Screw Specifications and Models: Dimensions Explained
2025-12-16
一. Basic Knowledge of Screws
1.1 Definition and Classification
Screw definition: A general term for fasteners with external threads, which achieve connection through the clamping force generated by the thread engagement.
Main classifications:
Machine screws: Used with nuts or threaded holes, 60° standard thread profile
Self-tapping screws: Can tap threads in soft materials on their own
Self-drilling screws: Have a drill bit at the end for direct drilling and tapping.
1.2 Key Parameter Illustration
Key dimensions:
- d = Nominal diameter (e.g., 6mm for M6)
- P = Pitch (distance between adjacent threads)
- L = Nominal length (excluding the head)
二. Comparison of Major Standard Systems
| Standard System | Code | Features | Common Applications | Examples |
| Chinese National Standard | GB/T | Domestic Mainstream | General Machinery | GB/T 5782 (Hexagonal Head Bolts) |
| International Standard | ISO | Globally Applicable | Export Products | ISO 4017 (Hex Head Bolts) |
| German Standard | DIN | Commonly Used in Europe | Precision Machinery | DIN 912 (Hex Socket Head Screws) |
| American Standard | ANSI | Imperial System | American Equipment | ANSI B18.3 |
| Japanese Standard | JIS | Asia Region | Electronic Products | JIS B1176 |
Note: Even if the product name is the same, the specific dimensions may vary slightly depending on the standard.
2.1 Comparison Examples of ANSI and ISO/GB Standards
| ANSI Specifications | Outer Diameter (inch/mm) | Pitch (TPI/mm) | Closest to ISO Specification | Outer Diameter Difference | Application Considerations |
| #10-24 UNC | 0.190"/4.83mm | 24TPI/1.06mm | M5×0.8 | +0.17mm | ANSI is slightly smaller and cannot be directly interchanged. |
| 1/4"-20 UNC | 0.250"/6.35mm | 20TPI/1.27mm | M6×1.0 | +0.35mm | The stress distribution is different, so recalculation is required. |
| 5/16"-18 UNC | 0.313"/7.94mm | 18TPI/1.41mm | M8×1.25 | -0.06mm | Similar dimensions but different pitch |
| 3/8"-16 UNC | 0.375"/9.53mm | 16TPI/1.59mm | M10×1.5 | -0.47mm | Not interchangeable, requires adapter. |
2.2 Cross-standard selection principles:
Priority Principle: Avoid mixing different standard systems as much as possible.
Equivalent Replacement: Strength must be recalculated; simple size-based replacement is not acceptable.
Conversion Solution: Use threaded adapters or redesign the interface.
Typical Case: When repairing American equipment, a 1/4"-20 bolt may be damaged. In an emergency, an M6×1.0 bolt can be used as a temporary replacement, but the original specification must be replaced as soon as possible.
三. Specifications and Model Details
3.1 Example of Model Number Interpretation
Example 1: M6×20-8.8
M6: Metric thread, diameter 6mm
×20: Length 20mm
8.8: Strength grade (tensile strength 800MPa, yield strength 640MPa)
Example 2: ST4.2×16-FPB
ST4.2: Self-tapping thread, diameter 4.2mm
×16: Length 16mm
F: Flat head
P: Phillips head
B: Black finish
3.2 Metric Thread Specification Table (Common Coarse Threads)
| Specifications | Pitch (mm) | Drilling Diameter (mm) | Hex Head Across Flats (mm) | Load Capacity Reference (kN)* |
| M3 | 0.5 | 2.5 | 5.5 | 1.8 |
| M4 | 0.7 | 3.3 | 7 | 3.3 |
| M5 | 0.8 | 4.2 | 8 | 5.3 |
| M6 | 1.0 | 5.0 | 10 | 7.4 |
| M8 | 1.25 | 6.8 | 13 | 13.6 |
| M10 | 1.5 | 8.5 | 16 | 21.6 |
| M12 | 1.75 | 10.2 | 18 | 31.2 |
| M16 | 2.0 | 14.0 | 24 | 58.0 |
*Bearing capacity is based on the minimum tensile load of grade 8.8 bolts according to GB/T 5782 standard.
