Metric Bolt Torque Chart: Tightening Guide for Grades 4.6, 8.8, 10.9 & 12.9

Getting bolt torque right matters. Too little and the joint works loose under vibration. Too much and you risk stretching the bolt, stripping the thread, or cracking the component. This guide gives you verified torque values for every common metric bolt grade — from the commercial-grade 4.6 through to high-tensile 12.9, plus stainless A2-70 and A4-80 — along with the adjustment factors you need for HDG, oiled, and anti-seize conditions.

How to Identify Your Bolt Grade

Before you can look up a torque value, you need to know your bolt's grade. Metric bolt grades are stamped on the head. The most common markings you'll encounter in Australian industry are:

• 4.6 — Commercial grade. General-purpose carbon steel, low strength. Often used in non-critical structural and general fabrication work where high-tensile fasteners are not required.
• 8.8 — High-tensile. The most widely used grade in Australian engineering and manufacturing. Identified by "8.8" on the bolt head.
• 10.9 — Very high-tensile. Used in high-load applications such as automotive, heavy machinery, and structural connections subject to dynamic loading.
• 12.9 — Ultra-high-tensile. The highest standard metric bolt grade. Socket head cap screws are commonly grade 12.9. Not for use in corrosive environments without appropriate coating.
• A2-70 / A4-80 — Stainless steel. A2 is 304 stainless; A4 is 316 marine grade. The number indicates tensile strength (700 MPa and 800 MPa respectively). Note: stainless bolts require lower torque values than carbon steel of equivalent strength — see the stainless table below.

No marking on the head? The bolt is likely a low-grade commercial fastener — treat it as 4.6 and do not apply high-tensile torque values. For a full guide to bolt markings and grade comparisons, see the AIMS bolt grade chart. The same grade designations and torque values in this guide also apply to threaded rod (allthread) — the torque spec is identical to a bolt of the same grade and diameter.

Metric Bolt Torque Chart — Grades 4.6 / 8.8 / 10.9 / 12.9

The values below are maximum tightening torques in Newton-metres (Nm) for metric coarse-thread bolts with clean, dry, uncoated steel threads (K = 0.2), tightened to 75% of proof load per ISO 898-1. If your threads are lubricated, plated, or coated, apply the adjustment factors in the next section.

Values calculated per ISO 898-1 at 75% proof load, K = 0.2 (dry uncoated steel). Reference values only — see disclaimer above.

Stainless Steel Bolt Torque Chart — A2-70 and A4-80

Stainless bolts require separate torque values for two reasons. First, their mechanical properties differ from carbon steel grades of the same approximate strength. Second — and more importantly — stainless-to-stainless threads are prone to galling (thread seizure from cold welding under load). Standard practice is to lubricate stainless threads with a copper-based anti-seize compound before assembly, which also changes the K-factor from 0.2 to approximately 0.13. The table below gives torque values for stainless with anti-seize applied.

Stainless values calculated with K = 0.13 (copper anti-seize applied), 75% proof load per ISO 3506. Always lubricate stainless threads to prevent galling. Reference values only.

Torque Adjustment Factors — Lubrication, Coatings and Plating

The main tables above assume clean, dry, uncoated steel threads — a K-factor of 0.2. In reality, bolt threads are often plated, lubricated, or treated. Each condition changes the friction coefficient and therefore the torque required to achieve the same preload. Applying the wrong K-factor for your thread condition is one of the most common causes of incorrect preloading — either stretching bolts by over-torquing an oiled thread with dry-thread values, or under-clamping a dry HDG thread that needs more torque than most charts show.

K-factor values based on VDI 2230 and industry reference data. Apply to Nm values from the main tables above.

Example: M12 Grade 8.8 bolt, hot-dip galvanised. Table value = 91 Nm (dry). Adjusted torque = 91 × 1.25 = 114 Nm.

Example: M16 Grade 10.9 bolt, oiled threads. Table value = 313 Nm (dry). Adjusted torque = 313 × 0.75 = 235 Nm.

Coarse Thread vs Fine Thread — Does Pitch Affect Torque?

Yes, but modestly. The standard metric coarse thread is what the tables above cover — it's what the vast majority of industrial bolts use. Metric fine pitch threads (MF) have a smaller thread pitch for the same diameter, which increases the threads-per-unit-length and slightly raises the friction component of the torque equation. Fine pitch bolts of the same grade typically require 8–12% higher torque to achieve the same preload as their coarse-pitch equivalents.

