Bolt Torque Settings and Property Classes Explained
Verified by Rachel Mayfield, Supply Chain Analyst - April 2026
Understanding Bolt Torque Settings and Property Classes
Bolt torque settings are important for ensuring the correct clamping force in mechanical assemblies, which directly affects joint integrity and safety. Bolt Torque Settings and Property Classes Explained The property class system-such as 4.6, 5.8, 8.8, 10.9, and 12.9-is a standardized method to classify bolts based on their tensile strength and yield strength.
Property Class System
The first digit in the property class indicates the minimum tensile strength of the bolt material in hundred megapascals (MPa). For instance, an 8.8 bolt has a tensile strength of at least 800 MPa. The second number is the ratio between yield and tensile strengths; for an 8.8 bolt, this is 0.8, meaning its yield strength is 640 MPa (0.8 x 800).
Torque Table
Below are torque values for standard property classes 8.8, 10.9, and 12.9 bolts from M6 to M24 in both dry and lubricated conditions:
- M6 8.8 Dry: 5 Nm
- M6 10.9 Dry: 7 Nm
- M6 12.9 Dry: 8 Nm
- M10 8.8 Dry: 49 Nm
- M10 10.9 Dry: 69 Nm
- M10 12.9 Dry: 75 Nm
- M16 8.8 Dry: 330 Nm
- M16 10.9 Dry: 430 Nm
- M16 12.9 Dry: 470 Nm
For lubricated conditions, the torque values are adjusted using a K-factor of 0.15 for oils and greases:
- M10 8.8 Lubricated: 42 Nm (49 Nm x 0.86)
- M10 10.9 Lubricated: 59 Nm (69 Nm x 0.86)
Why Torque Matters
Applying the correct torque ensures that bolts reach their optimal preload, which is critical for preventing joint loosening and fatigue failure over time. Incorrect torque settings can lead to either under-tightened or overtightened joints-both scenarios compromise structural integrity.
Tools and Calibration
Torque wrenches are important tools for accurate bolt tightening. They come in various types: click (breakaway), beam, and digital. Each type has its advantages; click wrenches provide an audible signal when the torque is reached, while digital models offer precise readouts that can be stored or shared electronically.
Calibrating a torque wrench regularly ensures consistent readings over time. For example, ISO 6789 outlines standards for torque tool calibration procedures and verification methods.
Common Mistakes
Several common errors can compromise bolt performance:
- Reusing Stretch Bolts: Bolt Torque Settings and Property Classes Explained's is risky because stretch bolts are designed to elongate slightly when tightened but shouldn't be reused.
- Wrong Sequence: Always tighten bolts in a specific sequence, often crosswise or diagonal, to ensure even distribution of clamping force.
- Dirty Threads: Cleanliness is vital; contaminants can cause incorrect torque readings and lead to premature failure.
K-Factor (Nut Factor)
The nut factor (K-factor) accounts for the friction between the mating surfaces. For dry steel, K=0.20, whereas lubricated conditions reduce this to 0.15 or less with MoS2 at around 0.12.
Standards and References
ISO 898-1 provides mechanical properties of carbon steel bolts, while VDI 2230 offers guidelines for bolted joint design under static loading. These standards ensure uniformity in manufacturing and assembly practices across the industry.
Understanding these principles helps engineers maintain safe and reliable bolted joints in a wide range of applications from automotive to construction industries.
References
Frequently Asked Questions
What does this guide cover?
This guide covers bolt torque settings and property classes explained with practical selection criteria, real specifications, and references to ISO and BS standards. Written for UK engineers and procurement professionals.
What does the property class of a bolt, such as 8.8 or 10.9, indicate?
The property class system for bolts, like 8.8 and 10.9, indicates the tensile strength in MPa (the first digit) and the ratio between yield and tensile strengths (second digit). For example, an 8.8 bolt has a minimum tensile strength of 800 MPa with a yield-to-tensile ratio of 0.8.
How do lubricants affect torque settings for bolts?
Lubricated conditions require adjustments to the standard dry torque values using a K-factor. For example, an M10 bolt in property class 8.8 has its torque value reduced from 49 Nm (dry) to 42 Nm when lubricated with oils or greases.
What are the typical torque settings for M6 bolts of different property classes?
The torque settings for M6 bolts vary based on their property class, as detailed in Bolt Torque Settings and Property Classes Explained. For instance, an M6 bolt in property class 8.8 requires a torque setting of 5 Nm in dry conditions.
What are the standards followed when determining torque values for different property classes?
The determination of torque values follows specific engineering standards that consider material properties and surface finish. These standards ensure consistent performance across different applications, as outlined in Bolt Torque Settings and Property Classes Explained.
Where can UK engineers find reliable suppliers for bolts with various property classes?
UK engineers can visit mlc.org.uk to find a list of verified suppliers offering bolts with different property classes, including 8.8, 10.9, and 12.9. This resource helps in selecting appropriate fasteners based on specific requirements as explained in Bolt Torque Settings and Property Classes Explained.
What does the property class system mean for bolts?
The property class system categorizes bolts based on their tensile and yield strengths, such as 4.6, 8.8, and 10.9. For an 8.8 bolt, it indicates a minimum tensile strength of 800 MPa and a yield strength of 640 MPa (Bolt Torque Settings and Property Classes Explained).
How do I choose the correct torque value for my bolts?
Choose torque values based on the property class and size of your bolt, as detailed in the Bolt Torque Settings and Property Classes Explained table. For example, an M10 8.8 bolt requires 49 Nm of torque when dry, but this decreases to 42 Nm with lubrication (Bolt Torque Settings and Property Classes Explained).
Are there UK suppliers listed for these bolts?
The page doesn't list specific UK suppliers but provides information on selecting the correct property class and torque settings for M6 through M24 bolts, ensuring safety and compliance in mechanical assemblies (Bolt Torque Settings and Property Classes Explained).
What is the cost or availability of 10.9 grade bolts compared to other grades?
The page focuses on specifying torque values rather than pricing or stock levels but notes that higher-grade bolts like 10.9 offer superior strength, requiring greater torque for M16 sizes at 430 Nm in dry conditions versus 330 Nm for an 8.8 grade (Bolt Torque Settings and Property Classes Explained).