engineering fundamentals Unified Screw Threads, Standard Series
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Nominal Thread SizeCovered Range
0-80 UNF to ¼-56 UNS Nominal thread sizes from 0-80 UNF to ¼-56 UNS
5/16-18 UNC to 9/16 -32 UN Nominal thread sizes from 5/16-18 UNC to 9/16 -32 UN
5/8-11 UNC to 7/8-32 UN Nominal thread sizes from 5/8-11 UNC to 7/8-32 UN
15/16-12 UN to 1 3/16-28 UN Nominal thread sizes from 15/16-12 UN to 1 3/16-28 UN
1 1/4-7 UNC to 1 9/16-20 UN Nominal thread sizes from 1 1/4-7 UNC to 1 9/16-20 UN
1 5/8-6 UN to 1 15/16-20 UN Nominal thread sizes from 1 5/8-6 UN to 1 15/16-20 UN
2-4½ UNC to 2¾-20 UN Nominal thread sizes from 2-4½ UNC to 2¾-20 UN
2 7/8-6 UN to 4-16 UN Nominal thread sizes from 2 7/8-6 UN to 4-16 UN
 
External Thread /Internal Thread
  Nominal Size, TPI, Series Class Allowance Maxa Major
Max Minor
Min Major
Min Minor
Min b Maxa Pitch Min Pitch UNRc Minor Dia
Major Dia(Min)
Ext. 2-41/2 UNC 1A 0.0029 1.9971 1.9641 - 1.8528 1.8385 1.7324
Int.   1B   1.7950 1.7590   1.8743 1.8557 2.0000
Ext. 2-41/2 UNC 2A 0.0029 1.9971 1.9751 1.9641 1.8528 1.8433 1.7324
Int.   2B   1.7950 1.7590   1.8681 1.8557 2.0000
Ext. 2-41/2 UNC 3A 0.0000 2.0000 1.9780 - 1.8557 1.8486 1.7353
Int.   3B   1.7861 1.7590   1.8650 1.8557 2.0000
Ext. 2-6 UN 2A 0.0026 1.9974 1.9792 - 1.8891 1.8805 1.7990
Int.   2B   1.8500 1.8200   1.9028 1.8917 2.0000
Ext. 2-6 UN 3A 0.0000 2.0000 1.9818 - 1.8917 1.8853 1.8016
Int.   3B   1.8396 1.8200   1.9000 1.8917 2.0000
Ext. 2-8 UN 2A 0.0023 1.9977 1.9827 1.9752 1.9165 1.9087 1.8489
Int.   2B   1.8900 1.8650   1.9289 1.9188 2.0000
Ext. 2-8 UN 3A 0.0000 2.0000 1.9850 - 1.9188 1.9130 1.8512
Int.   3B   1.8797 1.8650   1.9264 1.9188 2.0000
Ext. 2-10 UNS 2A 0.0020 1.9980 1.9851 - 1.9330 1.9265 1.8790
Int.   2B   1.9130 1.8920   1.9435 1.9350 2.0000
Ext. 2-12 UN 2A 0.0018 1.9982 1.9868 - 1.9441 1.9380 1.8990
Int.   2B   1.9280 1.9100   1.9538 1.9459 2.0000
Ext. 2-12 UN 3A 0.0000 2.0000 1.9886 - 1.9459 1.9414 1.9008
Int.   3B   1.9198 1.9100   1.9518 1.9459 2.0000
Ext. 2-14 UNS 2A 0.0017 1.9983 1.9880 - 1.9519 1.9462 1.9133
Int.   2B   1.9380 1.9230   1.9610 1.9536 2.0000
Ext. 2-16 UN 2A 0.0016 1.9984 1.9890 - 1.9578 1.9524 1.9239
Int.   2B   1.9460 1.9320   1.9664 1.9594 2.0000
Ext. 2-16 UN 3A 0.0000 2.0000 1.9906 - 1.9594 1.9554 1.9255
Int.   3B   1.9408 1.9320   1.9646 1.9594 2.0000
Ext. 2-18 UNS 2A 0.0015 1.9985 1.9898 - 1.9624 1.9573 1.9324
Int.   2B   1.9530 1.9400   1.9706 1.9639 2.0000
Ext. 2-20 UN 2A 0.0015 1.9985 1.9904 - 1.9660 1.9611 1.9390
