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UNC and UNF - Unified Coarce and Unified Fine Inch Threads

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

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:

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 - 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 34.925 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

Pitch is the distance between screw grooves and is commonly used with inch sized products and specified as threads per inch. Lead is the linear travel the nut makes per one screw revolution. The pitch and lead are equal.

Example - Typical designation of an UNC thread

  • 1/4” x 20 UNC

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 - 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 34.925 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

Related Topics

  • Fasteners

    Bolts, nuts and threaded rods - torque, tension and loads.

  • Mechanics

    The relationships between forces, acceleration, displacement, vectors, motion, momentum, energy of objects and more.

  • Threads

    Thread data - metric, unified, ACME and others.

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