This article focuses on one of the most common small-size threads—the M4 thread size, also known as the M4 metric thread.
We’ll walk you through its dimensions, drilling requirements, and practical application tips to help improve assembly efficiency and machining accuracy.
Understanding metric thread dimensions is key to achieving precision fits and smooth fastening performance.
一、Basic Parameters of M4 Thread (Based on ISO 724:2023)
| Thread Parameter | Value |
| Nominal Diameter (D) | 4.0 mm |
| Pitch (P) | 0.7 mm |
| Tapping Drill Diameter | 3.3 mm |
| Clearance Hole Diameter | 4.5 mm |
Choosing the correct drilling diameter is essential in everyday machining.
The 3.3 mm tapping drill ensures sufficient material for forming full threads, while the 4.5 mm clearance hole allows the screw to pass through smoothly and engage with an internal thread on the other side.
For accurate internal threading, selecting the correct tap drill for M4 threads ensures proper thread engagement and strength.
二、Explanation of Key Thread Dimensions
1. Major Diameter
The largest diameter across the thread crests. It determines the external contour of the bolt or threaded hole and is a key indicator for fit.
2. Minor Diameter
The smallest diameter between the thread roots. It directly affects thread strength, especially in load-bearing or frequent disassembly applications.
3. Pitch Diameter
The theoretical neutral surface of the thread where most engagement occurs. Poor fit is often due to mismatched pitch diameters.
4. Pitch
The standard pitch for M4 is 0.7 mm, which is the axial distance between two adjacent threads. Knowing the pitch helps confirm thread type and avoid mix-ups. The thread pitch for M4 threads plays a key role in matching fasteners and ensuring compatibility.
For accurate internal threading, selecting the correct tap drill for M4 threads ensures proper thread engagement and strength.
三、Tapping and Clearance Hole Recommendations
Before tapping internal threads, pre-drill using a 3.3 mm tapping drill bit. This size accounts for cutting allowance and ensures full thread formation, suitable for most materials like steel, aluminum, and plastic.
If only a through-hole is needed (without threading), use a 4.5 mm clearance drill bit，which serves as a standard clearance hole for M4 screw applications.
This is typical for designs where a screw passes through one part and threads into another.
Tips: Adjust drill diameter slightly based on material hardness to prevent stripping or overly tight threads.
四、How to Identify an M4 Thread
M4 indicates a thread with a 4 mm nominal diameter and 0.7 mm pitch.
The full designation is written as M4×0.7, the standard M4 screw thread specification used on engineering drawings.
五、Practical Machining Advice
When selecting a drill size for M4 tap, ensure it aligns with the material’s properties and thread depth requirements.
- Use carbide or high-cobalt drill bits for improved efficiency.
- Use taps with chip flutes to aid chip removal.
- Apply a small amount of cutting oil before tapping to extend tool life.
- After machining, use a thread plug gauge to inspect thread completeness and fit.
Proper CNC drilling for threaded holes also reduces tool wear and enhances repeatability in batch production.
FAQ
1. I want to use an M4 screw. What hole size should I design?
Do not use Ø4 mm. For M4 × 0.7 threads, the recommended tapping drill diameter is 3.3 mm. Ø4 mm is the major diameter of the M4 screw and doesn’t leave enough material for threading.
Refer to a standard tapping drill chart for M4 to verify recommended hole sizes before machining.
2. Should I model physical threads in 3D CAD?
Usually not necessary. Cosmetic threads are preferred.
Just clearly annotate thread specs in 2D drawings, e.g., M4 × 0.7 – 6H.
Modeling threads complicates the CAD model and can lead to manufacturing misunderstandings.
Be sure to check the required tapping depth for M4 thread based on the fastener length used.
3. What should I provide to the machine shop?
Provide two files:
1. STEP file (with only a 3.3 mm hole, no modeled thread)
2. 2D PDF drawing with full thread annotations
Though small, the M4 thread is widely used in precision assemblies, electronics, and automation equipment.
Understanding its standard dimensions and machining practices not only improves product quality but also prevents rework and assembly issues.
How can CNCFirst help you?
If you have any questions about part design or thread machining, feel free to contact us.
With rich CNC machining experience, we provide end-to-end support from drawing optimization to final delivery.