Int.   2B   1.9570 1.9460   1.9739 1.9675 2.0000
Ext. 2-20 UN 3A 0.0000 2.0000 1.9919 - 1.9675 1.9638 1.9405
Int.   3B   1.9537 1.9460   1.9723 1.9675 2.0000
Ext. 21/16-16 UNS 2A 0.0016 2.0609 2.0515 - 2.0203 2.0149 1.9864
Int.   2B   2.0090 1.9950   2.0289 2.0219 2.0625
Ext. 21/16-16 UNS 3A 0.0000 2.0625 2.0531 - 2.0219 2.0179 1.9880
Int.   3B   2.0033 1.9950   2.0271 2.0219 2.0625
Ext. 21/8-6 UN 2A 0.0026 2.1224 2.1042 - 2.0141 2.0054 1.9240
Int.   2B   1.9750 1.9450   2.0280 2.0167 2.1250
Ext. 21/8-6 UN 3A 0.0000 2.1250 2.1068 - 2.0167 2.0102 1.9266
Int.   3B   1.9646 1.9450   2.0251 2.0167 2.1250
Ext. 21/8-8 UN 2A 0.0024 2.1226 2.1076 2.1001 2.0414 2.0335 1.9738
Int.   2B   2.0150 1.9900   2.0540 2.0438 2.1250
Ext. 21/8-8 UN 3A 0.0000 2.1250 2.1100 - 2.0438 2.0379 1.9762
Int.   3B   2.0047 1.9900   2.0515 2.0438 2.1250
Ext. 21/8-12 UN 2A 0.0018 2.1232 2.1118 - 2.0691 2.0630 2.0240
Int.   2B   2.0530 2.0350   2.0788 2.0709 2.1250
Ext. 21/8-12 UN 3A 0.0000 2.1250 2.1136 - 2.0709 2.0664 2.0258
Int.   3B   2.0448 2.0350   2.0768 2.0709 2.1250
Ext. 21/8-16 UN 2A 0.0016 2.1234 2.1140 - 2.0828 2.0774 2.0489
Int.   2B   2.0710 2.0570   2.0914 2.0844 2.1250
Ext. 21/8-16 UN 3A 0.0000 2.1250 2.1156 - 2.0844 2.0803 2.0505
Int.   3B   2.0658 2.0570   2.0896 2.0844 2.1250
Ext. 21/8-20 UN 2A 0.0015 2.1235 2.1154 - 2.0910 2.0861 2.0640
Int.   2B   2.0820 2.0710   2.0989 2.0925 2.1250
Ext. 21/8-20 UN 3A 0.0000 2.1250 2.1169 - 2.0925 2.0888 2.0655
Int.   3B   2.0787 2.0710   2.0973 2.0925 2.1250
Ext. 23/16-16 UNS 2A 0.0016 2.1859 2.1765 - 2.1453 2.1399 2.1114
Int.   2B   2.1340 2.1200   2.1539 2.1469 2.1875
Ext. 23/16-16 UNS 3A 0.0000 2.1875 2.1781 - 2.1469 2.1428 2.1130
Int.   3B   2.1283 2.1200   2.1521 2.1469 2.1875
Ext. 21/4-41/2 UNC 1A 0.0029 2.2471 2.2141 - 2.1028 2.0882 1.9824
Int.   1B   2.0450 2.0090   2.1247 2.1057 2.2500
Ext. 21/4-41/2 UNC 2A 0.0029 2.2471 2.2251 2.2141 2.1028 2.0931 1.9824
Int.   2B   2.0450 2.0090   2.1183 2.1057 2.2500
Ext. 21/4-41/2 UNC 3A 0.0000 2.2500 2.2280 - 2.1057 2.0984 1.9853
Int.   3B   2.0361 2.0090   2.1152 2.1057 2.2500
Ext. 21/4-6 UN 2A 0.0026 2.2474 2.2292 - 2.1391 2.1303 2.0490
Int.   2B   2.1000 2.0700   2.1531 2.1417 2.2500
Ext. 21/4-6 UN 3A 0.0000 2.2500 2.2318 - 2.1417 2.1351 2.0516
Int.   3B   2.0896 2.0700   2.1502 2.1417 2.2500
Ext. 21/4-8 UN 2A 0.0024 2.2476 2.2326 2.2251 2.1664 2.1584 2.0988
Int.   2B   2.1400 2.1150   2.1792 2.1688 2.2500
Ext. 21/4-8 UN 3A 0.0000 2.2500 2.2350 - 2.1688 2.1628 2.1012
Int.   3B   2.1297 2.1150   2.1766 2.1688 2.2500
