UNC and UNF - Unified Coarce and Unified Fine Inch Threads (2024)

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

UNC and UNF - Unified Coarce and Unified Fine Inch Threads (1)

  • 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:

UNC and UNF - Unified Inch Coarse Screw Threads
Major Diameter
(in)
Threads per inch
(tpi)
Major DiameterTap Drill Size
(mm)
Pitch
(mm)
(in)(mm)
#1 - 64640.0731.8541.500.397
#2 - 56560.0862.1841.800.453
#3 - 48480.0992.5152.100.529
#4 - 40400.1122.8452.350.635
#5 - 40400.1253.1752.650.635
#6 - 32320.1383.5052.850.794
#8 - 32320.1644.1663.500.794
#10 - 24240.1904.8264.001.058
#12 - 24240.2165.4864.651.058
1/4" - 20200.2506.3505.351.270
5/16" - 18180.3137.9386.801.411
3/8" - 16160.3759.5258.251.587
7/16" - 14140.43811.1129.651.814
1/2" - 13130.50012.70011.151.954
9/16" - 12120.56314.28812.602.117
5/8" - 11110.62515.87514.052.309
3/4" - 10100.75019.05017.002.540
7/8" - 990.87522.22520.002.822
1" - 881.00025.40022.853.175
1 1/8" - 771.12528.57525.653.628
1 1/4" - 771.25031.75028.853.628
1 3/8" - 661.37534.92531.554.233
1 1/2" - 661.50038.10034.704.233
1 3/4" - 551.75044.45040.405.080
2" - 4 1/24 1/22.00050.80046.305.644
2 1/4" - 4 1/24 1/22.25057.15052.655.644
2 1/2" - 442.50063.50058.506.350
2 3/4" - 442.75069.85064.756.350
3" - 443.00076.20071.106.350
3 1/4" - 443.25082.55077.456.350
3 1/2" - 443.50088.90083.806.350
3 3/4" - 443.75095.25090.156.350
4" - 444.000101.60096.506.350

Example - Typical designation of an UNC thread

  • 1/4” x 20 UNC
  • US Bolts - Tightening Torques

UNF - Unified National Fine Threads

UNF threads ANSI B1.1:

UNC and UNF - Unified Inch Screw Fine Threads
Major Diameter
(in)
Threads per inch
(tpi)
Major DiameterTap Drill Size
(mm)
Pitch
(mm)
(in)(mm)
#0 - 80800.0601.5241.250.317
#1 - 72720.0731.8541.550.353
#2 - 64640.0862.1841.900.397
#3 - 56560.0992.5152.150.453
#4 - 48480.1122.8452.400.529
#5 - 44440.1253.1752.700.577
#6 - 40400.1383.5052.950.635
#8 - 36360.1644.1663.500.705
#10 - 32320.1904.8264.100.794
#12 - 28280.2165.4864.700.907
1/4" - 28280.2506.3505.500.907
5/16" - 24240.3137.9386.901.058
3/8" - 24240.3759.5258.501.058
7/16" - 20200.43811.1129.901.270
1/2" - 20200.50012.70011.501.270
9/16" - 18180.56314.28812.901.411
5/8" - 18180.62515.87514.501.411
3/4" - 16160.75019.05017.501.587
7/8" - 14140.87522.22520.401.814
1" - 12121.00025.40023.252.117
1 1/8" - 12121.12528.57526.502.117
1 1/4" - 12121.25031.75029.502.117
1 3/8" - 12121.37534.92532.752.117
1 1/2" - 12121.50038.10036.002.117

Example - Typical designation of an UNF thread

  • 1/2” x 20 UNF

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UNC and UNF - Unified Coarce and Unified Fine Inch Threads (2024)
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