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    2. ASME B18.2.1 Hex Bolts

    ASME B18.2.1 Hex Bolts

    Versatile hexagonal head bolts serving as the universal standard for general-purpose fastening in construction, manufacturing, and maintenance.

    ASME B18.2.1 Hex Bolts are the most common and widely used type of fastener in the world, characterized by a six-sided head and a partially or fully threaded shank. Designed to be used with a corresponding nut or driven into a tapped hole, they provide a reliable and removable method of joining two or more components. The hexagonal head shape offers an optimal balance between wrench accessibility and material usage, allowing for efficient installation in tight spaces while providing sufficient torque capability. These bolts are fundamental to virtually every industry, from assembling consumer electronics and automotive engines to constructing skyscrapers and bridges. The ASME B18.2.1 standard defines the exact dimensions for the head, thread length, and body diameter, ensuring that a hex bolt from any compliant manufacturer will fit standard tools and nuts perfectly.

    ASME B18.2.1 Hex Bolts

    ASME B18.2.1 Hex Bolts Dimensions (mm)

    Nominal Size Full-Size Body Diameter, E Width Across Flats, F Width Across Corners, G Head Height, H Radius of Fillet, R Nominal Thread Length, LT
    Max Min Basic Max Min Max Min Basic Max Min Max Min >152.4 mm <152.4 mm
    1/4 6.604 6.020 11.113 11.125 10.795 12.827 12.294 4.366 4.775 3.810 0.762 0.254 19.050 25.400
    5/16 8.230 7.569 12.700 12.700 12.294 14.656 14.021 5.556 5.969 4.953 0.762 0.254 22.225 28.575
    3/8 9.855 9.144 14.288 14.275 13.818 16.510 15.748 6.350 6.807 5.740 0.762 0.254 25.400 31.750
    7/16 11.481 10.693 15.875 15.875 15.316 18.339 17.450 7.541 8.026 6.909 0.762 0.254 28.575 34.925
    1/2 13.081 12.243 19.050 19.050 18.415 21.996 20.980 8.731 9.246 7.671 0.762 0.254 31.750 38.100
    5/8 16.307 15.367 23.813 23.825 23.012 27.508 26.238 10.716 11.278 9.601 1.524 0.508 38.100 44.450
    3/4 19.507 18.517 28.575 28.575 27.635 32.995 31.496 12.700 13.310 11.557 1.524 0.508 44.450 50.800
    7/8 22.733 21.641 33.338 33.325 32.233 38.506 36.754 14.684 15.342 13.487 1.524 0.508 50.800 57.150
    1 25.959 24.790 38.100 38.100 36.830 43.993 41.986 17.066 17.780 15.011 2.286 0.762 57.150 63.500
    1 1/8 29.185 27.889 42.863 42.875 41.427 49.505 47.219 19.050 19.812 16.713 2.286 0.762 63.500 69.850
    1 1/4 32.436 31.064 47.625 47.625 46.025 54.991 52.476 21.431 22.250 19.025 2.286 0.762 69.850 76.200
    1 3/8 35.662 34.163 52.388 52.375 50.648 60.503 57.734 23.019 23.876 20.574 2.286 0.762 76.200 82.550
    1 1/2 38.887 37.338 57.150 57.150 55.245 65.989 62.992 25.400 26.314 22.911 2.286 0.762 82.550 88.900
    1 5/8 42.113 40.411 61.913 61.925 59.842 71.501 66.446 26.194 28.346 24.841 2.286 0.762 88.900 95.250
    1 3/4 45.339 43.586 66.675 66.675 64.465 76.987 73.482 27.781 30.378 26.772 3.048 1.016 95.250 101.600
    1 7/8 48.565 46.711 71.438 71.425 69.063 82.499 78.715 31.750 32.410 28.702 3.048 1.016 101.600 107.950
    2 51.791 49.886 76.200 76.200 73.660 87.986 83.972 34.131 35.255 29.845 3.048 1.016 107.950 114.300
    2 1/4 58.547 56.236 85.725 85.725 82.855 98.984 94.463 38.100 39.319 33.706 4.826 1.524 120.650 127.000
    2 1/2 64.999 62.509 95.250 95.250 92.075 109.982 104.978 42.069 43.383 37.567 4.826 1.524 133.350 139.700
    2 3/4 71.806 68.859 104.775 104.775 101.295 120.980 115.468 46.038 47.473 41.453 4.826 1.524 146.050 152.400
    3 78.257 75.209 114.300 114.300 110.490 131.978 125.959 50.800 52.324 46.101 4.826 1.524 158.750 165.100
    3 1/4 84.709 81.534 123.825 123.825 119.685 142.977 136.449 55.563 57.175 49.174 4.826 1.524 171.450 177.800
    3 1/2 91.161 87.909 133.350 133.350 128.905 153.975 146.964 58.738 60.452 52.248 4.826 1.524 184.150 190.500
    3 3/4 97.993 94.640 142.875 142.875 138.100 164.973 157.429 63.500 65.329 56.921 4.826 1.524 196.850 203.200
    4 104.419 100.965 152.400 152.400 147.320 175.971 167.945 68.263 70.206 61.570 4.826 1.524 209.550 215.900