Ext. 21/4-10 UNS 2A 0.0020 2.2480 2.2351 - 2.1830 2.1765 2.1290
Int.   2B   2.1630 2.1420   2.1935 2.1850 2.2500
Ext. 21/4-12 UN 2A 0.0018 2.2482 2.2368 - 2.1941 2.1880 2.1490
Int.   2B   2.1780 2.1600   2.2038 2.1959 2.2500
Ext. 21/4-12 UN 3A 0.0000 2.2500 2.2386 - 2.1959 2.1914 2.1508
Int.   3B   2.1698 2.1600   2.2018 2.1959 2.2500
Ext. 21/4-14 UNS 2A 0.0017 2.2483 2.2380 - 2.2019 2.1962 2.1633
Int.   2B   2.1880 2.1730   2.2110 2.2036 2.2500
Ext. 21/4-16 UN 2A 0.0016 2.2484 2.2390 - 2.2078 2.2024 2.1739
Int.   2B   2.1960 2.1820   2.2164 2.2094 2.2500
Ext. 21/4-16 UN 3A 0.0000 2.2500 2.2406 - 2.2094 2.2053 2.1755
Int.   3B   2.1908 2.1820   2.2146 2.2094 2.2500
Ext. 21/4-18 UNS 2A 0.0015 2.2485 2.2398 - 2.2124 2.2073 2.1824
Int.   2B   2.2030 2.1900   2.2206 2.2139 2.2500
Ext. 21/4-20 UN 2A 0.0015 2.2485 2.2404 - 2.2160 2.2111 2.1890
Int.   2B   2.2070 2.1960   2.2239 2.2175 2.2500
Ext. 21/4-20 UN 3A 0.0000 2.2500 2.2419 - 2.2175 2.2137 2.1905
Int.   3B   2.2037 2.1960   2.2223 2.2175 2.2500
Ext. 25/16-16 UNS 2A 0.0017 2.3108 2.3014 - 2.2702 2.2647 2.2363
Int.   2B   2.2590 2.2450   2.2791 2.2719 2.3125
Ext. 25/16-16 UNS 3A 0.0000 2.3125 2.3031 - 2.2719 2.2678 2.2380
Int.   3B   2.2533 2.2450   2.2773 2.2719 2.3125
Ext. 23/8-6 UN 2A 0.0027 2.3723 2.3541 - 2.2640 2.2551 2.1739
Int.   2B   2.2260 2.1950   2.2782 2.2667 2.3750
Ext. 23/8-6 UN 3A 0.0000 2.3750 2.3568 - 2.2667 2.2601 2.1766
Int.   3B   2.2146 2.1950   2.2753 2.2667 2.3750
Ext. 23/8-8 UN 2A 0.0024 2.3726 2.3576 - 2.2914 2.2833 2.2238
Int.   2B   2.2650 2.2400   2.3043 2.2938 2.3750
Ext. 23/8-8 UN 3A 0.0000 2.3750 2.3600 - 2.2938 2.2878 2.2262
Int.   3B   2.2547 2.2400   2.3017 2.2938 2.3750
Ext. 23/8-12 UN 2A 0.0019 2.3731 2.3617 - 2.3190 2.3128 2.2739
Int.   2B   2.3030 2.2850   2.3290 2.3209 2.3750
Ext. 23/8-12 UN 3A 0.0000 2.3750 2.3636 - 2.3209 2.3163 2.2758
Int.   3B   2.2948 2.2850   2.3269 2.3209 2.3750
Ext. 23/8-16 UN 2A 0.0017 2.3733 2.3639 - 2.3327 2.3272 2.2988
Int.   2B   2.3210 2.3070   2.3416 2.3344 2.3750
Ext. 23/8-16 UN 3A 0.0000 2.3750 2.3656 - 2.3344 2.3303 2.3005
Int.   3B   2.3158 2.3070   2.3398 2.3344 2.3750
Ext. 23/8-20 UN 2A 0.0015 2.3735 2.3654 - 2.3410 2.3359 2.3140
Int.   2B   2.3320 2.3210   2.3491 2.3425 2.3750
Ext. 23/8-20 UN 3A 0.0000 2.3750 2.3669 - 2.3425 2.3387 2.3155
Int.   3B   2.3287 2.3210   2.3475 2.3425 2.3750
Ext. 27/16-16 UNS 2A 0.0017 2.4358 2.4264 - 2.3952 2.3897 2.3613
Int.   2B   2.3840 2.3700   2.4041 2.3969 2.4375
Ext. 27/16-16 UNS 3A 0.0000 2.4375 2.4281 - 2.3969 2.3928 2.3630
Int.   3B   2.3783 2.3700   2.4023 2.3969 2.4375