3.3 Self-tapping screw specification table
| Specifications | Thread Outer Diameter (mm) | Thread Pitch (mm) | Pre-drilled Hole Diameter (mm) | Applicable Plate Thickness (mm) |
| ST2.2 | 2.2 | 0.8 | 1.6-1.8 | 0.5-2.0 |
| ST2.9 | 2.9 | 1.1 | 2.2-2.4 | 0.8-3.0 |
| ST3.5 | 3.5 | 1.3 | 2.6-2.8 | 1.0-4.0 |
| ST4.2 | 4.2 | 1.4 | 3.1-3.3 | 1.5-5.0 |
| ST4.8 | 4.8 | 1.6 | 3.6-3.8 | 2.0-6.0 |
Principles for calculating pre-drilled hole diameter:
Mild steel/Low carbon steel: Hole diameter = Thread outer diameter × 0.75
Stainless steel/Hard materials: Hole diameter = Thread outer diameter × 0.80-0.85
Aluminum alloy/Soft metals: Hole diameter = Thread outer diameter × 0.70-0.75
Plastic/Wood: Hole diameter = Thread outer diameter × 0.60-0.70
四. Depth Technical Parameters
4.1 Detailed Explanation of Bolt Strength Grades
| Grade | Tensile Strength (MPa) | Yield Strength (MPa) | Application Scenarios | Materials |
| 4.8 | 400 | 320 | Light load | low carbon steel |
| 6.8 | 600 | 480 | General machinery | Medium carbon steel |
| 8.8 | 800 | 640 | Important Connections | Medium Carbon Steel Quenching |
| 10.9 | 1000 | 900 | High strength requirements | Alloy steel quenching |
| 12.9 | 1200 | 1080 | Special occasions | Alloy steel quenching |
4.2 Tightening Torque Calculation
Basic Formula:
T = K × d × F
Where:
T = Tightening Torque (N·m)
K = Torque Coefficient (generally taken as 0.15-0.20)
d = Nominal Thread Diameter (m)
F = Preload (N) = Yield Strength × Effective Area × 0.6-0.7
Common Tightening Torque Reference Table (8.8 grade, K=0.16):
4.3 Thread fit and tolerance
Fit Grades:
6H/6g: Standard fit (most commonly used)
6G/6h: Loose fit (easy to assemble)
4H/4h: Precision fit (high precision requirements)
五. Intelligent Selection Guide
5.1 Quick Selection Flowchart
Start Selection
↓ [Determine Connection Type]
├─→ Thick Metal Plate → Machine Bolt + Nut
├─→ Thin Metal Plate → Self-Tapping/Self-Drilling Screws
├─→ Plastic Parts → Plastic-Specific Self-Tapping Screws
└─→ Wood → Wood Screws
↓
[Calculate Required Strength]
├─→ Light Load (<50kg) → M4-M6
├─→ Medium Load (50-200kg) → M8-M10
└─→ Heavy Load (>200kg) → M12 and above
↓
[Determine Environmental Requirements]
├─→ Indoor Dry → Galvanized Carbon Steel
├─→ Outdoor/Humid → Stainless Steel 304
└─→ Marine/Chemical → Stainless Steel 316
↓
[Select Head and Slot Type]
├─→ Requires Recessed Finish → Countersunk Head
├─→ High Torque → Hex Head/Internal Hex Head
└─→ Standard Assembly → Cross Pan Head
5.2 Industry Application Cases
auto industry
Engine Assembly
Application Scenario: Cylinder Head Bolt Connection
Specification Selection: M10×1.25 fine thread, grade 12.9
Special Requirements: Tightening using yield point control method, angle monitoring method
Key Point: Using MoS2 coating to reduce friction coefficient variation
Chassis System
Application Scenario: Suspension System Connection
Specification Selection: M14×1.5, grade 10.9, Dacromet treatment
Special Requirements: Shock and vibration resistance, corrosion resistance
Example: A German car model uses M14×80 bolts with locking function for the lower control arm of the front suspension.
