NC, NF, NEF

to one inch in diameter

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These thread series are obsolete, having been replaced by the Unified thread series.

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.

For the history of these series, see source 1, below the table.

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   #73 160 164            
000   #63 120 #71           332
00   #55 90 #65           332
0 0.060 #52 80 364       532
1 0.073 #48 64 #52 72 #53   56 #54 532
2 0.086 #43 56 #50 64 #50   316
3 0.099 #38 48 #47 56 #45   316
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 34 516
7 0.151     30,
32,
36
31,
31,
18
516
8 0.164 #19 32 #29 36 #29   30,
40
30,
28
1132
9 0.177     24,
30,
32
29,
27,
26
1132
10 0.190 #11 24 #25 32 #21   28,
30
23,
22
38
12 0.216   24   28     32 13 716
14 0.242 D           20,
24
10,
7
716
¼″ 0.250 ¼″ or E 20 #7 28 #3 32 24,
27,
32
4,
3,
732
716
16 0.268     18, 20, 22 732″,
2
½″
18 0.294     18, 20 B,
D
916
516 0.3125 O 18 F 24 I 32 20, 27, 32 1764″,
J,
932
916
20 0.320             16, 18, 20 G,
1764″,
I
58
22 0.346             16, 18 932″,
L
58
24 0.372             16, 18 516″,
O
58
38 0.375 V 16 516 24 Q 32 20, 27   58
26 0.398 Y           14, 16 2164″,
R
1116
28 0.424 716           14, 16 T,
2364
1116
716 0.4375 716 14 U 20 2564 28 24, 27    
30 0.450 2964           14, 16 V,
2564
78
½″ 0.5000  ½″ 13 (see note 1) 2764 20 2964 28 12, 24, 27    
12             ¾″
916 0.5625 916 12 3164 18 3364 24 27    
58 0.6250 58 11 1732 18 3764 24 12,
27
   
1116  0.68753 1116         24      
¾″ 0.7500   10 2132 16   20 12,
27
  118
78 0.8750   9 4964 14   20 12,
18,
27
  1516
1″ 1.0000   8 78 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.

sources

1

This standard was largely abstracted by the Screw-Threads Division from the report of the Sectional Committee on the Standardization and Unification of Screw Threads, which was organized under the rules of procedure of the American Engineering Standards Committee by the American Society of Mechanical Engineers and the Society of Automotive Engineers. The report, which was prepared in collaboration with the National Screw Thread Commission, on which the A.S.M.E. and S.A.E. are the two civilian participating organizations, was approved by the A.S.M.E. in December, 1923, and by the S.A.E. in February, 1924. It was approved as an American Standard by the American Engineering Standards Committee in May, 1924.

The Coarse Screw-Thread Series, supplemented below the ¼-in. size by the standard established by the American Society of Mechanical Engineers in 1907, is the “United States Standard” Screw-Thread Series, soemtims termed the “American” or “National” Coarse Screw-Thread Series. The Coarse Screw-Thread Series is suited to bolts and screws, (1) where jar and vibration are not important factors, (2) where an average tensile srength is required, (3) where it is necessary to take the parts apart frequently and under unfavorable conditions, (4) where, in the case of cap-screws, the holes in which the screws are assembled are tapped in metals other than steel.

The Fine Screw-Thread Series, which was adopted by the Society in August, 1922, was formulated for fractional-inch sizes up to and including 1 in. by the Mechanical Branch of the Association of Licensed Automobile Manufacturers (A.L.A.M.), now the National Automobile Chamber of Commerece (N.A.C.C.), for automotive applications where U.S. Standard screw-threads were too coarse. In 1911 the A.L.A.M. made the Society (S.A.E.) the custodian of the A.L.A.M. standards, this series, extended to 1½ in., then becoming the S.A.E. Regular Screw-Thread series. This was subsequently extended by the S.A.E. to include fractional inch sizes above 2 in. The sizes below ¼ in. are in accordance with the series established by the American Society of Mechanical Engineers (A.S.M.E.). The present S.A.E. Fine Series is sometimes termed either the “American” or “National” Fine Series. …

The Fine Screw-Thread Series is suited to finished bolts and screws, (1) where jar and vibration are important factors, (2) where the tensile strength required is above the average, (3) where a fine adjustment is required (especially when castle nuts are used), (4) where the holes in which cap-screws are assembled are tapped in steel.

The Extra-Fine Screw-Thread Series was adopted by the Society in 1915 as the S.A.E. Fine Series for aeronautic and other applications where screw threads finer than the present S.A.E. Fine Screw-Thread Series are necessary. …

The Extra-Fine Screw-Thread Series is suited to parts, (1) in which the thickness of the metal supporting the thread is not great, as in tubing, (2) which have relatively light section, such as nuts, (3) which require fine adjustment, (4) where jar and vibration are very important factors, and (5) where assembly is made without the use of wrenches.

S.A.E. Handbook. 1928 Edition.
New York: Society of Automotive Engineers, Inc.
Pages 245, 246, 248 (footnote).

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