result
Elongation / reduction of area	Measures of ductility before fracture
Charpy impact energy	Notched impact performance at a stated temperature, important in low-temperature specifications such as A320

ASTM F606/F606M supplies fastener test methods. The product standard supplies the acceptance values and decides which tests apply.^[15]

For selection, use proof or yield strength to check the usable elastic/preload range, tensile strength to check ultimate axial capacity, and impact requirements when low-temperature toughness governs. The applicable product or application standard decides the acceptance criteria; the joint design decides which failure mode must be checked.

Strength is expressed as stress, but a bolt carries force:

Axial force ≈ stress × tensile stress area

Tensile stress area depends on diameter and pitch. An M16 × 2 thread has about 157 mm² of tensile stress area, compared with about 58 mm² for M10 × 1.5. A larger 8.8 bolt can therefore carry more axial force than a much smaller 12.9 bolt. Grade selection never replaces the diameter and thread calculation.

Can two grades with similar tensile strength be substituted?

Not from a comparison chart alone.

AISC provides a useful real example. SAE J429 Grade 5 and Grade 8 bolts may be strength equivalents of ASTM F3125 A325 and A490 bolts, respectively, but AISC rejects direct substitution. They are not interchangeable without the structural application's required geometry, assembly and documentation. The structural grades also control head size and thread length and carry more stringent quality-assurance and inspection requirements.^[16]

Before approving a cross-system change, compare:

thread system, diameter, pitch/TPI and tolerance class;
head, shank, thread length and reduced-loadability geometry;
proof, yield, tensile, hardness, ductility and diameter breakpoints;
application qualification for structural, pressure, temperature or anchor service;
matching nut, washer, tapped thread and bearing surface;
coating, lubricant, thread allowance and tightening method; and
marking, test frequency, lot traceability and inspection documents.

Use a conversion chart to find candidates for engineering review. Record an approved alternative by drawing revision or written deviation.

Does a higher grade make the joint safer?

Only if the joint can use the additional strength.

A higher grade can permit more preload without permanent bolt stretch. It can also move the weak point into a nut, a tapped aluminum thread, a washer or a soft flange. In a cyclic joint, retained preload, joint stiffness and prevention of interface separation often matter more than ultimate tensile strength.

Nuts and washers therefore have their own mechanical systems. ISO 898-2 defines nut property classes 04, 05, 5, 6, 8, 10 and 12 and distinguishes thin, regular and high nut styles. A nut marked 10 is not a bolt class 10.9 with the decimal removed. SAE J995 covers common inch steel nut grades, while ASTM application standards use companion specifications such as A194/A194M. ISO 898-3 supplies mechanical requirements for applicable flat washers.^{[20] [21] [22] [23]}

Torque is another separate decision. Coating, topcoat, oil, wax, anti-seize and reuse change thread and bearing friction, so the same wrench torque can produce very different preload. ISO 16047 provides controlled torque/clamp-force testing; NASA's Fastener Design Manual explains why torque coefficients depend on the actual assembly condition.^{[24] [25]}

Coating choice becomes more sensitive as strength and hardness rise. ISO 4042 covers electroplated fasteners and measures intended to reduce hydrogen-embrittlement risk. ISO 10683 notes the particular use of non-electrolytic zinc-flake coatings on fasteners at or above 1,000 MPa to avoid hydrogen introduced by electrodeposition.^{[26] [27]}

The opposite blanket rule is also wrong: “8.8 and above can never be hot-dip galvanized.” ISO 10684 provides a controlled route for specified coarse-thread fasteners from M8 to M64 up to bolt class 10.9 and nut class 12. That scope is not general approval for every 8.8+ product or service; the listed size, pitch, dimensional and process requirements still have to be met.^[28]

Verify the delivered grade

A head mark identifies the claimed system and grade. It does not replace lot records.

Check that the required property-class or grade mark and manufacturer identification are present for the product and size. Then connect the physical lot to the order, material/heat records, heat-treatment route, coating condition and actual required test results.

ISO 16228 defines fastener inspection documents including declaration F2.1 and test reports F2.2, F3.1 and F3.2. The required document type should be stated when ordering. ISO 3269 provides a reference purchaser acceptance procedure when no prior inspection agreement exists, but excludes special-purpose and specially engineered applications needing more advanced control or traceability.^{[29] [30]}

Check the standard edition across markets

Related national standards can use familiar designations without being identical editions. GB/T 3098.1-2010 modifies ISO 898-1:2009 rather than identically adopting current ISO 898-1:2013. GB/T 3098.2-2025 modifies ISO 898-2:2022 for nuts, and GB/T 3098.6-2023 modifies ISO 3506-1:2020 for stainless fasteners.^{[17] [18] [19]}

State the governing national standard and edition on the order instead of relying on an edition-free claim such as “GB/T 10.9 equals ISO 10.9.”

What should go on the RFQ or drawing?

Avoid descriptions such as high tensile bolt, Grade 8 equivalent or 316 stainless bolt. State:

product form and dimensional standard;
complete thread designation, fit/tolerance, length and thread length;
governing mechanical or application standard, grade/property class and edition;
separate material designation when chemistry is controlled;
matching nut standard and grade/class, plus washer requirements;
coating standard, finish, lubricant and friction window where controlled;
service temperature, corrosive exposure, cyclic/shear duty and impact requirements;
tightening method or target preload for controlled joints; and
marking, inspection-document type, actual test results and lot traceability.

Examples of meaningful starting points are:

“M12 × 1.75-6g, ISO 4014, property class 10.9 to ISO 898-1, zinc flake to ISO 10683”;
“1/2-13 UNC-2A hex cap screw, SAE J429 Grade 8”;
“ASTM A193/A193M Grade B7, 1-8 UNC stud, with ASTM A194/A194M Grade 2H nuts”; and
“ASTM F3125/F3125M Grade A325 Type 1 heavy-hex structural bolt assembly”.

Complete each callout with size-specific dimensions, finish details, quantity, documentation and the requirements of the actual joint.

Continue by system: use the metric 8.8/10.9/12.9 guide, the SAE Grade 5/Grade 8 comparison, or the A2-70/A4-80 stainless guide. Return to How to Specify a Bolt to complete the size, thread, finish and inspection fields.