Ext. 21/2-4 UNC 1A 0.0031 2.4969 2.4612 - 2.3345 2.3190 2.1992
Int.   1B   2.2670 2.2290   2.3578 2.3376 2.5000
Ext. 21/2-4 UNC 2A 0.0031 2.4969 2.4731 2.4612 2.3345 2.3241 2.1992
Int.   2B   2.2670 2.2290   2.3511 2.3376 2.5000
Ext. 21/2-4 UNC 3A 0.0000 2.5000 2.4762 - 2.3376 2.3298 2.2023
Int.   3B   2.2594 2.2290   2.3477 2.3376 2.5000
Ext. 21/2-6 UN 2A 0.0027 2.4973 2.4791 - 2.3890 2.3800 2.2989
Int.   2B   2.3500 2.3200   2.4033 2.3917 2.5000
Ext. 21/2-6 UN 3A 0.0000 2.5000 2.4818 - 2.3917 2.3850 2.3016
Int.   3B   2.3396 2.3200   2.4004 2.3917 2.5000
Ext. 21/2-8 UN 2A 0.0024 2.4976 2.4826 2.4751 2.4164 2.4082 2.3488
Int.   2B   2.3900 2.3650   2.4294 2.4188 2.5000
Ext. 21/2-8 UN 3A 0.0000 2.5000 2.4850 - 2.4188 2.4127 2.3512
Int.   3B   2.3797 2.3650   2.4268 2.4188 2.5000
Ext. 21/2-10 UNS 2A 0.0020 2.4980 2.4851 - 2.4330 2.4263 2.3790
Int.   2B   2.4130 2.3920   2.4437 2.4350 2.5000
Ext. 21/2-12 UN 2A 0.0019 2.4981 2.4867 - 2.4440 2.4378 2.3989
Int.   2B   2.4280 2.4100   2.4540 2.4459 2.5000
Ext. 21/2-12 UN 3A 0.0000 2.5000 2.4886 - 2.4459 2.4413 2.4008
Int.   3B   2.4198 2.4100   2.4519 2.4459 2.5000
Ext. 21/2-14 UNS 2A 0.0017 2.4983 2.4880 - 2.4519 2.4461 2.4133
Int.   2B   2.4380 2.4230   2.4612 2.4536 2.5000
Ext. 21/2-16 UN 2A 0.0017 2.4983 2.4889 - 2.4577 2.4522 2.4238
Int.   2B   2.4460 2.4320   2.4666 2.4594 2.5000
Ext. 21/2-16 UN 3A 0.0000 2.5000 2.4906 - 2.4594 2.4553 2.4255
Int.   3B   2.4408 2.4320   2.4648 2.4594 2.5000
Ext. 21/2-18 UNS 2A 0.0016 2.4984 2.4897 - 2.4623 2.4570 2.4323
Int.   2B   2.4530 2.4400   2.4708 2.4639 2.5000
Ext. 21/2-20 UN 2A 0.0015 2.4985 2.4904 - 2.4660 2.4609 2.4390
Int.   2B   2.4570 2.4460   2.4741 2.4675 2.5000
Ext. 21/2-20 UN 3A 0.0000 2.5000 2.4919 - 2.4675 2.4637 2.4405
Int.   3B   2.4537 2.4460   2.4725 2.4675 2.5000
Ext. 25/8-6 UN 2A 0.0027 2.6223 2.6041 - 2.5140 2.5050 2.4239
Int.   2B   2.4750 2.4450   2.5285 2.5167 2.6250
Ext. 25/8-6 UN 3A 0.0000 2.6250 2.6068 - 2.5167 2.5099 2.4266
Int.   3B   2.4646 2.4450   2.5255 2.5167 2.6250
Ext. 25/8-8 UN 2A 0.0025 2.6225 2.6075 - 2.5413 2.5331 2.4737
Int.   2B   2.5150 2.4900   2.5545 2.5438 2.6250
Ext. 25/8-8 UN 3A 0.0000 2.6250 2.6100 - 2.5438 2.5376 2.4762
Int.   3B   2.5047 2.4900   2.5518 2.5438 2.6250
Ext. 25/8-12 UN 2A 0.0019 2.6231 2.6117 - 2.5690 2.5628 2.5239
Int.   2B   2.5530 2.5350   2.5790 2.5709 2.6250
Ext. 25/8-12 UN 3A 0.0000 2.6250 2.6136 - 2.5709 2.5663 2.5258
Int.   3B   2.5448 2.5350   2.5769 2.5709 2.6250
Ext. 25/8-16 UN 2A 0.0017 2.6233 2.6139 - 2.5827 2.5772 2.5488
