UNC, UNF, UNEF, UNS

screw thread gauges to 1 inch in diameter

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Index to fastener thread series

Content

Table

History

Nomenclature

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series) are the inch-based thread series currently used in the United States, and have been since 1949.1 They superceded the American National Series (NC, NF, etc.)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 80 364  — 532
1 0.0730 #48 64 #52 72 #53 #54 532
2 0.0860 #43 56 #50 64 #50 316
3 0.0990 #37 48 #47 56 #45 316
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 #34 516
8 0.1640 #18 32 #29 36 #29 30,
40
#30,
#28
1132
10 0.190 #9 24 #25 32 #21   28,
36,
40,
48,
56,
30
#23,
#22
38
12 0.216 #2 24   28   32 36,
40,
48,
56
#13 716
¼″ 0.2500 F 20 #7 28 #3 32 24,
27,
32,
36,
40,
48,
56
4,
3,
732
716
516 0.3125 P 18 F 24 I 32 20,
27,
28,
36,
40
48
1764″,
J,
932
916
38 0.375 W 16 516 24 Q 32 18,
27,
36,
40
  58
0.390 0.390 25/64 27    
716 0.4375 29⁄64 14 U 20 2564 28 18,
24,
27
   
½″ 0.5000  33/64″ 13 (see note 2) 2764 20 2964 28 12,
14,
18,
24,
27
  ¾″
916 0.5625 916 12 3164 18 3364 24 14,
27
   
58 0.6250 58 11 1732 18 3764 24 14,
27
   
1116  0.68753 1116         24    
¾″ 0.7500 ¾ 10 2132 16   20 14,
18,
24
  118
1316 0.8125 1316 20    
78 0.8750 78 9 4964 14   20 10,
18,
24,
27
  1516
1516 0.9375 1516 20    
1″ 1.0000 1″ 8 78 14   20 10,
14,
18,
24,
27
  1½″

History

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. 

drawing of unified screw thread form

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.

Nomenclature

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.

Standards

British Standard 1580 Unified (ISO, inch) screw threads: Parts 1 and 2 (1962); Part 3 (1965).

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