M18 bolts are commonly used as metric fasteners in the industrial sector, and their dimensional accuracy, pitch selection and torque control have a direct impact on the safety and stability of equipment operation. In this article, we will discuss everything you need to know when choosing M18 bolts.
What Is an M18 bolt?
M18 bolts are medium-to-large metric fasteners typically used in structural connections requiring high load-bearing capacity. Common performance grades include 8.8, 10.9, and 12.9.
- M：Metric thread
- 18：Nominal thread diameter
M18 Bolt Size Specification Table
| Parameter | Standard Value (mm) | Typical Tolerance | Project Description |
|---|---|---|---|
| Nominal Diameter D | 18 | ±0.13 | Match with M18 nut/female hole |
| Coarse Pitch P | 2.5 | ±0.076 | Universal assembly for efficient installation |
| Fine Pitch P | 1.5 | ±0.058 | Superior anti-loosening performance |
| Thread Length L | 30–200 (Customizable) | ±1.0 / ±1.5 | Select based on clamping thickness |
| Hexagon Opposite Sides S | 27 | ±0.5 | For 27mm wrench |
| Head Height K | 11.5 | ±0.3 | Affecting torque transmission |
| Groove depth (cross) | 9.8 | ±0.2 | Prevent slipping |
Note:
- High Strength Grade: Grade 10.9 M18 bolts with tensile strength ≥1040MPa, suitable for wind turbine tower connections.
- Corrosion Resistance Treatment: Dacromet coating with salt spray test endurance ≥1000 hours, suitable for harsh environments like offshore platforms.
- Customized Solutions: Capable of producing extended bolts with thread lengths up to 500mm or countersunk screws with head diameters reduced to 24mm.
What Is the Thread Pitch of an M18 Screw?
According to ISO standards (ISO 68-1), the coarse thread pitch of M18 screws is 2.5mm. This is the most widely used specification globally, suitable for most general mechanical assembly scenarios. Its advantages include:
International Standard
- High Efficiency: 2.5mm feed per revolution, ideal for rapid assembly;
- Strong Compatibility: Fully interchangeable with mainstream standards like GB, DIN, and JIS;
- Low Cost: High production volume reduces mold amortization costs, resulting in more competitive pricing.
Typical applications: Construction scaffolding, general machinery casings, pipe connections, and other cost-sensitive scenarios with moderate load requirements.
Fine-Pitch Thread Specifications
When higher self-locking or finer adjustment is required, M18 screws are available with fine threads with a pitch of 1.5 mm.Advantages of fine threads include:
- Improved anti-loosening performance: Smaller thread lead angle reduces loosening in vibrating environments;
- Precision control: Reduced feed per revolution makes it suitable for fine-adjustment mechanisms (e.g., optical equipment, medical instruments);
- Thin-walled connections: Fine-pitch threads cause less weakening of hole walls for the same bolt diameter, ideal for lightweight designs.
Typical applications: Automotive engine compartments, aerospace equipment, precision instruments, and other scenarios with extremely high reliability requirements.
Coarse-Thread vs Fine-Thread
- Coarse Thread (2.5mm): Fast installation, strong contamination resistance, suitable for construction and engineering machinery
- Fine Thread (1.5mm): Superior anti-loosening performance, more uniform load distribution, suitable for vibration-prone and high-precision applications (engines, rail transit)
Custom Pitch
BD Fastener can develop non-standard pitch specifications according to customer requirements, such as:
- 1.0mm ultra-fine thread: Suitable for semiconductor equipment and ultra-precision machining centers;
- 3.0mm coarse thread: Designed for high-load applications such as mining machinery and heavy-duty lifting equipment.
Technical: Custom cold heading dies and thread rolling wheels, combined with optimized heat treatment processes, ensure the strength of non-standard threads matches that of standard parts.
Multi-threaded Design
For high-speed rotation or rapid feed requirements, BD Fastener can produce double-start threads (pitch 1.25mm) or triple-start threads (pitch 0.83mm), featuring:
- Increased feed rate: A single rotation achieves feed equivalent to multiple pitch lengths;
- Uniform load distribution: Multiple threads disperse stress, extending service life.
Typical Applications: CNC machine tool spindles, automated production line conveying systems
BD Fastener: The Technical Benchmark for Pitch Control
Full-Process Precision Control
- Cold heading forming: Pitch deviation controlled within ±0.03mm;
- Thread rolling: Precise control of rolling depth via servo motor;
- Inspection process: Equipped with a comprehensive thread measuring instrument capable of simultaneously inspecting 12 parameters including pitch, thread angle, and cumulative pitch error.
Material and Process Innovation
- High-Strength Materials: Developed high-strength alloy steel maintaining Grade 10.9 strength even at 1.5mm fine thread pitch;
- Surface Treatment: Utilizes Dacromet coating (8-12μm thickness), passing 1000 hours of salt spray testing without corrosion, suitable for marine environments;
- Intelligent Monitoring: Customized M18 bolts with built-in sensors for wind power clients, providing real-time feedback on thread pitch changes to prevent loosening failures.
Global Certification System
- Certified to ISO 9001, IATF 16949, CE, RoHS, and other standards;
- The laboratory holds CNAS accreditation and can issue test reports compliant with ASTM F1470 standards.
Typical Cases:
- Supplied M18 aluminum alloy bolts for Tesla’s Shanghai Gigafactory, passing temperature difference tests from -40°C to 150°C with thread pitch deformation <0.01mm;
- Customized M18×1.5 stainless steel bolts for aerospace launch site ground equipment, meeting military standards.
