Stainless Steel Bolt Torque Chart: A2-70, A4-70 and A4-80 Explained
An M8 A2-70 or A4-70 stainless bolt has a general dry tightening value of about 25 Nm. M10 is about 50 Nm and M12 about 88 Nm. For A4-80, the equivalent guide values are higher: 32 Nm for M8, 69 Nm for M10 and 117 Nm for M12.
Those figures are reference values for clean, dry, coarse metric threads. They are not a design specification. Lubrication, anti-seize, surface finish, joint stiffness and the required preload can change the correct setting substantially. For structural, pressure, lifting, braking or other safety-critical assemblies, use the equipment or design authority's procedure.
Dry stainless steel bolt torque chart
The values below are commonly published recommended tightening torques for A2 and A4 stainless fasteners. A2-70 and A4-70 share a strength class, so their guide torque is the same. A2-80 and A4-80 share the higher class 80 values.
| Thread | A2-70 / A4-70 dry | A2-80 / A4-80 dry | Common spanner size |
|---|---|---|---|
| M3 | 1.35 Nm | 1.85 Nm | 5.5 mm |
| M4 | 3 Nm | 4 Nm | 7 mm |
| M5 | 6.1 Nm | 8 Nm | 8 mm |
| M6 | 10 Nm | 13.5 Nm | 10 mm |
| M8 | 25 Nm | 32 Nm | 13 mm |
| M10 | 50 Nm | 69 Nm | 17 mm |
| M12 | 88 Nm | 117 Nm | 19 mm |
| M14 | 140 Nm | 185 Nm | 22 mm |
| M16 | 215 Nm | 290 Nm | 24 mm |
| M18 | 380 Nm | 410 Nm | 27 mm |
| M20 | 435 Nm | 585 Nm | 30 mm |
Spanner sizes shown are common current ISO hex sizes. Older DIN parts and special head forms can differ, so check the actual fastener. The DIN, ISO and BS standards guide explains why nominally similar bolts can have different head dimensions.
What A2-70, A4-70 and A4-80 mean
The first part identifies the stainless family and grade. A2 is the common 304-type austenitic stainless family. A4 is the molybdenum-bearing 316-type family with improved resistance to chloride pitting. Our A2 vs A4 guide covers where each belongs.
The number is the property class:
- Class 70: minimum tensile strength of 700 N/mm² and minimum 0.2% proof stress of 450 N/mm² for the standard range covered by ISO 3506.
- Class 80: minimum tensile strength of 800 N/mm² and minimum 0.2% proof stress of 600 N/mm².
A4-80 therefore combines A4 corrosion resistance with a higher mechanical property class. It does not mean the installer should simply apply more torque to every A4-80 fastener. The joint, nut, washer, tapped material and thread engagement all need to support the intended preload.
Why torque is only an indirect measure
A tightened bolt stretches slightly and clamps the joint. That clamp load, or preload, is the useful result. Torque is only the input used to create it. Most of the applied torque is consumed overcoming friction under the head or nut and in the threads. Only a smaller part stretches the bolt.
Change the friction and the same wrench setting can produce a different preload. Common causes include:
- dry, waxed, oiled or anti-seize-coated threads;
- plain, polished, passivated or contaminated surfaces;
- a washer versus a nut turning directly on the joint;
- a new nut versus a reused or damaged thread;
- slow hand tightening versus a high-speed impact tool.
This is why copying a dry torque into a lubricated joint can overload it. Do not apply a generic percentage reduction without knowing the lubricant's tested nut factor and the design preload. Use the fastener, equipment or lubricant maker's approved data.
Dry or lubricated stainless threads?
Dry stainless threads are more vulnerable to galling, especially with prevailing-torque nuts, high installation speed or long thread engagement. Galling is adhesive wear: high local pressure breaks the passive film, metal transfers between the thread flanks and the fastener can lock solid.
A compatible anti-galling lubricant can make assembly more consistent and reduce seizure risk. It also changes friction, which is exactly why the torque must be recalculated or taken from the lubricant supplier. See how to stop stainless steel bolts seizing before assembling a critical A2 or A4 joint.
How to use the chart safely
- Read the head marking and product data. Confirm diameter, pitch, grade and property class. Do not infer A4-80 from colour or appearance.
- Check the mating part. The nut or tapped hole must have adequate grade and thread engagement. Avoid mixing an unknown low-strength nut with a class 80 bolt.
- Define the thread condition. Dry, lubricated and coated are different specifications.
- Find the equipment value. Use the design or manufacturer's torque where one exists. Use this chart only as general dry guidance.
- Use a calibrated tool. Apply torque smoothly with the socket square to the fastener. Do not add an extension to the handle unless the tool calculation allows it.
- Inspect the assembly. Stop if torque rises erratically, the nut binds or the thread shows damage.
Common torque mistakes
Using carbon-steel values for stainless
Carbon-steel property classes such as 8.8 and 10.9 are not interchangeable with A2-70 or A4-80. Match the material and property class shown on the fastener.
Using an impact wrench for final torque
An impact wrench is fast, but its delivered torque depends on air pressure, battery state, joint stiffness and trigger time. Snug the joint if permitted, then finish with the specified calibrated method.
Reusing a nyloc nut without checking
A nylon insert adds prevailing torque that is not the same as clamp-producing torque. Reuse can change that drag. Follow the joint specification, particularly where locking performance matters.
Ignoring aluminium threads
A stainless bolt may be stronger than the aluminium tapped hole. The female thread can strip before the bolt approaches its generic chart value. Our guide to stainless fasteners in aluminium covers both thread damage and galvanic exposure.
Continue through the stainless fixing series
Build a matched stainless joint
- M10 x 30 A2-70 hexagon set screw - a common fully threaded workshop size covered by the 50 Nm dry reference.
- M8 x 30 A4-80 hexagon set screw - higher property class with A4 corrosion resistance for suitable marine assemblies.
- M10 A4 Form A flat washer - provides a controlled bearing surface under a matching M10 joint.
Frequently asked questions
What is the torque for an M8 A2-70 stainless bolt?
About 25 Nm is a common dry-thread reference value. Use the equipment maker's specification where available, and reduce or recalculate for lubrication using tested data.
What is the torque for an M10 A4-80 bolt?
About 69 Nm is a common dry reference for a coarse M10 A4-80 fastener. The joint design, nut, washer and lubrication can require a different setting.
Are A2-70 and A4-70 torque values the same?
General charts normally give the same value because both are property class 70. Their main difference is corrosion resistance, not the nominal class strength.
Should stainless steel bolts be lubricated?
Lubrication is often used to reduce galling, but it increases preload at a given torque. Use a compatible product and its verified torque or nut-factor data.
Can I use this chart for structural bolts?
No. Structural and safety-critical joints need the specified fastener system, preload method, inspection and installation procedure. A generic chart is not a structural design.
Next step: photograph the fastener marking, record whether the thread is dry or lubricated, and check the equipment specification before setting the wrench. Browse matching hexagon set screws, hexagon nuts and flat washers.