In practice, if you're using metric fine pitch bolts (common in precision machinery, automotive, and hydraulic components) and the manufacturer has not provided a torque specification, add approximately 10% to the coarse-pitch table values above. However, manufacturer specifications should always take priority — fine pitch bolts are often used in precision applications where specific torque values are critical.

Not sure whether you have coarse or fine pitch? Count the thread pitch with a thread gauge, or refer to the metric fastener thread standards guide.

Socket Head Cap Screws — Torque vs Hex Head Bolts

Socket head cap screws (also called Allen head bolts or cap screws) are almost always Grade 12.9 for metric sizes. However, their recommended tightening torque is typically set at about 80% of the calculated maximum — because the small hex socket drive is prone to rounding if over-driven, and because SHCS are commonly used in tapped blind holes where thread engagement length matters more than absolute preload.

As a working rule: use 80% of the Grade 12.9 values from the main table for standard socket head cap screws in steel, unless the manufacturer specifies otherwise. For example, M10 Grade 12.9 table value = 84 Nm → SHCS working torque ≈ 67 Nm. For aluminium tapped holes, reduce further — typically to 50–60% of the steel value to avoid stripping the softer thread. See the socket head cap screw guide for full selection and torque guidance.

K-Factor and Nut Factor Explained

The K-factor (also called nut factor or torque coefficient) is the single most important variable in bolt torque calculations — and the one most often misunderstood. It's a dimensionless constant that accounts for all the friction in the joint: under-head friction, thread friction, and a small contribution from thread geometry.

The torque formula is: T = K × F × d

Where T is tightening torque (Nm), K is the nut factor, F is the desired bolt preload (N), and d is the nominal bolt diameter (m). K is emphatically not a material property — it's an empirical value that depends on thread surface condition, lubrication, plating, thread quality, and the condition of the mating surfaces.

Why does this matter? Because K can vary from 0.10 (PTFE-coated fasteners) to 0.35 (corroded or rough threads), and this variation is multiplied directly through the torque calculation. A bolt tightened to 100 Nm with K = 0.20 achieves very different preload than the same bolt tightened to 100 Nm with K = 0.13. For most general industrial work, K = 0.20 (dry uncoated steel) is the correct baseline. For anything critical, verify the K-factor for your specific thread condition before specifying a torque value.

Over-Tightening and Under-Tightening — What Goes Wrong

Both failure modes are common and both are preventable with correct torque application.

Over-tightening stretches the bolt beyond its yield point, permanently reducing its cross-sectional area. Once a bolt yields, it loses its elastic clamping capacity — it cannot be returned to correct preload by retightening, and must be replaced. Repeated over-tightening in aluminium tapped holes strips the thread entirely, often ruining the component. In brittle materials (cast iron, some plastics), the compressive stress under the bolt head can cause cracking around the hole. Galvanised bolts are particularly susceptible because the rougher HDG thread means most mechanics instinctively stop tightening before the bolt has reached the higher torque actually required — but some overcompensate and go too far.

Under-tightening is statistically more common and often more dangerous, because the failure is progressive rather than immediate. An under-torqued joint works loose under vibration (the Junker effect), reducing clamping load progressively until the joint either separates or the bolt shears under the resulting bending load. Self-loosening under vibration is virtually eliminated by correct preload — the friction in a properly torqued joint is sufficient to prevent rotation. For vibration-critical applications, combine correct torque with an appropriate threadlocker or locking fastener system.

How to Use a Torque Wrench Correctly

A torque wrench is only as accurate as its calibration and the technique of the person using it. A few things to get right:

Choose the right range. A torque wrench is most accurate at 20–80% of its rated maximum. Using a 500 Nm wrench to torque an M8 bolt to 26 Nm puts you at 5% of range — accuracy drops to ±20% or worse. Use a wrench rated for the torque you're actually applying. For M4–M12 fasteners, a 5–50 Nm wrench is appropriate. For M16–M24, use a 100–500 Nm rated wrench.

Pull, don't push. Apply force to the handle in a smooth, steady pull. Jerking or pushing reduces accuracy. For a click-type wrench, stop immediately when you hear and feel the click — continuing to apply force after the click over-torques the bolt.

Account for extensions. Adding a socket extension does not change torque as long as the extension is in line with the drive. If you use a side extension to reach an awkward bolt, you introduce a lever arm that changes the effective torque applied — calculate accordingly.