Torque for M18 bolts
| Performance Grade | Recommended Torque (Nm) | Typical Applications |
|---|---|---|
| 4.8 | 80 – 100 | Light-load structure |
| 8.8 | 220 – 250 | Automotive chassis, construction machinery |
| 10.9 | 275 – 310 | Wind power, pressure vessels |
| 12.9 | 320 – 350 | High-end equipment, rail transit |
Special Operating Condition Torque Adjustment Rules:
- Galvanized bolts: Torque reduced by 10–15%
- Stainless steel (A2-70): Approximately 180–200 Nm
- High-temperature environments (>200°C): Preload torque must be increased by 20% to compensate for thermal relaxation effects
Tool Selection
- Manual torque wrench: Suitable for small-batch assembly, accuracy ±5%
- Electric torque wrench: Recommended for production lines, accuracy ±3% (e.g., Atlas Copco equipment supplied with BD fasteners)
- Hydraulic impact wrench: Suitable for high-torque applications (>500Nm), requires counterforce arm
Construction Process
- Clean threads: Use a wire brush to remove metal shavings and oil residue from the hole.
- Lubricate: Apply molybdenum disulfide lubricant (coefficient of friction 0.12-0.18) to the threaded area.
- Initial Tightening Stage: Apply 50% of the recommended torque to ensure the bolt is vertically aligned.
- Final Tightening Stage: Apply the remaining torque within 30 seconds to prevent thread seizure.
- Marking Inspection: Use a paint marker to apply an anti-loosening mark on the bolt head.
M18 Bolt Tensile Strength Standard
Performance Grade and Material Association
The pull-out force of an M18 bolt is closely related to its performance grade. For example, the yield strength of an 8.8-grade bolt reaches 640 MPa, while that of a 10.9-grade bolt reaches as high as 900 MPa. Taking a 10.9-grade M18 bolt as an example, its theoretical pull-out force can be calculated using the formula: F = σs × A.
Where σs is the yield strength (900 MPa) and A is the stress-bearing cross-sectional area of the thread (approximately 192 mm² for an M18 bolt), the calculated theoretical pull-out force is approximately 172.8 kN. In practical applications, a safety factor of 1.5 to 2 times is typically applied, resulting in an actual pull-out force range of 86.4 to 115.2 kN.
International Standards and Industry Regulations
- ISO 898-1: Specifies the mechanical properties of bolts, defining the tensile strength and yield strength for different grades of bolts.
- DIN 267: German Industrial Standard, setting specific requirements for bolt tightening torque and pull-out force.
- ASTM A325: American Society for Testing and Materials standard, applicable to high-strength structural bolts.
For example, under ISO standards, a Grade 10.9 M18 bolt must achieve a minimum pull-out force of 153 kN (calculated based on the stress-area ratio and yield strength).
Differentiated Requirements for Application Scenarios
- Automotive Manufacturing: Bolts must exhibit high fatigue resistance, with pull-out strength meeting dynamic load testing requirements.
- Wind Power Generation: Bolts must withstand vibration and impact in extreme environments, with pull-out strength standards 20%-30% higher than standard applications.
- Petrochemical Industry: High corrosion resistance is required; bolts must be made of stainless steel or special alloys, with pull-out strength validated through salt spray testing.
What Size Pilot Hole Should Be Drilled for an M18 Screw?
- Standard M18 coarse thread
| Material | Recommended Pilot Hole Diameter |
|---|---|
| Steel / Cast Iron | 16.2 – 16.5 mm |
| Aluminum Alloy | 16.5 – 16.8 mm |
| Copper | 16.7 – 17.0 mm |
- M18 Fine Thread
The pilot hole diameter must be calculated based on the specific pitch, typically slightly smaller than the coarse-thread pilot hole. Use a standard tapping manual or formula to confirm.
BD Fastener: A Benchmark Company in the Fastener Industry
BD Fastener factory is located in Yongnian District, Handan City, Hebei Province, covering an area of more than 20,000 square meters, with an investment of more than 20 million yuan, and an annual production capacity of more than 10,000 tons, covering more than 60 countries and regions around the world. With the concept of “pioneering and innovation, beyond service”, the company has built a whole chain quality control system from R&D to production.
Advanced Production and Testing Equipment
- Introduced multi-station cold heading machine, large hot heading machine, thread rolling machine and other equipment for the automatic production of bolts of M6-M39 specifications.
- Equipped with magnetic particle flaw detector, metallographic analyzer, direct-reading spectrometer and other testing equipment to ensure that the products comply with GB, DIN, ASTM, ISO, JIS and other standards.
- Customized specification diameter covers M20-M140mm and length up to 1400mm.
Diversified Product Portfolio
- Bolt Types: Including fishtail bolts, hex flange bolts, wheel hub bolts, etc., with mechanical properties ranging from Grade 8.8 to Grade 12.9.
- Nut Types: Hex nuts, heavy-duty nuts, wing nuts, etc., available in carbon steel and stainless steel (304/316L).
- Custom Non-Standard: Manufacture specialized fasteners per client drawings to meet requirements in aerospace, medical devices, and other sectors.
Global Certification and Quality Assurance
- Certified under ISO 9001 Quality Management System and IATF 16949 Automotive Industry Standard.
- Export products comply with EU CE certification and US UL certification.