    ASME B18.2.1 Hex Bolts Length Tolerance (mm)

    Nominal Length Size
    1/4 to 3/8 7/16 and 1/2 9/16 to 3/4 7/8 and 1 1 1/8 to 1 1/2 Over 1 1/2
    Up to 25.4 mm, incl. +0.508
    0.762    		 +0.508
    0.762    		 +0.508
    0.762    		 ... ... ...
    Over 25.4 mm to 63.5 mm, incl. +0.508
    1.016    		 +1.016
    1.524    		 +1.524
    2.032    		 +2.032
    2.54    		 +3.048
    3.048    		 +4.572
    4.572
    Over 63.5 mm to 101.6 mm, incl. +1.016
    1.524    		 +1.524
    2.032    		 +2.032
    2.54    		 +2.54
    3.556    		 +4.064
    4.064    		 +5.08
    5.08
    Over 101.6 mm to 152.4 mm, incl. +1.524
    2.54    		 +2.032
    2.54    		 +2.54
    2.54    		 +3.048
    4.064    		 +4.572
    4.572    		 +5.588
    5.588
    Longer than 152.4 mm +2.54
    4.572    		 +3.048
    4.572    		 +3.556
    4.572    		 +4.064
    5.08    		 +5.588
    5.588    		 +6.096
    6.096

    ASME B18.2.1 Hex Bolts Dimensions (Inch)

    Nominal Size Full-Size Body Diameter, E Width Across Flats, F Width Across Corners, G Head Height, H Radius of Fillet, R Nominal Thread Length, LT
    Max Min Basic Max Min Max Min Basic Max Min Max Min >6" <6"
    1/4 0.260 0.237 7/16 0.438 0.425 0.505 0.484 11/64 0.188 0.150 0.03 0.01 0.750 1.000
    5/16 0.324 0.298 1/2 0.500 0.484 0.577 0.552 7/32 0.235 0.195 0.03 0.01 0.875 1.125
    3/8 0.388 0.360 9/16 0.562 0.544 0.650 0.620 1/4 0.268 0.226 0.03 0.01 1.000 1.250
    7/16 0.452 0.421 5/8 0.625 0.603 0.722 0.687 19/64 0.316 0.272 0.03 0.01 1.125 1.375
    1/2 0.515 0.482 3/4 0.750 0.725 0.866 0.826 11/32 0.364 0.302 0.03 0.01 1.250 1.500
    5/8 0.642 0.605 15/16 0.938 0.906 1.083 1.033 27/64 0.444 0.378 0.06 0.02 1.500 1.750
    3/4 0.768 0.729 1 1/8 1.125 1.088 1.299 1.240 1/2 0.524 0.455 0.06 0.02 1.750 2.000
    7/8 0.895 0.852 1 5/16 1.312 1.269 1.516 1.447 37/64 0.604 0.531 0.06 0.02 2.000 2.250
    1 1.022 0.976 1 1/2 1.500 1.450 1.732 1.653 43/64 0.700 0.591 0.09 0.03 2.250 2.500
    1 1/8 1.149 1.098 1 11/16 1.688 1.631 1.949 1.859 3/4 0.780 0.658 0.09 0.03 2.500 2.750
    1 1/4 1.277 1.223 1 7/8 1.875 1.812 2.165 2.066 27/32 0.876 0.749 0.09 0.03 2.750 3.000
    1 3/8 1.404 1.345 2 1/16 2.062 1.994 2.382 2.273 29/32 0.940 0.810 0.09 0.03 3.000 3.250
    1 1/2 1.531 1.470 2 1/4 2.250 2.175 2.598 2.480 1 1.036 0.902 0.09 0.03 3.250 3.500
    1 5/8 1.658 1.591 2 7/16 2.438 2.356 2.815 2.616 1 3/32 1.116 0.978 0.09 0.03 3.500 3.750
    1 3/4 1.785 1.716 2 5/8 2.625 2.538 3.031 2.893 1 5/32 1.196 1.054 0.12 0.04 3.750 4.000
    1 7/8 1.912 1.839 2 13/16 2.812 2.719 3.248 3.099 1 1/4 1.276 1.130 0.12 0.04 4.000 4.250
    2 2.039 1.964 3 3.000 2.900 3.464 3.306 1 11/32 1.388 1.175 0.12 0.04 4.250 4.500
    2 1/4 2.305 2.214 3 3/8 3.375 3.262 3.897 3.719 1 1/2 1.548 1.327 0.19 0.06 4.750 5.000
    2 1/2 2.559 2.461 3 3/4 3.750 3.625 4.330 4.133 1 21/32 1.708 1.479 0.19 0.06 5.250 5.500
    2 3/4 2.827 2.711 4 1/8 4.125 3.988 4.763 4.546 1 13/16 1.869 1.632 0.19 0.06 5.750 6.000
    3 3.081 2.961 4 1/2 4.500 4.350 5.196 4.959 2 2.060 1.815 0.19 0.06 6.250 6.500
    3 1/4 3.335 3.210 4 7/8 4.875 4.712 5.629 5.372 2 3/16 2.251 1.936 0.19 0.06 6.750 7.000
    3 1/2 3.589 3.461 5 1/4 5.250 5.075 6.062 5.786 2 5/16 2.380 2.057 0.19 0.06 7.250 7.500
    3 3/4 3.858 3.726 5 5/8 5.625 5.437 6.495 6.198 2 1/2 2.572 2.241 0.19 0.06 7.750 8.000
    4 4.111 3.975 6 6.000 5.800 6.928 6.612 2 11/16 2.764 2.424 0.19 0.06 8.250 8.500