Calibration. Click-type torque wrenches should be calibrated annually or every 5,000 cycles, whichever comes first. Store them wound back to the lowest setting — leaving a click wrench at high torque setting compresses the spring and accelerates drift. Beam wrenches and dial wrenches do not require calibration management in the same way, but check that the zero returns correctly before each use.

Sequence for multi-bolt joints. For flanges, covers, and head bolts, tighten in a cross pattern (star or cross sequence) in three passes: 30%, 70%, 100% of final torque. This ensures even clamping load distribution and prevents gasket distortion.

When to Follow Manufacturer Specifications Instead of This Chart

This chart is a general reference. It is not appropriate for the following situations — always use manufacturer-specified torque values or consult a qualified engineer:

Structural steel connections. AS 4100 (Steel Structures) and AS 4600 (Cold-Formed Steel) specify installation torque and procedures for structural bolts. 8.8/S and 10.9 structural bolts used in friction-type joints have specific snug-tight and full-pretension procedures that go beyond a simple torque value.

Lifting and rigging equipment. Any fastener in a lifting application — eye bolts, shackle pins, crane superstructure, hoist mountings — must be torqued and locked to the manufacturer's specification. No generic chart applies. See the SWL vs WLL vs MBL guide for load rating context.

Pressure systems and hydraulic connections. Threaded fittings in hydraulic and pneumatic circuits must be torqued per fitting manufacturer specifications. Applying bolt torque values to hydraulic fittings will almost certainly cause leaks or thread damage.

Cylinder head bolts and engine fasteners. These are almost always torque-to-yield and require torque-angle sequences specified by the engine manufacturer. Replace them after any removal.

Proprietary fastener systems. Huck bolts, Superbolt tensioners, hydraulic bolt tensioning systems, and similar proprietary solutions have their own installation specifications that override ISO 898-1 calculations.

Australian Standards for Metric Fasteners

For Australian industry, the key standards governing metric fastener mechanical properties and assembly are:

AS/NZS 1110.1 and AS/NZS 1110.2 — Mechanical and physical properties of metric bolts, screws, and studs. These are the Australian adoptions of ISO 898-1 and ISO 898-2. The proof load stress values used in this guide's torque calculations are taken from these standards.

AS 4100 — Steel Structures. Governs structural bolt grades, installation method (snug-tight vs fully pretensioned), and minimum edge distances for bolted connections in structural steel. References bolt grades 8.8/S, 10.9/S, and 12.9/HF.

AS/NZS 1554 series — Structural steel welding standards, which set requirements where bolted and welded connections are used together.

AS/NZS 3992 — Pressure equipment, which sets requirements for bolted pressure vessel and flange connections.

For general industrial maintenance and non-structural applications, there is no mandatory Australian standard requiring use of specific torque values. However, Safe Work Australia guidelines require that fastened joints be assembled in accordance with the manufacturer's instructions or, where none exist, to industry-recognised practice — which this guide supports.

Frequently Asked Questions

What is the torque for an M10 bolt Grade 8.8?

For a clean, dry M10 Grade 8.8 bolt, the reference torque is 52 Nm. If the threads are oiled, reduce to approximately 39 Nm. If hot-dip galvanised, increase to approximately 65 Nm. Always confirm with the equipment manufacturer's specification if one exists.

What is the torque for an M8 bolt Grade 8.8?

For a clean, dry M8 Grade 8.8 bolt, the reference torque is 26 Nm. With lubricated threads, approximately 20 Nm. With HDG threads, approximately 33 Nm. M8 is one of the most commonly used fastener sizes in Australian light industrial and fabrication work.

What is the torque for an M12 bolt Grade 8.8?

For a clean, dry M12 Grade 8.8 bolt, the reference torque is 91 Nm. HDG adjustment: 91 × 1.25 = 114 Nm. Oiled: 91 × 0.75 = 68 Nm. M12 is common in structural connections, machinery frames, and equipment mounting plates.

What is the torque for an M16 bolt Grade 8.8?

For a clean, dry M16 Grade 8.8 bolt, the reference torque is 226 Nm. This typically requires a 1/2" or 3/4" drive torque wrench rated for at least 280 Nm. For structural applications under AS 4100, follow the snug-tight and pretensioning procedures rather than a generic torque value.

What is the torque for an M20 bolt Grade 8.8?

For a clean, dry M20 Grade 8.8 bolt, the reference torque is 426 Nm. At this size, a 3/4" drive torque wrench is typically required. Confirm this is not a structural connection requiring AS 4100 pretensioning procedures before applying a generic torque value.

