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The reference dimensions (outer diameters) of R, Rp, Rc, PT, G, and PF for pipe screws are shown in the following table.
Reference dimensions and pitch of pipe screws
| Tube size Direction(A) | Size (B) | Reference dimension (outer diameter) (mm) | Pitch (mm) |
| 1/16-28 | 7.723 | 0.9071 | |
| 6A | 1/8-28 | 9.728 | 0.9071 |
| 8A | 1/4-19 | 13.157 | 1.3368 |
| 10A | 3/8-19 | 16.662 | 1.3368 |
| 15A | 1/2-14 | 20.955 | 1.8143 |
| 20A | 3/4-14 | 26.441 | 1.8143 |
| 25A | 1-11 | 33.249 | 2.3091 |
| 32A | 1・1/4-11 | 41.910 | 2.3091 |
| 40A | 1・1/2-11 | 47.803 | 2.3091 |
| 50A | 2-11 | 59.614 | 2.3091 |
| 65A | 2・1/2-11 | 75.184 | 2.3091 |
| 80A | 3-11 | 87.884 | 2.3091 |
| 90A | 3・1/2-11 | 100.330 | 2.3091 |
| 100A | 4-11 | 113.030 | 2.3091 |
| 125A | 5-11 | 138.430 | 2.3091 |
| 150A | 6-11 | 163.830 | 2.3091 |
| 175A | 7-11 | 189.230 | 2.3091 |
| 200A | 8-11 | 214.630 | 2.3091 |
| 225A | 9-11 | 240.030 | 2.3091 |
| 250A | 10-11 | 265.430 | 2.3091 |
| 300A | 12-11 | 316.230 | 2.3091 |
The millimeter conversion of the outer diameter of the unified screw can be calculated by size ×25.4, but the millimeter conversion of the outer diameter of the pipe screw cannot be calculated and is determined by the standard as shown in the table above.
Example)
・The outer diameter of 1/4-20UNC is 1/4×25.4 = 6.350mm
The outer diameter of PT1/4-19 is 13.157mm (cannot be calculated).
Therefore, the tool diameter of the Tap of the pipe screw is larger than the value converted to millimeters.
In addition, the pitch of both unified screws and pipe screws can be calculated by the number of peaks between 1 inch.
(FAQID:368)