M18 Thread Specs
Standard: ISO 261 (ISO Metric Coarse)
This reference guide outlines the technical dimensions and tapping parameters for the standard M18 metric thread.
Part of the ISO Metric Coarse series defined by ISO 261, the thread features a nominal diameter of 18 mm and a pitch of 2.5 mm.
For machining or fabrication, the recommended tap drill size is 15.5 mm (for 75% thread depth) or 15.66 mm (for 65% thread depth).
Dimensional Specifications (mm)
Nominal Diameter
18 mm
Pitch
2.5 mm
Threads per mm
0.4
Major Diameter
18 mm
Pitch Diameter (d₂)
16.376 mm
Minor Diameter (d₁) Ext
15.294 mm
Minor Diameter (D₁) Int
15.655 mm
Thread Height
2.165 mm
Root Radius
0.361 mm
Drilling & Strength Specs
Tap Drill (75%)
15.5 mm
Tap Drill (65%)
15.66 mm
Stress Area
192.47 mm²
Stress Diameter
14.933 mm
Recommended Tightening Torque
| Property Class | Dry (Nm) | Lubricated (Nm) |
|---|---|---|
| Class 8.8 | 359 | 269.3 |
| Class 10.9 | 528 | 396 |
| Class 12.9 | 634 | 475.5 |
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Frequently Asked Questions
What is the tap drill size for a M18 thread?
For standard thread engagement (approx. 75%), the recommended tap drill size for M18 is 15.5 mm. For a slightly looser 65% thread engagement, a 15.66 mm drill bit is recommended.
What is the pitch of a M18 thread?
The thread pitch for a standard M18 thread is 2.5 mm, which equates to approximately 0.4 threads per millimeter.
What are the major and pitch diameters for M18?
The nominal major diameter of M18 is 18 mm. The calculated pitch diameter (d₂) is 16.376 mm per the ISO 68-1/ISO 261 standards.