Int.   2B   2.5710 2.5570   2.5916 2.5844 2.6250
Ext. 25/8-16 UN 3A 0.0000 2.6250 2.6156 - 2.5844 2.5803 2.5505
Int.   3B   2.5658 2.5570   2.5898 2.5844 2.6250
Ext. 25/8-20 UN 2A 0.0015 2.6235 2.6154 - 2.5910 2.5859 2.5640
Int.   2B   2.5820 2.5710   2.5991 2.5925 2.6250
Ext. 25/8-20 UN 3A 0.0000 2.6250 2.6169 - 2.5925 2.5887 2.5655
Int.   3B   2.5787 2.5710   2.5975 2.5925 2.6250
Ext. 23/4-4 UNC 1A 0.0032 2.7468 2.7111 - 2.5844 2.5686 2.4491
Int.   1B   2.5170 2.4790   2.6082 2.5876 2.7500
Ext. 23/4-4 UNC 2A 0.0032 2.7468 2.7230 2.7111 2.5844 2.5739 2.4491
Int.   2B   2.5170 2.4790   2.6013 2.5876 2.7500
Ext. 23/4-4 UNC 3A 0.0000 2.7500 2.7262 - 2.5876 2.5797 2.4523
Int.   3B   2.5094 2.4790   2.5979 2.5876 2.7500
Ext. 23/4-6 UN 2A 0.0027 2.7473 2.7291 - 2.6390 2.6299 2.5489
Int.   2B   2.6000 2.5700   2.6536 2.6417 2.7500
Ext. 23/4-6 UN 3A 0.0000 2.7500 2.7318 - 2.6417 2.6349 2.5516
Int.   3B   2.5896 2.5700   2.6506 2.6417 2.7500
Ext. 23/4-8 UN 2A 0.0025 2.7475 2.7325 2.7250 2.6663 2.6580 2.5987
Int.   2B   2.6400 2.6150   2.6796 2.6688 2.7500
Ext. 23/4-8 UN 3A 0.0000 2.7500 2.7350 - 2.6688 2.6625 2.6012
Int.   3B   2.6297 2.6150   2.6769 2.6688 2.7500
Ext. 23/4-10 UNS 2A 0.0020 2.7480 2.7351 - 2.6830 2.6763 2.6290
Int.   2B   2.6630 2.6420   2.6937 2.6850 2.7500
Ext. 23/4-12 UN 2A 0.0019 2.7481 2.7367 - 2.6940 2.6878 2.6489
Int.   2B   2.6780 2.6600   2.7040 2.6959 2.7500
Ext. 23/4-12 UN 3A 0.0000 2.7500 2.7386 - 2.6959 2.6913 2.6508
Int.   3B   2.6698 2.6600   2.7019 2.6959 2.7500
Ext. 23/4-14 UNS 2A 0.0017 2.7483 2.7380 - 2.7019 2.6961 2.6633
Int.   2B   2.6880 2.6730   2.7112 2.7036 2.7500
Ext. 23/4-16 UN 2A 0.0017 2.7483 2.7389 - 2.7077 2.7022 2.6738
Int.   2B   2.6960 2.6820   2.7166 2.7094 2.7500
Ext. 23/4-16 UN 3A 0.0000 2.7500 2.7406 - 2.7094 2.7053 2.6755
Int.   3B   2.6908 2.6820   2.7148 2.7094 2.7500
Ext. 23/4-18 UNS 2A 0.0016 2.7484 2.7397 - 2.7123 2.7070 2.6832
Int.   2B   2.7030 2.6900   2.7208 2.7139 2.7500
Ext. 23/4-20 UN 2A 0.0015 2.7485 2.7404 - 2.7160 2.7109 2.6890
Int.   2B   2.7070 2.6960   2.7241 2.7175 2.7500
Ext. 23/4-20 UN 3A 0.0000 2.7500 2.7419 - 2.7175 2.7137 2.6905
Int.   3B   2.7037 2.6960   2.7225 2.7175 2.7500
a For Class 2A threads with coating (plating) the max. is increased by the allowance, to the basic size. This is the same value as Class 3A.
b For uncoated hot-rolled, but not standard fasteners with cold rolled threads
c UN series external thread, Max.Diameter = Basic - Allowance, for Class 1A and 2A
  = Basic for Class 3A
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