M20 Thread Specs
Standard: ISO 261 (ISO Metric Coarse)
This reference guide outlines the technical dimensions and tapping parameters for the standard M20 metric thread.
Part of the ISO Metric Coarse series defined by ISO 261, the thread features a nominal diameter of 20 mm and a pitch of 2.5 mm.
For machining or fabrication, the recommended tap drill size is 17.5 mm (for 75% thread depth) or 17.66 mm (for 65% thread depth).
Dimensional Specifications (mm)
Nominal Diameter
20 mm
Pitch
2.5 mm
Threads per mm
0.4
Major Diameter
20 mm
Pitch Diameter (d₂)
18.376 mm
Minor Diameter (d₁) Ext
17.294 mm
Minor Diameter (D₁) Int
17.655 mm
Thread Height
2.165 mm
Root Radius
0.361 mm
Drilling & Strength Specs
Tap Drill (75%)
17.5 mm
Tap Drill (65%)
17.66 mm
Stress Area
244.79 mm²
Stress Diameter
16.933 mm
Recommended Tightening Torque
| Property Class | Dry (Nm) | Lubricated (Nm) |
|---|---|---|
| Class 8.8 | 507 | 380.3 |
| Class 10.9 | 745 | 558.8 |
| Class 12.9 | 894 | 670.5 |
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Frequently Asked Questions
What is the tap drill size for a M20 thread?
For standard thread engagement (approx. 75%), the recommended tap drill size for M20 is 17.5 mm. For a slightly looser 65% thread engagement, a 17.66 mm drill bit is recommended.
What is the pitch of a M20 thread?
The thread pitch for a standard M20 thread is 2.5 mm, which equates to approximately 0.4 threads per millimeter.
What are the major and pitch diameters for M20?
The nominal major diameter of M20 is 20 mm. The calculated pitch diameter (d₂) is 18.376 mm per the ISO 68-1/ISO 261 standards.