Aerospace
Airframe Structure
Application Scenarios: Aluminum Alloy Skin Riveting
Specification Selection: NAS1351C4-6 (1/4"-28 UNF Titanium Alloy Bolts)
Special Requirements: Lightweight, High Strength, Fatigue Resistance
Technical Specifications: Tensile Strength ≥1100MPa, 45% lighter than steel
Engine Hot-Section Components
Application Scenarios: Turbine Blade Installation
Specification Selection: Inconel 718 Material, M6×20
Operating Temperature: 650°C
Special Treatment: Silver Coating to Prevent Seizure
Electronic Equipment
PCB Board Fixing
Application Scenarios: Circuit board to chassis connection
Specifications: M2.5×5, nickel-plated brass
Special Requirements: Non-magnetic, conductive grounding
Assembly Torque: 0.35 N·m (to prevent PCB deformation)
Precision Instruments
Application Scenarios: Optical instrument adjustment mechanism
Specifications: M3×0.35 fine thread, 316 stainless steel
Special Process: Electrolytic polishing, burr-free
Precision Grade: 6g
Medical Devices
Orthopedic Implants
Material Requirements: Titanium alloy Ti6Al4V, biocompatible
Surface Treatment: Anodizing, roughness Ra0.8
Typical Size: HA3.5×14 (self-tapping type)
Sterilization Requirements: High temperature and high pressure sterilization
5.3 Selection Recommendations for Special Environments
Vibration Environment
Use screws with locking function (nylon locking rings)
Use spring washers or locking washers
Consider using anti-loosening coatings (such as Loctite)
High Temperature Environment
<250°C: Stainless Steel 310
<550°C: High Temperature Alloy A286
550°C: Nickel-based Alloy Inconel
Clean Environment
Select grease-free screws
Use passivated or electropolished surfaces
Avoid the risk of plating peeling
六. Common Problems and Solutions
6.1 Prevention of thread stripping
Cause Analysis:
Excessive tightening torque
Improper thread fit
Material strength mismatch
Solutions:
Use a torque wrench to control the torque
Check thread specification compatibility
Upgrade the thread strength grade
6.2 Guidelines for Corrosion-Resistant Material Selection
| Environmental conditions | Recommended materials | Surface treatment | Expected lifespan |
| Indoor drying | Carbon steel | 5-8μm galvanized | 5-10 years |
| Outdoor atmospheric conditions | Carbon steel | Hot-dip galvanized >45μm | 15-20 years |
| Slight corrosion | 304 Stainless Steel | Passivation | 20-30 years |
| Marine environment | 316 stainless steel | Passivated | Over 30 years |
| Strong acids and bases | Hastelloy alloy | - | Hastelloy alloy |
6.3 Assembly Precautions
Pre-drilling Requirements
Machine screws: Hole diameter = Thread diameter + 0.1-0.2mm
Self-tapping screws: Hole diameter = 0.7-0.8 × Thread diameter
Thread Length Selection
Steel: ≥ 1.0 × Thread diameter
Cast iron: ≥ 1.5 × Thread diameter
Aluminum alloy: ≥ 2.0 × Thread diameter
Tightening Sequence
For multi-bolt connections, use diagonal tightening.
Gradually reach the target torque in 2-3 passes.
七. Practical Quick Reference Tools
7.1 Imperial-Metric Conversion Table
| Imperial units | Approximately equivalent to metric units (mm) | Closest to metric units |
| #4-40 | 2.84 | M3 |
| #6-32 | 3.51 | M3.5 |
| #8-32 | 4.17 | M4 |
| #10-24 | 4.83 | M5 |
| 1/4"-20 | 6.35 | M6 |
| 5/16"-18 | 7.94 | M8 |
| 3/8"-16 | 9.53 | M10 |
| 1/2"-13 | 12.70 | M12 |
7.2 Quick Estimation Formula
Bolt Quantity Estimation:
n = (1.5 × F) / (f × d²)
Where:
n = Number of bolts
F = Total load (N)
f = Allowable stress per unit area (MPa)
d = Bolt diameter (mm)
Recommended Safety Factor:
Static load: 1.5-2.5
Dynamic load: 2.5-4.0
Impact load: 4.0-6.0
八. Summary and Recommendations
8.1 Core Principles of Selection
Safety First: Choose larger over smaller.
Standards First: Prioritize standard parts to reduce costs.
Environmental Compatibility: Materials must be suitable for the operating environment.
Ease of Maintenance: Consider ease of replacement later.
Industry Standards: Follow industry-specific technical standards.
8.2 Procurement Recommendations
Establish a list of commonly used specifications in stock.
Select reputable suppliers.
Require material certificates and test reports.
Conduct small-batch trials before bulk purchases.
Special industries (such as aerospace and medical) require certified suppliers.
