Bolt Tightening Torque Chart: Recommended Torque Values for Metric Bolts (8.8 & 10.9)

Why Bolt Torque Matters

Under-tightening a bolted joint causes loosening under vibration; over-tightening risks thread stripping, bolt fracture, or permanent damage to the clamped parts. A correct torque value ensures the bolt reaches its intended clamping force (preload) — typically 70–90% of the bolt's proof load — which is what actually holds the joint together, not the friction of the threads.

The universally used approximation is:

T = K × D × F

T = tightening torque (Nm)

K = friction coefficient (nut factor): 0.20 dry / 0.15 zinc-plated / 0.12 lubricated

D = nominal bolt diameter (m)

F = desired preload (N), usually 70–90% of proof load

This formula appears in most engineering references (e.g., Engineering Toolbox fastener resources and bolt manufacturer guides). It is an approximation — for critical joints (pressure vessels, structural steel per EN 1090, automotive safety parts), always follow the specified tightening method and check with a torque wrench calibration.

Recommended Torque Values — Grade 8.8 (Dry, K = 0.2)

Recommended Torque Values — Grade 10.9 (Dry, K = 0.2)

Values are reference figures for plain, dry threads (K = 0.2) and standard steel surfaces. They are not a substitute for a design calculation. For safety-critical applications, use the preload specified in ISO 898-1 or your structural code.

How Lubrication and Finish Change the Numbers

The nut factor K is the biggest source of torque variation. The same M12 bolt can require very different torque depending on surface condition:

Two practical consequences:

1. Never re-use torque values across finishes. Switching from plain to zinc-plated bolts without reducing torque risks overtightening and thread stripping.

2. Galvanized and lubricated joints need separate charts. Hot-dip galvanized threads are rougher and need higher torque for the same preload; lubricated threads need lower torque for the same preload.

Torque vs. Turn: When a Chart Is Not Enough

For critical joints, torque control alone has accuracy limits (typically ±25% on preload) because of friction scatter. Higher-accuracy methods:

• Torque + angle control — tighten to a snug point, then rotate a defined angle

• Hydraulic tensioning — for large-diameter bolts (M24+)

• Ultrasonic bolt measurement — measures actual elongation

If a project specifies "torque + angle" or "tension control," follow that procedure instead of the torque table.

What Grade Do You Actually Need?

• Grade 8.8 — tensile strength 800 MPa. The workhorse for general machinery, automotive, and structural applications. Best balance of strength, ductility, and cost.

• Grade 10.9 — tensile strength 1000 MPa. For higher-stress and dynamic-load joints where 8.8 is insufficient.

Property classes follow ISO 898-1 (the first digit × 100 = tensile strength in MPa; the second digit = yield ratio). For a deeper comparison, see our separate guide on Grade 8.8 vs 10.9 bolts.

How to Choose the Right Bolt for Your Application

1. Determine the required preload from your joint design

2. Select the property class (8.8 / 10.9 / 12.9) that delivers it with a safety margin

3. Choose the finish for the environment (plain, zinc, hot-dip galvanized, Dacromet, stainless)

4. Verify the standard (DIN 933 / ISO 4017 / GB/T 5783 for full-thread hex bolts)

5. Apply the correct torque or angle procedure on assembly

Ningbo Weifeng Fastener Co., Ltd. manufactures hex bolts in grades 4.8–12.9, in carbon steel and stainless steel (304/316), with zinc, black oxide, hot-dip galvanized, or Dacromet finishes, to DIN 933, DIN 931, ISO 4014, ISO 4017, GB/T 5782 and GB/T 5783. We are a China-based fastener manufacturer serving buyers and distributors worldwide since 2003 — browse our hex bolt range or send us your inquiry with your specifications and we will confirm the right product and torque data for your application.

FAQ

What torque should I use for an M8 8.8 bolt? Approximately 25 Nm for dry threads (K = 0.2). Reduce to ~20 Nm if zinc-plated, or verify with your application's specification.

How much torque for an M12 10.9 bolt? Approximately 120 Nm dry. Use the 10.9 column of the chart above — never use 8.8 values for a 10.9 bolt, as the higher-strength bolt is designed for a higher preload.

Is torque the same as preload? No. Torque is the input you apply; preload is the clamping force actually achieved. Friction consumes 40–50% of applied torque, which is why K-factor charts exist.

Can I use this chart for stainless steel bolts? Stainless steel has different friction behavior and a higher galling risk. Use dedicated stainless torque guidance and consider thread lubricant (anti-seize) to prevent galling.

Why is my bolt loosening even at the correct torque? Likely causes: insufficient preload for the vibration level, no locking element (nylon insert nut, thread locker, or serrated flange), or incorrect K-factor used during assembly.

Do you provide bolts with a specified tightening procedure? We supply fasteners in the specified grade and finish, and can share material certificates and standard compliance documents. For application-specific torque procedures, please provide your joint design and we will advise on the appropriate standard.

Sources: ISO 898-1 (mechanical properties of fasteners), DIN 933 / DIN 931, ISO 4014 / ISO 4017, GB/T 5782 / GB/T 5783, Engineering Toolbox fastener references, common industry torque references. Values above are reference data for general use — always validate against your engineering specification.