    ASME B18.2.1 Hex Bolts Length Tolerance (Inch)

    Nominal Length Nominal Size
    1/4 to 3/8 7/16 and 1/2 9/16 to 3/4 7/8 and 1 1 1/8 to 1 1/2 Over 1 1/2
    Up to 1 in., incl. +0.02
    0.03    		 +0.02
    0.03    		 +0.02
    0.03    		 ... ... ...
    Over 1 in. to 2 1/2 in., incl. +0.02
    0.04    		 +0.04
    0.06    		 +0.06
    0.08    		 +0.08
    0.10    		 +0.12
    0.12    		 +0.18
    0.18
    Over 2 1/2 in. to 4 in., incl. +0.04
    0.06    		 +0.06
    0.08    		 +0.08
    0.10    		 +0.10
    0.14    		 +0.16
    0.16    		 +0.20
    0.20
    Over 4 in. to 6 in., incl. +0.06
    0.10    		 +0.08
    0.10    		 +0.10
    0.10    		 +0.12
    0.16    		 +0.18
    0.18    		 +0.22
    0.22
    Longer than 6 in. +0.10
    0.18    		 +0.12
    0.18    		 +0.14
    0.18    		 +0.16
    0.20    		 +0.22
    0.22    		 +0.24
    0.24

    In-Depth Engineering Reference

    What is the difference between ASME B18.2.1 hex bolts and heavy hex bolts?

    ASME B18.2.1 covers BOTH series. Standard hex bolts have a width-across-flats of 1.5×D (1½ times bolt diameter). Heavy hex bolts have a width-across-flats of approximately 1.625×D (slightly larger flats) and proportionally thicker heads, making them better for high-torque structural and pressure bolting applications.

    Can ASTM A193 B7 hex bolts be made to ASME B18.2.1 dimensions?

    Yes — ASTM A193 is the MATERIAL specification (chemistry + mechanicals + heat treatment); ASME B18.2.1 is the DIMENSIONAL specification (head size, body diameter, thread length). The two are routinely combined: B7 hex bolts dimensioned per ASME B18.2.1 are the standard high-temperature flange stud.

    What size range is in ASME B18.2.1?

    ASME B18.2.1 covers nominal diameters from ¼ inch through 4 inches, with bolt lengths typically from ½ inch to 24 inches. Larger or longer bolts move into custom (special order per ASME B18.2.3 family / B18.31 stud series).

    Frequently Asked Questions

    What does ASME B18.2.1 hex bolts cover?
    ASME B18.2.1 is the inch dimensional standard for hex bolts. It defines head dimensions, body diameter, thread length, total length, drive recess, and tolerances so any compliant hex bolts interchanges with standard nuts, washers and wrenches across manufacturers.
    What size range is available for hex bolts under ASME B18.2.1?
    hex bolts under ASME B18.2.1 covers 1/4 in to 4 in nominal diameter, length 1/2 in to 24 in. TorqBolt manufactures the full range in ASTM A193 Grades B7, B7M, B8, B8M and the full ASTM A320 cryogenic counterpart on customer specification.
    What is the metric/inch counterpart of ASME B18.2.1 hex bolts?
    The companion specification for hex bolts is ASME B18.2.3 metric series. Both standards share equivalent functional dimensions but use the corresponding measurement system (inch vs metric ISO M-series).
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