Unified National Coarse Thread | UNC Thread Chart

  Unified National Coarse Thread | UNC Thread

The Unified Thread Standard (UTS) defines a standard thread form and series along with allowances, tolerances, and designations, for screw threads commonly used in the United States and Canada.

It is the main standard for bolts, nuts, and a wide variety of other threaded fasteners used in these countries. It has the same 60° profile as the ISO metric screw thread, but the characteristic dimensions of each UTS thread (outer diameter and pitch) were chosen as an inch fraction rather than a millimeter value. The UTS is currently controlled by ASME/ANSI in the United States.

  Unified National Coarse Thread | UNC Tolerance classes

A classification system exists for ease of manufacture and interchangeability of fabricated threaded items. Most (but certainly not all) threaded items are made to a classification standard called the Unified Screw Thread Standard Series. This system is analogous to the fits used with assembled parts.

Class 1 threads are loosely fitting threads intended for ease of assembly or use in a dirty environment.

Class 2 threads are the most common. They are designed to maximize strength considering typical machine shop capability and machine practice.

Class 3 threads are used for closer tolerances.

Class 4 thread fit are even tighter than a Class 3.

Class 5 fit is not a loose thread. It is an interference thread, used on items like spring shackles on an automobile.

The letter suffix "A" or "B" denotes whether the threads are external or internal, respectively. Classes 1A, 2A, 3A apply to external threads; Classes 1B, 2B, 3B apply to internal threads.

Thread class refers to the acceptable range of pitch diameter for any given thread. The pitch diameter is indicated as Dp in the figure shown above. There are several methods that are used to measure the pitch diameter. The most common method used in production is by way of a go/no go gauge.

  Thread Count (TPI)

TPI stands for Threads Per Inch, Means the total number of threads per inch measured along the length of a fastener. TPI is used only with American fasteners. Metric Fasteners use a thread Pitch. In general, smaller fasteners have finer threads, so the thread count is higher.

  Thread Pitch

Metric fasteners are specified with a thread pitch instead of a thread count. The thread pitch is the distance between threads expressed in millimeters. In general, smaller fasteners have finer thread so they have lower thread pitch.

  UNC Thread Charts

UNC Thread Chart

  UNC Thread Chart in Inch

Major diameter in inchThread density (d), Threads Per Inch and Thread Pitch (p)Preferred cutting tap drill size
Coarse (UNC)Fine (UNF)Extra fine (UNEF)
inch diameter TPI Pitch TPI Pitch TPI Pitch Coarse Fine Extra fine
#0 0.0600 - - 80 0.012500 - - - 3/64 -
#1 0.0730 64 0.015625 72 0.013888 - - #53 (0.060) #53 (0.060) -
#2 0.0860 56 0.017857 64 0.015625 - - #50 (0.070) #50 (0.070) -
#3 0.0990 48 0.020833 56 0.017857 - - #47 (0.079) #45 (0.082) -
#4 0.1120 40 0.025000 48 0.020833 - - #43 (0.089) #42 (0.094) -
#5 0.1250 40 0.025000 44 0.022727 - - #38 (0.102) #37 (0.104) -
#6 0.1380 32 0.031250 40 0.025000 - - #36 (0.107) #33 (0.113) -
#8 0.1640 32 0.031250 36 0.027778 - - #29 (0.136) #29 (0.136) -
#10 0.1900 24 0.041667 32 0.031250 - - #25 (0.150) #21 (0.159) -
#12 0.2160 24 0.041667 28 0.035714 32 0.031250 #16 (0.177) #14 (0.182) 3/16 (0.187)
1/4 0.2500 20 0.050000 28 0.035714 32 0.031250 #7 (0.201) #3 (0.213) 7/32 (0.218)
5/16 0.3125 18 0.055556 24 0.041667 32 0.031250 F (.257) I (.272) 9/32 (0.281)
3/8 0.3750 16 0.062500 24 0.041667 32 0.031250 5/16 (0.312) Q (.332) 11/32 (0.343)
7/16 0.4375 14 0.071428 20 0.050000 28 0.035714 U (.368) 25/64 (0.390) Y (.404)
1/2 0.5000 13 0.076923 20 0.050000 28 0.035714 27/64 (0.421) 29/64 (0.453) 15/32 (0.468)
9/16 0.5625 12 0.083333 18 0.055556 24 0.041667 31/64 (0.484) 1/2 (0.5) 33/64 (0.515)
5/8 0.6250 11 0.090909 18 0.055556 24 0.041667 17/32 (0.531) 9/16 (0.562) 37/64 (0.578)
3/4 0.7500 10 0.100000 16 0.062500 20 0.050000 21/32 (0.656) 11/16 (0.687) 45/64 (0.703)
7/8 0.8750 9 0.111111 14 0.071428 20 0.050000 49/64 (0.765) 51/64 (0.796) 53/64 (0.828)
1 1.0000 8 0.125000 12(*) 0.083333 20 0.050000 7/8 (0.857) 59/64 (0.921) 61/64 (0.953)