Do I need to reduce torque for lubricated bolts?

Yes — significantly. Lubricating threads reduces the K-factor from approximately 0.20 to 0.15, which means the same torque produces about 33% more preload. Applying dry-thread torque values to an oiled bolt will over-tension it. Reduce torque by approximately 25% when threads are lightly oiled with machine oil. With anti-seize (copper or moly), reduce by approximately 35%.

What torque should I use for hot-dip galvanised (HDG) bolts?

Hot-dip galvanised bolts have a rougher zinc coating that increases thread friction, raising the K-factor to approximately 0.25 vs 0.20 for bare steel. This means you need to apply approximately 25% more torque than the table values to achieve the same preload. Example: M12 Grade 8.8 HDG = 91 × 1.25 = 114 Nm. Many maintenance tradespeople under-torque HDG bolts because they feel stiffer at lower torque values — this is the friction, not the preload. Use a calibrated torque wrench, not feel.

What torque should I use for stainless steel bolts?

Use the stainless torque table above rather than the carbon steel grades. Always apply copper-based anti-seize compound to stainless threads before assembly to prevent galling (thread seizure). If assembling stainless-into-steel rather than stainless-into-stainless, galling risk is lower but anti-seize is still recommended. The A2-70 and A4-80 values in this guide already assume anti-seize is applied.

What happens if I overtighten a bolt?

The bolt stretches beyond its yield point, permanently losing its ability to provide correct clamping force. In threaded holes (as opposed to through-bolts with nuts), overtightening can strip the thread — especially in aluminium or cast iron. In flanged joints, overtightening can crush the gasket beyond its recovery range. Once a bolt has been yielded, replace it — retightening will not restore correct preload, and the bolt's fatigue life is compromised.

What happens if I undertighten a bolt?

The joint lacks sufficient clamping force and can work loose under vibration, thermal cycling, or dynamic loading. Self-loosening is the primary failure mode — the bolt gradually rotates itself out of the joint. In machinery, this creates fretting wear, progressive loosening of adjacent fasteners, and ultimately joint failure. Under-torqued bolts in pressurised systems or lifting equipment create serious safety risks. Use a torque wrench, not feel — the difference between 50 Nm and 80 Nm of torque is imperceptible by hand on an M10 bolt.

What bolt grade should I use if there's no marking on the head?

Treat it as Grade 4.6 and apply the corresponding torque values. Unmarked bolts are typically low-grade commercial fasteners. Do not apply Grade 8.8 or higher torque values to an unmarked bolt — it may not have the proof load to sustain the preload, and could yield or fracture. For any application requiring Grade 8.8 or higher, use properly marked, certified fasteners from a reputable supplier.

Do I always need a torque wrench?

For non-critical connections under M8, experienced tradespeople often estimate by feel — but this introduces variability of ±30–50%. For anything M10 and above, structural, pressure-bearing, or vibration-critical, use a calibrated torque wrench. For M16 and above, a torque wrench is effectively mandatory — the clamping loads are too high to judge accurately by feel, and the consequences of a mistake are proportionally greater.

What is the difference between coarse and fine pitch torque?

Fine pitch metric bolts (MF series) require approximately 8–12% higher torque than coarse pitch bolts of the same grade and diameter to achieve the same preload. In practice, if no manufacturer specification exists, add 10% to the coarse-thread table values for fine-pitch fasteners. Fine pitch bolts are more commonly found in precision machinery, automotive applications, and hydraulic components than in general industrial fastening.

What is proof load and how does it relate to torque?

Proof load is the maximum tensile force a bolt can sustain without permanent deformation — it's below the yield strength and represents the safe working region of the bolt's elastic range. The torque tables in this guide are calculated to achieve 75% of proof load as preload, which is the standard industrial target: high enough to resist self-loosening, well short of yielding the fastener. The ISO 898-1 proof load for Grade 8.8 is 600 MPa (for diameters up to M16), giving a target preload of 450 MPa — translated to a torque via the K-factor equation.

Should I use this chart for thread-forming screws into plastic or aluminium?

No. Thread-forming screws (self-tapping, thread-rolling) create their own mating thread and have completely different torque requirements. Applying bolt torque values will strip the formed thread. Use torque values from the screw manufacturer's datasheet, or follow assembly guidelines for the specific material and hole size. As a general guide, thread-forming screw torque is typically 30–60% of a tapped bolt of the same diameter.