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UNC and UNF - Unified Inch Screw Threads

ANSI/ASME B1.1 Unified National Thread - UNC Coarse series and UNF Fine series

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UNC and UNF - screw threads

  • UNC – Unified National Coarse Thread - comparable with the ISO metric thread
  • UNF – Unified National Fine Thread

  • Female and Male UNC/UNF threads are both parallel
  • The UNC/UNF flank angle is 60°

Unified threads comes in three different classes:

  1. for applications where a liberal tolerance is required to permit easy assembly even with slightly nicked threads
  2. most commonly used class for general applications
  3. for applications where closeness of fit and/or accuracy of thread elements are important

UNC - Unified Coarse Threads

UNC threads according ANSI B1.1:

Major Diameter
(in)
Threads per inchMajor Diameter
(in)
Major Diameter
(mm)
Tap Drill Size
(mm)
Pitch
(mm)
#1 - 64 64 0.073 1.854 1.50 0.397
#2 - 56 56 0.086 2.184 1.80 0.453
#3 - 48 48 0.099 2.515 2.10 0.529
#4 - 40 40 0.112 2.845 2.35 0.635
#5 - 40 40 0.125 3.175 2.65 0.635
#6 - 32 32 0.138 3.505 2.85 0.794
#8 - 32 32 0.164 4.166 3.50 0.794
#10 - 24 24 0.190 4.826 4.00 1.058
#12 - 24 24 0.216 5.486 4.65 1.058
1/4" - 20 20 0.250 6.350 5.35 1.270
5/16" - 18 18 0.313 7.938 6.80 1.411
3/8" - 16 16 0.375 9.525 8.25 1.587
7/16" - 14 14 0.438 11.112 9.65 1.814
1/2" - 13 13 0.500 12.700 11.15 1.954
9/16" - 12 12 0.563 14.288 12.60 2.117
5/8" - 11 11 0.625 15.875 14.05 2.309
3/4" - 10 10 0.750 19.050 17.00 2.540
7/8" - 9 9 0.875 22.225 20.00 2.822
1" - 8 8 1.000 25.400 22.85 3.175
1 1/8" - 7 7 1.125 28.575 25.65 3.628
1 1/4" - 7 7 1.250 31.750 28.85 3.628
1 3/8" - 6 6 1.375 43.925 31.55 4.233
1 1/2" - 6 6 1.500 38.100 34.70 4.233
1 3/4" - 5 5 1.750 44.450 40.40 5.080
2" - 4 1/2 4 1/2 2.000 50.800 46.30 5.644
2 1/4" - 4 1/2 4 1/2 2.250 57.150 52.65 5.644
2 1/2" - 4 4 2.500 63.500 58.50 6.350
2 3/4" - 4 4 2.750 69.850 64.75 6.350
3" - 4 4 3.000 76.200 71.10 6.350
3 1/4" - 4 4 3.250 82.550 77.45 6.350
3 1/2" - 4 4 3.500 88.900 83.80 6.350
3 3/4" - 4 4 3.750 95.250 90.15 6.350
4" - 4 4 4.000 101.600 96.50 6.350

Example - Typical designation of an UNC thread

  • 1/4” x 20 UNC

UNF - Unified National Fine Threads

UNF threads ANSI B1.1:

Major Diameter
(in)
Threads per inchMajor Diameter 
(in)
Major Diameter
(mm)
Tap Drill Size
(mm)
Pitch
(mm)
#0 - 80 80 0.060 1.524 1.25 0.317
#1 - 72 72 0.073 1.854 1.55 0.353
#2 - 64 64 0.086 2.184 1.90 0.397
#3 - 56 56 0.099 2.515 2.15 0.453
#4 - 48 48 0.112 2.845 2.40 0.529
#5 - 44 44 0.125 3.175 2.70 0.577
#6 - 40 40 0.138 3.505 2.95 0.635
#8 - 36 36 0.164 4.166 3.50 0.705
#10 - 32 32 0.190 4.826 4.10 0.794
#12 - 28 28 0.216 5.486 4.70 0.907
1/4" - 28 28 0.250 6.350 5.50 0.907
5/16" - 24 24 0.313 7.938 6.90 1.058
3/8" - 24 24 0.375 9.525 8.50 1.058
7/16" - 20 20 0.438 11.112 9.90 1.270
1/2" - 20 20 0.500 12.700 11.50 1.270
9/16" - 18 18 0.563 14.288 12.90 1.411
5/8" - 18 18 0.625 15.875 14.50 1.411
3/4" - 16 16 0.750 19.050 17.50 1.587
7/8" - 14 14 0.875 22.225 20.40 1.814
1" - 12 12 1.000 25.400 23.25 2.117
1 1/8" - 12 12 1.125 28.575 26.50 2.117
1 1/4" - 12 12 1.250 31.750 29.50 2.117
1 3/8" - 12 12 1.375 43.925 32.75 2.117
1 1/2" - 12 12 1.500 38.100 36.00 2.117

Example - Typical designation of an UNF thread

  • 1/2” x 20 UNF
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