🔧 What is the tightening torque for fasteners? Is it better to tighten it tighter?

Simple and straightforward answer: It’s not the tighter the better! Overtightening is more dangerous than not tightening – it can cause plastic elongation (yielding) of the bolt, thread slippage, and even on-site or delayed fracture.

1、 Why can’t we ‘twist desperately’?

The tightening of bolts relies on pre tightening force (clamping force), not torque itself:
• Loosen too much → After being loaded, the connecting parts separate and the nuts loosen and vibrate

Tightening too tightly (exceeding the yield point) → The bolt is stretched and “dead”, losing its elastic rebound ability, and is prone to breakage under impact

-Electric/pneumatic wrenches are prone to overtightening by 30% to 50% depending on the hand feel, and torque wrenches must be used for important parts

✅ Correct method: According to the bolt specifications+performance level+friction conditions, check the standard torque table to set the target value, and tighten step by step.

2、 Recommended tightening torque reference for commonly used carbon steel bolts (dry/non lubricated, grade 8.8)

Specification 8.8 Recommended torque (N · m) 10.9 Recommended torque (N · m) 12.9 Recommended torque (N · m)

M6 10～12 14～17 17～20

M8 25～30 35～42 45～50

M10 50～60 70～85 95～110

M12 85～100 120～145 165～190

M14 130～160 190～230 260～310

M16 210～250 300～360 410～480

M20 410～490 590～700 800～950

⚠️ The above are reference values for steel to steel, dry state, and standard rough surface (the lower limit of ISO/GB recommended range is conservative).

-Apply lubricating grease/molybdenum disulfide → Friction ↓ → Same torque, excessive pre tightening force, need to reduce torque by about 20%~30%

-With flat/spring washers, surface galvanized/Dacromet different → torque may fluctuate by ± 15%

3、 Key points for correct tightening operation ✅

1. Use a torque wrench (especially for M12 and above or important connections)
2. Multi bolt connection → step-by-step+cross sequence tightening:
-Step 1: Pre tighten all bolts (hand tighten+slightly firm)
Step 2: 50% target torque, in diagonal/star order

Step 3: 100% target torque, re tighten in the same order again

3. The material/length/grade of the same connecting bolt must be consistent, and mixing is prohibited
4. Important stress nodes (steel structure/cylinder head/flange) are recommended to be subjected to torque+angle method or hydraulic tension according to design specifications, not just torque

4、 Special situation reminder ⚠️

Situation handling

Stainless steel bolts have a high coefficient of friction, and the lower limit of torque for carbon steel of the same specification can be set. After applying anti bite paste, the pre tightening force control can be appropriately increased

Aluminum/soft connectors are prone to crushing, with torque values ranging from 70% to 80%. If necessary, increase the size of the gasket

Reusing old bolts and high-strength bolts is not recommended for secondary use (pre tightening force attenuation+minor damage), and key parts must be replaced with new ones

Considering the thermal expansion difference under high temperature conditions, thermal re tightening or the use of heat-resistant bolts may be necessary

5、 One sentence summary ✅

Check the standard torque according to specifications and grades → use a torque wrench → tighten diagonally step by step → never forcefully twist by hand

Twist to the specified torque=just right, then twist again=dangerous ❌