  UNC Thread in MM

Major diameter in mmThread density (d), Threads Per Inch and Thread Pitch (p)Preferred cutting tap drill size
Coarse (UNC)Fine (UNF)Extra fine (UNEF)
inch diameter TPI Pitch TPI Pitch TPI Pitch Coarse Fine Extra fine
#0 1.5240 - - 80 0.3175 - - - 3/64 -
#1 1.8542 64 0.3969 72 0.3528 - - #53 (1.524) #53 (1.524) -
#2 2.1844 56 0.4536 64 0.3969 - - #50 (1.778) #50 (1.778) -
#3 2.5146 48 0.5292 56 0.4536 - - #47 (2.006) #45 (2.083) -
#4 2.8448 40 0.6350 48 0.5292 - - #43 (2.260) #42 (2.387) -
#5 3.1750 40 0.6350 44 0.5773 - - #38 (2.5908) #37 (2.6416) -
#6 3.5052 32 0.7938 40 0.6350 - - #36 (2.717) #33 (2.870) -
#8 4.1656 32 0.7938 36 0.7056 - - #29 (3.454) #29 (3.454) -
#10 4.8260 24 1.0583 32 0.7938 - - #25 (3.810) #21 (4.038) -
#12 5.4864 24 1.0583 28 0.9071 32 0.7938 #16 (4.496) #14 (4.623) 3/16 (4.762)
1/4 6.3500 20 1.2700 28 0.9071 32 0.7938 #7 (5.105) #3 (5.410) 7/32 (5.556)
5/16 7.9375 18 1.4111 24 1.0583 32 0.7938 F (6.527) I (6.908) 9/32 (7.143)
3/8 9.5250 16 1.5875 24 1.0583 32 0.7938 5/16 (7.937) Q (8.433) 11/32 (8.731)
7/16 11.1125 14 1.8143 20 1.2700 28 0.9071 U (9.347) 25/64 (9.921) Y (10.261)
1/2 12.7000 13 1.9538 20 1.2700 28 0.9071 27/64 (10.715) 29/64(11.509) 15/32 (11.906)
9/16 14.2875 12 2.1167 18 1.4111 24 1.0583 31/64 (12.303) 1/2 (12.7) 33/64 (13.096)
5/8 15.8750 11 2.3091 18 1.4111 24 1.0583 17/32 (13.493) 9/16 (14.287) 37/64 (14.684)
3/4 19.0500 10 2.5400 16 1.5875 20 1.2700 21/32 (16.668) 11/16 (17.462) 45/64 (17.859)
7/8 22.2250 9 2.8222 14 1.8143 20 1.2700 49/64 (19.446) 51/64 (20.240) 53/64 (21.034)
1 25.4000 8 3.1750 12(*) 2.1167 20 1.2700 7/8 (22.225) 59/64 (23.415) 61/64 (24.209)

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