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    2. ASME B18.2.1

    ASME B18.2.1 Hex Bolts Dimensions and Specifications

    Standard Dimensions for Hex Head Bolts (ANSI/ASME B18.2.1)

    ASME B18.2.1 Standard covers the dimensional requirements, tolerances, and specifications for hex head bolts and screws used in general engineering and industrial applications. This standard defines key parameters such as head dimensions, thread lengths, and mechanical compatibility to ensure interchangeability and performance. On this page, you will find complete ASME B18.2.1 hex bolt dimensions, size charts, and technical details for accurate selection and application.

    Procurement engineers specifying stainless hex bolts dimensions for general-service applications use ASME B18.2.1 inch-series tolerances on ASTM A193 Grade B8 material. Typical service: chemical processing flange joints, pressure-vessel cover plates, valve bonnet fastening, and atmospheric storage-tank assembly. The standard six-sided head with across-flats wrench engagement is the default fastener pattern for 1/4-inch through 4-inch nominal diameters in 304 stainless. See the B8 Class 1 annealed condition for sour-gas duty.

    ASME B18.2.1 Hex Bolts

    ASME B18.2.1 Dimensional Tables for ASTM A193 B8 Hex Bolts

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

    ASME B18.2.1 Hex Bolts Grip Gaging Lengths (Inch)

