Index to fastener thread series
Content
Table
These series (and the UNR, UNJ and fixed-pitch series) are the inch-based thread series currently used in the United States, and have been since 1949.1 They superceded the American National Series (NC, NF, etc.) Pink shading indicates secondary sizes whose use is discouraged by standards organizations. See this table for diameters of twist drills.
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 |
UNC tpi |
Tap Drill for UNC | UNF tpi | Tap Drill for UNF | UNEF tpi |
UNS tpi |
Tap drill for UNS | Nut Size |
|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0.0600 | #52 | — | — | 80 | 3⁄64″ | — | — | — | 5⁄32″ |
| 1 | 0.0730 | #48 | 64 | #52 | 72 | #53 | — | — | #54 | 5⁄32″ |
| 2 | 0.0860 | #43 | 56 | #50 | 64 | #50 | — | — | — | 3⁄16″ |
| 3 | 0.0990 | #37 | 48 | #47 | 56 | #45 | — | — | — | 3⁄16″ |
| 4 | 0.1120 | #32 | 40 | #43 | 48 | #42 | — | 32, 36 |
#45, #44 |
¼″ |
| 5 | 0.1250 | #30 | 40 | #39 | 44 | #37 | — | 36 | #40 | ¼″ |
| 6 | 0.1380 | #27 | 32 | #36 | 40 | #33 | — | 36 | #34 | 5⁄16″ |
| 8 | 0.1640 | #18 | 32 | #29 | 36 | #29 | — | 30, 40 |
#30, #28 |
11⁄32″ |
| 10 | 0.190 | #9 | 24 | #25 | 32 | #21 | 28, 36, 40, 48, 56, 30 |
#23, #22 |
3⁄8″ | |
| 12 | 0.216 | #2 | 24 | 28 | 32 | 36, 40, 48, 56 |
#13 | 7⁄16″ | ||
| ¼″ | 0.2500 | F | 20 | #7 | 28 | #3 | 32 | 24, 27, 32, 36, 40, 48, 56 |
4, 3, 7⁄32″ |
7⁄16″ |
| 5⁄16″ | 0.3125 | P | 18 | F | 24 | I | 32 | 20, 27, 28, 36, 40 48 |
17⁄64″, J, 9⁄32 |
9⁄16″ |
| 3⁄8″ | 0.375 | W | 16 | 5⁄16″ | 24 | Q | 32 | 18, 27, 36, 40 |
5⁄8″ | |
| 0.390 | 0.390 | 25/64 | — | — | — | — | — | 27 | ||
| 7⁄16″ | 0.4375 | 29⁄64″ | 14 | U | 20 | 25⁄64″ | 28 | 18, 24, 27 |
||
| ½″ | 0.5000 | 33/64″ | 13 (see note 2) | 27⁄64″ | 20 | 29⁄64″ | 28 | 12, 14, 18, 24, 27 |
¾″ | |
| 9⁄16″ | 0.5625 | 9⁄16″ | 12 | 31⁄64″ | 18 | 33⁄64″ | 24 | 14, 27 |
||
| 5⁄8″ | 0.6250 | 5⁄8″ | 11 | 17⁄32″ | 18 | 37⁄64″ | 24 | 14, 27 |
||
| 11⁄16″ | 0.68753 | 11⁄16 | 24 | — | ||||||
| ¾″ | 0.7500 | ¾ | 10 | 21⁄32″ | 16 | 20 | 14, 18, 24 |
11⁄8″ | ||
| 13⁄16″ | 0.8125 | 13⁄16″ | — | — | — | — | 20 | — | ||
| 7⁄8″ | 0.8750 | 7⁄8″ | 9 | 49⁄64″ | 14 | 20 | 10, 18, 24, 27 |
15⁄16″ | ||
| 15⁄16″ | 0.9375 | 15⁄16″ | — | — | — | — | 20 | — | ||
| 1″ | 1.0000 | 1″ | 8 | 7⁄8″ | 14 | 20 | 10, 14, 18, 24, 27 |
1½″ |
The differences between American and British thread forms became a painful problem during the Second World War, especially in manufacturing and repairing airplane engines. In 1948 representatives of Britain, Canada and the United States agreed on a Unified Standard.
In the compromise the British accepted the 60° thread angle, and the Americans accepted rounded roots and optionally rounded crests. Five classes of fit were defined. The new fasteners continued to fit, for most practical purposes, ones manufactured under the old American National Standards. NC became UNC, NF became UNF, etc.
The UNS series is a catch-all category for threads which have the American Standard form, but whose pitches are not in the Unified Coarse (UNC) or Unified Fine (UNF) series.
1. B1.1-1949
2. In the Unified National Series the ½ inch size has 13 threads per inch, but the American Standard was 12 threads per inch.
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Last revised: 9 December 2005.