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20UN - 60 Degree Unified Threads

The 20 thread series (20-UN) have been used more or less widely in industry for various applications where the Standard Coarse, Fine or Extra-fine Series were not as applicable. They are now recognized as Standard Unified Thread Series in a specified selection of diameters for each pitch.

5/16" - 1 5/16" 1 3/8" - 1 15/16" 2" - 3"

20UN Threads

Nominal Size External (b) Internal (b)
Threads per Inch and Series Designation Class Allowance Major Diameter Minor Diameter UNR Minor Dia.,c Max (Ref.) Class Minor Diameter Pitch Diameter Major Diameter
Max (d) Min Min (e) Max (d) Min Min Max Min Max Min
5/16" - 20 UN 2A 0.0012 7.907 7.701 - 7.082 6.98 6.396 2B 6.553 6.858 7.112 7.244 7.938
5/16" - 20 UN 3A 0 7.938 7.732 - 7.112 7.036 6.426 3B 6.553 6.807 7.112 7.211 7.938
3/8" - 20 UN 2A 0.0012 9.495 9.289 - 8.669 8.565 7.983 2B 8.153 8.433 8.7 8.837 9.525
3/8" - 20 UN 3A 0 9.525 9.319 - 8.7 8.621 8.014 3B 8.153 8.374 8.7 8.801 9.525
9/16" -20 UN 2A 0.0013 14.254 14.049 - 13.429 13.322 12.743 2B 12.903 13.208 13.462 13.602 14.288
9/16" -20 UN 3A 0 14.288 14.082 - 13.462 13.381 12.776 3B 12.903 13.111 13.462 13.566 14.288
5/8" - 20 UN 2A 0.0013 15.842 15.636 - 15.016 14.907 14.331 2B 14.503 14.783 15.05 15.192 15.875
5/8" - 20 UN 3A 0 15.875 15.669 - 15.05 14.968 14.364 3B 14.503 14.699 15.05 15.156 15.875
11/16" - 20 UN 2A 0.0013 17.429 17.224 - 16.604 16.495 15.918 2B 16.078 16.383 16.637 16.779 17.463
11/16" - 20 UN 3A 0 17.463 17.257 - 16.637 16.556 15.951 3B 16.078 16.286 16.637 16.744 17.463
1 1/16" - 20 UN 2A 0.0014 26.952 26.746 - 26.126 26.012 25.441 2B 25.603 25.908 26.162 26.312 26.988
1 1/16" - 20 UN 3A 0 26.988 26.782 - 26.162 26.076 25.476 3B 25.603 25.811 26.162 26.274 26.988
1 1/8" - 20 UN 2A 0.0014 28.539 28.334 - 27.714 27.6 27.028 2B 27.203 27.483 27.75 27.899 28.575
1 1/8" - 20 UN 3A 0 28.575 28.369 - 27.75 27.663 27.064 3B 27.203 27.399 27.75 27.861 28.575
1 3/8" - 20 UN 2A 0.0014 30.127 29.921 - 29.301 29.182 28.616 2B 28.778 29.083 29.337 29.492 30.163
1 3/8" - 20 UN 3A 0 30.163 29.957 - 29.337 29.248 28.651 3B 28.778 28.986 29.337 29.451 30.163
1 1/4 -20 UN 2A 0.0014 31.714 31.509 - 30.889 30.77 30.203 2B 30.378 30.658 30.925 31.079 31.75
1 1/4 -20 UN 3A 0 31.75 31.544 - 30.925 30.836 30.239 3B 30.378 30.574 30.925 31.039 31.75
1 5/16" - 20 UN 2A 0.0014 33.302 33.096 - 32.476 32.357 31.791 2B 31.953 32.258 32.512 32.667 33.338
1 5/16" - 20 UN 3A 0 33.338 33.132 - 32.512 32.423 31.826 3B 31.953 32.161 32.512 32.626 33.338
1 3/8" - 20 UN 2A 0.0014 34.889 34.684 - 34.064 33.945 33.378 2B 33.553 33.833 34.1 34.254 34.925
1 3/8" - 20 UN 3A 0 34.925 34.719 - 34.1 34.011 33.414 3B 33.553 33.749 34.1 34.214 34.925
1 7/16" - 20 UN 2A 0.0014 36.477 36.271 - 35.651 35.53 34.966 2B 35.128 35.433 35.687 35.844 36.513
1 7/16" - 20 UN 3A 0 36.513 36.307 - 35.687 35.596 35.001 3B 35.128 35.336 35.687 35.804 36.513
1 1/2" - 20 UN 2A 0.0014 38.064 37.859 - 37.239 37.117 36.553 2B 36.728 37.008 37.275 37.432 38.1
1 1/2" - 20 UN 3A 0 38.1 37.894 - 37.275 37.183 36.589 3B 36.728 36.924 37.275 37.391 38.1