    Nominal Length, Lg max. / Lb min. Nominal Size
    1/4 5/16 3/8 7/16 1/2 9/16 5/8 3/4 7/8 1 1-1/8 1-1/4 1-3/8 1-1/2
    1-1/4 Lg Max. 0.50 Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread
    Lb Min. 0.25
    1-3/8 Lg Max. 0.63 0.50 Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread
    Lb Min. 0.38 0.22
    1-1/2 Lg Max. 0.75 0.62 0.50 Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread
    Lb Min. 0.50 0.35 0.19
    1-5/8 Lg Max. 0.88 0.75 0.62 Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread
    Lb Min. 0.62 0.47 0.31
    1-3/4 Lg Max. 1.00 0.88 0.75 0.63 Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread
    Lb Min. 0.75 0.60 0.44 0.27
    1-7/8 Lg Max. 1.12 1.00 0.88 0.75 0.63 Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread
    Lb Min. 0.88 0.72 0.56 0.39 0.24
    2 Lg Max. 1.25 1.12 1.00 0.88 0.75 Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread
    Lb Min. 1.00 0.85 0.69 0.52 0.38
    2-1/8 Lg Max. 1.38 1.25 1.12 1.00 0.88 0.75 Full thread Full thread Full thread Full thread Full thread Full thread Full thread Full thread
    Lb Min. 1.12 0.97 0.81 0.64 0.49 0.33
    2-1/4 Lg Max. 1.50 1.38 1.25 1.12 1.00 0.88 0.75 Full thread Full thread Full thread Full thread Full thread Full thread Full thread
    Lb Min. 1.25 1.10 0.94 0.77 0.62 0.45 0.30
    2-3/8 Lg Max. 1.62 1.50 1.38 1.25 1.12 1.00 0.88 Full thread Full thread Full thread Full thread Full thread Full thread Full thread
    Lb Min. 1.38 1.22 1.06 0.89 0.74 0.58 0.42
    2-1/2 Lg Max. 1.75 1.62 1.50 1.38 1.25 1.12 1.00 Full thread Full thread Full thread Full thread Full thread Full thread Full thread
    Lb Min. 1.50 1.35 1.19 1.02 0.86 0.75 0.55
    2-5/8 Lg Max. 1.88 1.75 1.62 1.50 1.38 1.25 1.12 1.12 Full thread Full thread Full thread Full thread Full thread Full thread
    Lb Min. 1.62 1.47 1.31 1.14 0.99 0.83 0.67 0.67
    2-3/4 Lg Max. 2.00 1.88 1.75 1.62 1.50 1.38 1.25 1.25 Full thread Full thread Full thread Full thread Full thread Full thread
    Lb Min. 1.75 1.60 1.44 1.27 1.12 0.96 0.80 0.80
    2-7/8 Lg Max. 2.12 2.00 1.88 1.75 1.62 1.50 1.38 1.38 1.00 0.88 Full thread Full thread Full thread Full thread
    Lb Min. 1.88 1.72 1.56 1.39 1.24 1.08 0.92 0.92 0.50 0.32
    3 Lg Max. 2.52 2.12 2.00 1.88 1.75 1.62 1.50 1.50 1.12 1.00 Full thread Full thread Full thread Full thread
    Lb Min. 2.00 1.85 1.69 1.52 1.36 1.21 1.05 1.05 0.62 0.44
    3-1/4 Lg Max. 2.50 2.38 2.25 2.12 2.00 1.88 1.75 1.50 1.25 1.00 Full thread Full thread Full thread Full thread
    Lb Min. 2.25 2.10 1.94 1.77 1.62 1.46 1.30 1.00 0.75 0.38
    3-1/2 Lg Max. 2.75 2.62 2.50 2.38 2.25 2.12 2.00 1.75 1.25 1.25 Full thread Full thread Full thread Full thread
    Lb Min. 2.50 2.35 2.19 2.02 1.86 1.71 1.55 1.25 0.69 0.62
    3-3/4 Lg Max. 3.00 2.88 2.75 2.62 2.50 2.38 2.25 2.00 1.50 1.50 1.25 Full thread Full thread Full thread
    Lb Min. 2.75 2.60 2.44 2.27 2.12 1.96 1.80 1.50 0.94 0.88 0.54
    4 Lg Max. 3.25 3.12 3.00 2.88 2.75 2.62 2.50 2.25 1.75 1.75 1.50 1.25 Full thread Full thread
    Lb Min. 3.00 2.85 2.69 2.52 2.36 2.21 2.05 1.75 1.19 1.12 0.79 0.54
    4-1/4 Lg Max. 3.50 3.38 3.25 3.12 3.00 2.88 2.75 2.50 2.00 2.00 1.75 1.50 Full thread Full thread
    Lb Min. 3.25 3.10 2.94 2.77 2.62 2.46 2.30 2.00 1.44 1.38 1.04 0.79
    4-1/2 Lg Max. 3.75 3.62 3.50 3.38 3.25 3.12 3.00 2.75 2.25 2.25 2.00 1.75 1.50 Full thread
    Lb Min. 3.50 3.35 3.19 3.02 2.86 2.71 2.55 2.25 1.69 1.62 1.29 1.04 0.79
    4-3/4 Lg Max. 4.00 3.88 3.75 3.62 3.50 3.38 3.25 3.00 2.50 2.50 2.25 2.00 1.75 1.50
    Lb Min. 3.75 3.60 3.44 3.27 3.12 2.96 2.80 2.50 1.94 1.88 1.54 1.29 1.04 0.67
    5 Lg Max. 4.25 4.12 4.00 3.88 3.75 3.62 3.50 3.25 2.75 2.75 2.50 2.25 2.00 1.75
    Lb Min. 4.00 3.85 3.69 3.52 3.36 3.21 3.05 2.75 2.19 2.12 1.79 1.54 1.29 0.92
