Preferred Sizes
ISO 261 specifies a detailed list of preferred combinations of outer diameter D and pitch P for ISO metric screw threads.
ISO 262 specifies a shorter list of thread dimensions – a subset of ISO 261.
| Nominal diameter D (mm) ISO 261 |
Pitch P (mm) |
||
|---|---|---|---|
| 1st choice ISO 262 |
2nd choice |
coarse | fine |
| 1 | 0.25 | ||
| 1.2 | 0.25 | ||
| 1.4 | 0.3 | ||
| 1.6 | 0.35 | ||
| 1.8 | 0.35 | ||
| 2 | 0.4 | ||
| 2.5 | 0.45 | ||
| 3 | 0.5 | ||
| 3.5 | 0.6 | ||
| 4 | 0.7 | ||
| 5 | 0.8 | ||
| 6 | 1 | ||
| 7 | 1 | ||
| 8 | 1.25 | 1 | |
| 10 | 1.5 | 1.25 or 1 | |
| 12 | 1.75 | 1.5 or 1.25 | |
| 14 | 2 | 1.5 | |
| Nominal diameter D (mm) |
Pitch P (mm) |
||
|---|---|---|---|
| 1st choice |
2nd choice |
coarse | fine |
| 16 | 2 | 1.5 | |
| 18 | 2.5 | 2 or 1.5 | |
| 20 | 2.5 | 2 or 1.5 | |
| 22 | 2.5 | 2 or 1.5 | |
| 24 | 3 | 2 | |
| 27 | 3 | 2 | |
| 30 | 3.5 | 2 | |
| 33 | 3.5 | 2 | |
| 36 | 4 | 3 | |
| 39 | 4 | 3 | |
| 42 | 4.5 | 3 | |
| 45 | 4.5 | 3 | |
| 48 | 5 | 3 | |
| 52 | 5 | 4 | |
| 56 | 5.5 | 4 | |
| 60 | 5.5 | 4 | |
| 64 | 6 | 4 | |
The “coarse” pitch is the commonly-used default pitch for a given diameter. In addition, one or two smaller “fine” pitches are defined, for use in applications where the height of the normal “coarse” pitch would be unsuitable (e.g., threads in thin-walled pipes). The terms “coarse” and “fine” have (in this context) no relation to the manufacturing quality of the thread.
In addition to coarse and fine threads, there is another division of extra fine, or “superfine” threads, with a very fine pitch thread. Superfine pitch metric threads are occasionally used in automotive components, such as suspension struts, and are commonly used in the aviation manufacturing industry. This is because extra fine threads are more resistant to coming loose from vibrations.
Read more about this topic: ISO Metric Screw Thread
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