1 9/16" - 20 UN 2A 0.0014 39.652 39.446 - 38.826 38.705 38.141 2B 38.303 38.608 38.862 39.019 39.688
1 9/16" - 20 UN 3A 0 39.688 39.482 - 38.862 38.771 38.176 3B 38.303 38.511 38.862 38.979 39.688
1 5/8" - 20 UN 2A 0.0014 41.239 41.034 - 40.414 40.292 39.728 2B 39.903 40.183 40.45 40.607 41.275
1 5/8" - 20 UN 3A 0 41.275 41.069 - 40.45 40.358 39.764 3B 39.903 40.099 40.45 40.566 41.275
1 11/16" - 20 UN 2A 0.0015 42.824 42.619 - 41.999 41.877 41.313 2B 41.478 41.783 42.037 42.197 42.863
1 11/16" - 20 UN 3A 0 42.863 42.657 - 42.037 41.946 41.351 3B 41.478 41.686 42.037 42.156 42.863
1 3/4" - 20 UN 2A 0.0015 44.412 44.206 - 43.586 43.464 42.901 2B 43.078 43.358 43.625 43.785 44.45
1 3/4" - 20 UN 3A 0 44.45 44.244 - 43.625 43.533 42.939 3B 43.078 43.274 43.625 43.744 44.45
1 13/16" - 20 UN 2A 0.0015 45.999 45.794 - 45.174 45.052 44.488 2B 44.653 44.958 45.212 45.372 46.038
1 13/16" - 20 UN 3A 0 46.038 45.832 - 45.212 45.121 44.526 3B 44.653 44.861 45.212 45.331 46.038
1 7/8" - 20 UN 2A 0.0015 47.587 47.381 - 46.761 46.639 46.076 2B 46.253 46.533 46.8 46.96 47.625
1 7/8" - 20 UN 3A 0 47.625 47.419 - 46.8 46.708 46.114 3B 46.253 46.449 46.8 46.919 47.625
1 15/16" - 20 UN 2A 0.0015 49.174 48.969 - 48.349 48.224 47.663 2B 47.828 48.133 48.387 48.55 49.213
1 15/16" - 20 UN 3A 0 49.213 49.007 - 48.387 48.293 47.701 3B 47.828 48.036 48.387 48.509 49.213
2" - 20 UN 2A 0.0015 50.762 50.556 - 49.936 49.812 49.251 2B 49.428 49.708 49.975 50.137 50.8
2" - 20 UN 3A 0 50.8 50.594 - 49.975 49.881 49.289 3B 49.428 49.624 49.975 50.096 50.8
2 1/8" - 20 UN 2A 0.0015 53.937 53.731 - 53.111 52.987 52.426 2B 52.603 52.883 53.15 53.312 53.975
2 1/8" - 20 UN 3A 0 53.975 53.769 - 53.15 53.056 52.464 3B 52.603 52.799 53.15 53.271 53.975
2 1/4" - 20 UN 2A 0.0015 57.112 56.906 - 56.286 56.162 55.601 2B 55.778 56.058 56.325 56.487 57.15
2 1/4" - 20 UN 3A 0 57.15 56.944 - 56.325 56.228 55.639 3B 55.778 55.974 56.325 56.446 57.15
2 3/8" - 20 UN 2A 0.0015 60.287 60.081 - 59.461 59.332 58.776 2B 58.953 59.233 59.5 59.667 60.325
2 3/8" - 20 UN 3A 0 60.325 60.119 - 59.5 59.403 58.814 3B 58.953 59.149 59.5 59.627 60.325
2 1/2" - 20 UN 2A 0.0015 63.462 63.256 - 62.636 62.507 61.951 2B 62.128 62.408 62.675 62.842 63.5
2 1/2" - 20 UN 3A 0 63.5 63.294 - 62.675 62.578 61.989 3B 62.128 62.324 62.675 62.802 63.5
2 5/8" - 20 UN 2A 0.0015 66.637 66.431 - 65.811 65.682 65.126 2B 65.303 65.583 65.85 66.017 66.675
2 5/8" - 20 UN 3A 0 66.675 66.469 - 65.85 65.753 65.164 3B 65.303 65.499 65.85 65.977 66.675
2 3/4" - 20 UN 2A 0.0015 69.812 69.606 - 68.986 68.857 68.301 2B 68.478 68.758 69.025 69.192 69.85
2 3/4" - 20 UN 3A 0 69.85 69.644 - 69.025 68.928 68.339 3B 68.478 68.674 69.025 69.152 69.85
2 7/8" - 20 UN 2A 0.0016 72.984 72.779 - 72.159 72.027 71.473 2B 71.653 71.933 72.2 72.372 73.025
2 7/8" - 20 UN 3A 0 73.025 72.819 - 72.2 72.1 71.514 3B 71.653 71.849 72.2 72.329 73.025
3" - 20 UN 2A 0.0016 76.159 75.954 - 75.334 75.202 74.648 2B 74.828 75.108 75.375 75.547 76.2
3" - 20 UN 3A 0 76.2 75.994 - 75.375 75.275 74.689 3B 74.828 75.024 75.375 75.504 76.2