    5-1/4 Lg Max. 4.50 4.38 4.25 4.12 4.00 3.88 3.75 3.50 3.00 3.00 2.75 2.50 2.25 2.00
    Lb Min. 4.25 4.10 3.94 3.77 3.62 3.46 3.30 3.00 2.44 2.38 2.04 1.79 1.54 1.17
    5-1/2 Lg Max. 4.75 4.62 4.50 4.38 4.25 4.12 4.00 3.75 3.25 3.25 3.00 2.75 2.50 2.25
    Lb Min. 4.50 4.35 4.19 4.02 3.87 3.71 3.55 3.25 2.69 2.62 2.29 2.04 1.79 1.42
    5-3/4 Lg Max. 5.00 4.88 4.75 4.63 4.50 4.38 4.25 4.00 3.50 3.50 3.25 3.00 2.75 2.50
    Lb Min. 4.75 4.60 4.44 4.27 4.12 3.96 3.80 3.50 2.94 2.88 2.54 2.29 2.04 1.67
    6 Lg Max. 5.25 5.12 5.00 4.88 4.75 4.62 4.50 4.25 3.75 3.75 3.50 3.25 3.00 2.75
    Lb Min. 5.00 4.85 4.69 4.52 4.36 4.21 4.05 3.75 3.19 3.12 2.79 2.54 2.29 1.92
    6-1/4 Lg Max. 5.25 5.12 5.00 4.88 4.75 4.62 4.50 4.25 4.00 3.75 3.75 3.25 3.25 2.75
    Lb Min. 5.00 4.85 4.69 4.52 4.36 4.21 4.05 3.75 3.44 3.12 3.04 2.54 2.54 1.92
    6-1/2 Lg Max. 5.50 5.38 5.25 5.12 5.00 4.88 4.75 4.50 4.00 4.00 4.00 3.50 3.25 3.00
    Lb Min. 5.25 5.10 4.94 4.77 4.62 4.46 4.30 4.00 3.44 3.38 3.29 2.79 2.54 2.17
    6-3/4 Lg Max. 5.75 5.62 5.50 5.38 5.25 5.12 5.00 4.75 4.25 4.25 4.25 3.75 3.50 3.25
    Lb Min. 5.50 5.35 5.19 5.02 4.86 4.71 4.55 4.25 3.69 3.63 3.54 3.04 2.79 2.42
    7 Lg Max. 6.00 5.88 5.75 5.62 5.50 5.38 5.25 5.00 4.50 4.50 4.50 4.00 3.75 3.50
    Lb Min. 5.75 5.60 5.44 5.27 5.12 4.96 4.80 4.50 3.94 3.88 3.79 3.29 3.04 2.67
    7-1/4 Lg Max. 6.25 6.12 6.00 5.88 5.75 5.62 5.50 5.25 4.75 4.75 4.75 4.25 4.00 3.75
    Lb Min. 6.00 5.85 5.69 5.52 5.36 5.20 5.05 4.75 4.19 4.12 4.04 3.54 3.29 2.92
    7-1/2 Lg Max. 6.50 6.38 6.25 6.12 6.00 5.88 5.75 5.50 5.00 5.00 5.00 4.50 4.25 4.00
    Lb Min. 6.25 6.10 5.94 5.77 5.62 5.46 5.30 5.00 4.44 4.38 4.29 3.79 3.54 3.17
    7-3/4 Lg Max. 6.75 6.62 6.50 6.38 6.25 6.12 6.00 5.75 5.25 5.25 5.25 4.75 4.50 4.25
    Lb Min. 6.50 6.35 6.19 6.02 5.87 5.71 5.55 5.25 4.69 4.52 4.54 4.04 3.79 3.42
    8 Lg Max. 7.00 6.88 6.75 6.62 6.50 6.38 6.25 6.00 5.50 5.50 5.50 5.00 4.75 4.50
    Lb Min. 6.75 6.60 6.44 6.27 6.12 5.96 5.80 5.50 4.94 4.88 4.79 4.29 4.04 3.67
    8-1/4 Lg Max. 7.25 7.12 7.00 6.88 6.75 6.62 6.50 6.25 5.75 5.75 5.75 5.25 5.00 4.75
    Lb Min. 7.00 6.85 6.69 6.52 6.36 6.21 6.05 5.75 5.19 5.12 5.04 4.54 4.29 3.92
    8-1/2 Lg Max. 7.50 7.38 7.25 7.12 7.00 6.88 6.75 6.50 6.00 6.00 6.00 5.50 5.25 5.00
    Lb Min. 7.25 7.10 6.94 6.77 6.62 6.46 6.30 6.00 5.44 5.38 5.29 4.79 4.54 4.17
    8-3/4 Lg Max. 7.75 7.62 7.50 7.38 7.25 7.12 7.00 6.75 6.25 6.25 6.25 5.75 5.50 5.25
    Lb Min. 7.50 7.35 7.19 7.02 6.86 6.71 6.55 6.25 5.69 5.62 5.54 5.04 4.79 4.42
    9 Lg Max. 8.00 7.88 7.75 7.62 7.50 7.38 7.25 7.00 6.50 6.50 6.50 6.00 5.75 5.50
    Lb Min. 7.75 7.60 7.44 7.27 7.12 6.96 6.80 6.50 5.94 5.88 5.79 5.29 5.04 4.67
    9-1/4 Lg Max. 8.25 8.12 8.00 7.88 7.75 7.62 7.50 7.25 6.75 6.75 6.75 6.25 6.00 5.75
    Lb Min. 8.00 7.85 7.69 7.52 7.36 7.21 7.05 6.75 6.19 6.12 6.04 5.54 5.29 4.92
    9-1/2 Lg Max. 8.50 8.38 8.25 8.12 8.00 7.88 7.75 7.50 7.00 7.00 7.00 6.50 6.25 6.00
    Lb Min. 8.25 8.10 7.94 7.77 7.62 7.46 7.30 7.00 6.44 6.38 6.29 5.79 5.54 5.17
    9-3/4 Lg Max. 8.75 8.62 8.50 8.38 8.25 8.12 8.00 7.75 7.25 7.25 7.25 6.75 6.50 6.25
    Lb Min. 8.50 8.35 8.19 8.02 7.86 7.71 7.55 7.25 6.89 6.62 6.54 6.04 5.79 5.42
    10 Lg Max. 9.00 8.88 8.75 8.62 8.50 8.38 8.25 8.00 7.50 7.50 8.25 7.00 6.75 6.50
    Lb Min. 8.75 8.60 8.44 8.27 8.12 7.96 7.80 7.50 6.94 6.88 7.54 6.29 6.04 5.67
    11 Lg Max. 10.00 9.88 9.75 9.62 9.50 9.38 9.25 9.00 8.75 8.50 9.25 8.00 7.75 7.50
    Lb Min. 9.75 9.60 9.44 9.27 9.12 8.96 8.80 8.50 8.19 7.88 8.54 7.29 7.04 6.67
    12 Lg Max. 11.00 10.88 10.75 10.62 10.50 10.38 10.25 10.00 9.75 9.50 9.25 9.00 8.75 8.50
    Lb Min. 10.75 10.60 10.44 10.27 10.12 9.96 9.80 9.50 9.19 8.88 8.54 8.29 8.04 7.67
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