These thread series are obsolete. For the current series, see Unified.
When the American Standards Assn. (now ANSI) was formed, one of its first projects was to pull together a set of thread standards for the United States that would be consistent and would provide for most needs. The thread form and most sizes were based on the old United States Standard. Sizes below ¼ inch were added from the 1907 American Society of Mechanical Engineers standard. A fine-thread Series (NF) was taken from the "Regular Screw-Thread Series" of the Society of Automotive Engineers, with sizes below ¼ inch added from the A.S.M.E. "Fine-thread Series." Later the SAE Extra-Fine was added as the American National Extra Fine Thread Series (NEF).
The sizes in the first column that are not followed by an inch mark are American Standard Screw gauges. Pink shading indicates sizes no longer in use or whose use is discouraged by standards organizations. See this table for diameters of twist drills.
The NS series is a catch-all category for threads which have the American Standard form, but whose pitches are not in the National Coarse or National Fine series.
The tap drill sizes are listed in the order in which the threads per inch (tpi) are listed in the previous column.
| Gage and Frac- tional Sizes |
Major diam. (inches) |
Clear- ance Drill |
NC tpi | Tap Drill for NC | NF tpi | Tap Drill for NF | NEF | NS | Tap drill for NS | Nut Size |
|---|---|---|---|---|---|---|---|---|---|---|
| 0000 | #73 | 160 | 1⁄64″ | |||||||
| 000 | #63 | 120 | #71 | 3⁄32″ | ||||||
| 00 | #55 | 90 | #65 | 3⁄32″ | ||||||
| 0 | 0.060 | #52 | — | — | 80 | 3⁄64″ | 5⁄32″ | |||
| 1 | 0.073 | #48 | 64 | #52 | 72 | #53 | 56 | #54 | 5⁄32″ | |
| 2 | 0.086 | #43 | 56 | #50 | 64 | #50 | — | — | 3⁄16″ | |
| 3 | 0.099 | #38 | 48 | #47 | 56 | #45 | — | — | 3⁄16″ | |
| 4 | 0.112 | #33 | 40 | #43 | 48 | #42 | 32, 36 |
45, 44 |
¼″ | |
| 5 | 0.125 | #30 | 40 | #39 | 44 | #37 | 36 | 40 | ¼″ | |
| 6 | 0.138 | #28 | 32 | #36 | 40 | #33 | 36 | 34 | 5⁄16″ | |
| 7 | 0.151 | — | — | — | — | 30, 32, 36 |
31, 31, 1⁄8″ |
5⁄16″ | ||
| 8 | 0.164 | #19 | 32 | #29 | 36 | #29 | 30, 40 |
30, 28 |
11⁄32″ | |
| 9 | 0.177 | — | — | — | — | 24, 30, 32 |
29, 27, 26 |
11⁄32″ | ||
| 10 | 0.190 | #11 | 24 | #25 | 32 | #21 | 28, 30 |
23, 22 |
3⁄8″ | |
| 12 | 0.216 | 24 | 28 | 32 | 13 | 7⁄16″ | ||||
| 14 | 0.242 | D | 20, 24 |
10, 7 |
7⁄16″ | |||||
| ¼″ | 0.250 | ¼″ or E | 20 | #7 | 28 | #3 | 32 | 24, 27, 32 |
4, 3, 7⁄32″ |
7⁄16″ |
| 16 | 0.268 | — | — | — | — | 18, 20, 22 | 7⁄32″, 2 |
½″ | ||
| 18 | 0.294 | — | — | — | — | 18, 20 | B, D |
9⁄16″ | ||
| 5⁄16″ | 0.3125 | O | 18 | F | 24 | I | 32 | 20, 27, 32 | 17⁄64″, J, 9⁄32 |
9⁄16″ |
| 20 | 0.320 | 16, 18, 20 | G, 17⁄64″, I |
5⁄8″ | ||||||
| 22 | 0.346 | 16, 18 | 9⁄32″, L |
5⁄8″ | ||||||
| 24 | 0.372 | 16, 18 | 5⁄16″, O |
5⁄8″ | ||||||
| 3⁄8″ | 0.375 | V | 16 | 5⁄16″ | 24 | Q | 32 | 20, 27 | 5⁄8″ | |
| 26 | 0.398 | Y | 14, 16 | 21⁄64″, R |
11⁄16″ | |||||
| 28 | 0.424 | 7⁄16″ | 14, 16 | T, 23⁄64″ |
11⁄16″ | |||||
| 7⁄16″ | 0.4375 | 7⁄16″ | 14 | U | 20 | 25⁄64″ | 28 | 24, 27 | ||
| 30 | 0.450 | 29⁄64″ | 14, 16 | V, 25⁄64″ |
7⁄8″ | |||||
| ½″ | 0.5000 | ½″ | 13 (see note 1) | 27⁄64″ | 20 | 29⁄64″ | 28 | 12, 24, 27 | ||
| 12 | ¾″ | |||||||||
| 9⁄16″ | 0.5625 | 9⁄16″ | 12 | 31⁄64″ | 18 | 33⁄64″ | 24 | 27 | ||
| 5⁄8″ | 0.6250 | 5⁄8″ | 11 | 17⁄32″ | 18 | 37⁄64″ | 24 | 12, 27 |
||
| 11⁄16″ | 0.68753 | 11⁄16 | 24 | |||||||
| ¾″ | 0.7500 | 10 | 21⁄32″ | 16 | 20 | 12, 27 |
11⁄8″ | |||
| 7⁄8″ | 0.8750 | 9 | 49⁄64″ | 14 | 20 | 12, 18, 27 |
15⁄16″ | |||
| 1″ | 1.0000 | 8 | 7⁄8″ | 14 | 20 | 12, 27 |
1½″ |
1. In the Unified Series the ½ inch size has 12 threads per inch, but the American Standard retains 13 threads per inch.
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Last revised: 9 December 2005.