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Recommended hole size limits before threading to provide for optimum strength of fastenings and tapping conditions are shown in the following Table for classes 1B, 2B, and 3B. The hole size limit before threading, and the tolerances between them, are derived from the minimum and maximum minor diameters of the internal threads using the following rules: For lengths of engagement in the range to and including 1/3d, where d equals nominal diameter, the minimum hole size will be equal to the minimum minor diameter of the internal thread and the maximum hole size will be larger by one-half the minor diameter tolerance. For the range from 1/3d to 2/3d, the minimum and maximum hole sizes will each be one quarter of the minor diameter tolerance larger than the corresponding limits for the length of engagement to and including 1/3d. For the range from 2/3d to 11/2d the minimum hole size will be larger than the minimum minor diameter of the internal thread by one-half the minor diameter tolerance and the maximum hole size will be equal to the maximum minor diameter. For the range from 11/2d to 3d the minimum and maximum hole sizes will each be one-quarter of the minor diameter tolerance of the internal thread larger than the corresponding limits for the 2/3d to 11/2d length of engagement. From the foregoing it will be seen that the difference between limits in each range is the same and equal to one-half of the minor diameter tolerance. This is a general rule, except that the minimum differences for sizes below 1/4 inch are equal to the minor diameter tolerances calculated on the basis of lengths of engagement to and including 1/3d. Also, for lengths of engagement greater than 1/3d and for sizes 1/4 inch and larger the values are adjusted so that the difference between limits is never less than 0.004 inch. For diameter-pitch combinations other than those given below, the foregoing rules should be applied to the screw thread tolerances or derived from the formulas given on table to determine the hole size limits. Selection of Tap Drills: In selecting standard drills to produce holes within the limits given on table it should be recognized that drills have a tendency to cut oversize. The material on table may be used as a guide to the expected amount of oversize

Classes 1B and 2B Class 3B Length of Engagement (d = Nominal Size of Thread) Thread Size To and Including 1/3d Above 1/3d to 2/3d Above 2/3d to 1-1/2d Above1-1/3d to3d To and Including 1/3d Above 1/3d to 2/3d Above 2/3d to 1-1/2d Above 1-1/2d to 3d Recommended Hole Size Limits Min.* Max. Min. Max. Min. Max. Min. Max. Min.* Max. Min. Max. Min. Max.† Min. Max. 0-80 1-64 1-72 2-56 2-64 .0465 .0561 .0580 .0667 .0691 .0500 .0599 .0613 .0705 .0724 .0479 .0585 .0596 .0686 .0707 .0514 .0623 .0629 .0724 .0740 .0479 .0585 .0602 .0699 .0720 .0514 .0623 .0635 .0737 .0753 .0479 .0585 .0602 .0699 .0720 .0514 .0623 .0635 .0737 .0753 .0465 .0561 .0580 .0667 .0691 .0500 .0599 .0613 .0705 .0724 .0479 .0585 .0596 .0686 .0707 .0514 .0623 .0629 .0724 .0740 .0479 .0585 .0602 .0699 .0720 .0514 .0623 .0635 .0737 .0753 .0479 .0585 .0602 .0699 .0720 0.0514 0.0623 0.0635 0.0737 0.0753 3-48 3-56 4-40 4-48 5-40 .0764 .0797 .0849 .0894 .0979 .0804 .0831 .0894 .0931 .1020 .0785 .0814 .0871 .0912 .1000 .0825 .0848 .0916 .0949 .1041 .0805 .0831 .0894 .0931 .1021 .0845 .0865 .0939 .0968 .1062 .0806 .0833 .0902 .0939 .1036 .0846 .0867 .0947 .0976 .1077 .0764 .0797 .0849 .0894 .0979 .0804 .0831 .0894 .0931 .1020 .0785 .0814 .0871 .0912 .1000 .0825 .0848 .0916 .0949 .1041 .0805 .0831 .0894 .0931 .1021 .0845 .0865 .0939 .0968 .1062 .0806 .0833 .0902 .0939 .1036 0.0846 0.0867 0.0947 0.0976 0.1077 5-44 6-32 6-40 8-32 8-36 .1004 .1040 .1110 .1300 .1340 .1042 .1090 .1150 .1340 .1380 .1023 .1060 .1130 .1320 .1360 .1060 .1120 .1170 .1370 .1400 .1042 .1090 .1150 .1340 .1380 .1079 .1140 .1190 .1390 .1420 .1060 .1120 .1170 .1370 .1400 .1097 .1170 .1210 .1410 .1440 .1004 .1040 .1110 .1300 .1340 .1042 .1091 .1148 .1345 .1377 .1023 .1066 .1128 .1324 .1359 .1060 .1115 .1167 .1367 .1397 .1042 .1091 .1147 .1346 .1378 .1079 .1140 .1186 .1389 .1416 .1060 .1115 .1166 .1367 .1397 0.1097 0.1164 0.1205 0.1410 0.1435 10-24 10-32 12-24 12-28 12-32 .1450 .1560 .1710 .1770 .1820 .1500 .1600 .1760 .1820 .1860 .1480 .1580 .1740 .1790 .1840 .1540 .1620 .1790 .1840 .1880 .1500 .1600 .1760 .1820 .1860 .1560 .1640 .1810 .1860 .1900 .1520 .1620 .1780 .1840 .1880 .1590 .1660 .1840 .1880 .1920 .1450 .1560 .1710 .1770 .1820 .1502 .1601 .1758 .1815 .1858 .1475 .1581 .1733 .1794 .1837 .1528 .1621 .1782 .1836 .1877 .1502 .1601 .1758 .1815 .1855 .1555 .1641 .1807 .1857 .1895 .1528 .1621 .1782 .1836 .1873 0.1581 0.1661 0.1831 0.1878 0.1913 1/4-20 1/4-28 1/4-32 1/4-36 5/16-18 .1960 .2120 .2160 .2220 .2520 .2020 .2160 .2200 .2240 .2590 .1990 .2130 .2180 .2210 .2550 .2040 .2180 .2220 .2250 .2620 .2020 .2160 .2200 .2240 .2590 .2070 .2200 .2240 .2260 .2650 .2040 .2180 .2220 .2250 .2620 .2100 .2220 .2260 .2280 .2680 .1960 .2110 .2160 .2200 .2520 .2013 .2152 .2196 .2243 .2577 .1986 .2131 .2172 .2199 .2551 .2040 .2171 .2212 .2243 .2604 .2013 .2150 .2189 .2214 .2577 .2067 .2190 .2229 .2258 .2630 .2040 .2169 .2206 .2229 .2604 0.2094 0.2209 0.2246 0.2273 0.2657 5/16-24 5/16-32 5/16-36 3/8-16 3/8-24 .2670 .2790 .2820 .3070 .3300 .2720 .2830 .2860 .3140 .3350 .2700 .2810 .2840 .2310 .3330 .2750 .2850 .2880 .3180 .3380 .2720 .2830 .2850 .3140 .3350 .2770 .2860 .2890 .3210 .3400 .2750 .2850 .2870 .3180 .3380 .2800 .2890 .2910 .3250 .3430 .2670 .2790 .2820 .3070 .3300 .2714 .2817 .2863 .3127 .3336 .2694 .2792 .2824 .3101 .3314 .2734 .2832 .2863 .3155 .3354 .2714 .2807 .2837 .3128 .3332 .2754 .2847 .2877 .3182 .3372 .2734 .2822 .2850 .3155 .3351 0.2774 0.2862 0.2890 0.3209 0.3391 3/8-32 3/8-36 7/16-14 7/16-20 7/16-28 .3410 .3450 .3600 .3830 .3990 .3450 .3490 .3680 .3890 .4030 .3430 .3460 .3640 .3860 .4010 .3470 .3500 .3720 .3910 .4060 .3450 .3470 .3680 .3890 .4030 .3490 .3520 .3760 .3950 .4070 .3470 .3490 .3720 .3910 .4060 .3510 .3530 .3800 .3970 .4100 .3410 .3450 .3600 .3830 .3990 .3441 .3488 .3660 .3875 .4020 .3415 .3449 .3630 .3855 .3995 .3455 .3488 .3688 .3896 .4035 .3429 .3461 .3659 .3875 .4011 .3469 .3501 .3717 .3916 .4051 .3444 .3474 .3688 .3896 .4017 0.3484 0.3514 0.3746 0.3937 0.4067 1/2-13 1/2-12 1/2-20 1/2-28 .4170 .4100 .4460 .4610 .4260 .4140 .4520 .4670 .4210 .4140 .4490 .4630 .4300 .4240 .4540 .4680 .4260 .4140 .4520 .4660 .4340 .4280 .4570 .4700 .4300 .4240 .4540 .4680 .4380 .4330 .4600 .4720 .4170 .4100 .4460 .4610 .4225 .4161 .4498 .4645 .4196 .4129 .4477 .4620 .4254 .4192 .4517 .4660 .4226 .4160 .4497 .4636 .4284 .4223 .4537 .4676 .4255 .4192 .4516 .4652 0.4313 0.4255 0.4556 0.4692 All dimensions are in inches. For basis of recommended hole size limits see table. As an aid in selecting suitable drills, see tap drill sizes for universal and m-profile screw threads and tap drill sizes for forming taps. For amount of expected drill oversize, see table. *This is the minimum minor diameter specified in thread tables 7, 8, 9. †This is the maximum minor diameter specified in thread tables 7, 8, 9.