UNC and UNF - Unified Coarce and Unified Fine Inch 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:
- for applications where a liberal tolerance is required to permit easy assembly even with slightly nicked threads
- most commonly used class for general applications
- 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 inch (tpi) | Major Diameter | Tap 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:
| Major Diameter (in) | Threads per inch (tpi) | Major Diameter | Tap 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
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