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    2. ASTM A193 B8 Torque Chart

    ASTM A193 B8 Bolt Torque Chart: Reference Values + ASME PCC-1 Method

    Class 1 and Class 2 Stud Bolt Torque; Reference Use Only, Follow Equipment OEM Spec for Safety-Critical Joints

    ⚠ Critical safety notice; read before using any value on this page

    A generic torque chart cannot substitute for joint-specific torque calculation. Bolt preload depends on the friction K-factor (which varies 0.10-0.35 based on lubrication, plating, gasket compression, and thread condition), the joint stiffness ratio, the gasket type, and the equipment manufacturer's procedure. For ASME B16.5 flange joints in pressure service, follow ASME PCC-1 Annex K (Recommended Bolt Load Calculations) or the equipment OEM's tightening specification; not a generic table. Misapplied torque can under-load (joint leak) or over-load (gasket crush, stud yield, flange distortion). TorqBolt provides this reference as a starting point only; the procurement engineer is responsible for verifying the value against the joint design.

    Bolt torque on ASTM A193 B8 stud bolts is dominated by friction, not metallurgy. Two B8 Class 2 studs from the same heat lot, tightened with the same torque wrench, can develop preloads that differ by 30% if one is lubricated with anti-seize and the other is plain; that range comes entirely from the K-factor difference. This page reproduces the reference torque values you will find in handbooks and procurement quick-references, but the working procurement engineer should treat the values as a starting point, not an authoritative number.

    The reference values below assume a K-factor of 0.20 (the conservative middle value for lubricated stainless threading) and a target preload of approximately 50% of yield strength. They are computed from the standard torque-preload relationship T = K × d × F, where T is torque in lb-ft, K is the friction coefficient, d is the nominal bolt diameter in inches, and F is the desired preload force in pounds. The B8 Class 1 yield is 30 ksi minimum (A193 Table 2); the B8 Class 2 yield is 100 ksi minimum in diameters up to 3/4 inch and decreases at larger diameters.

    Reference Torque Values . B8 Class 1 (Solution-Annealed, 30 ksi YS)

    K-factor 0.20 (lubricated), preload at 50% of yield. Adjust by lubrication K and equipment spec.

    Nominal DiameterThread PitchTensile Stress Area (in²)Preload at 50% YS (lb)Reference Torque (lb-ft)
    1/4 in20 UNC0.03184772.0
    3/8 in16 UNC0.077511637.3
    1/2 in13 UNC0.1419212917.7
    5/8 in11 UNC0.2260339035.3
    3/4 in10 UNC0.3340501062.6
    7/8 in9 UNC0.46206930101.1
    1 in8 UNC0.60609090151.5
    1 1/8 in7 UNC0.763011445214.6
    1 1/4 in7 UNC0.969014535302.8
    1 3/8 in6 UNC1.155017325396.9
    1 1/2 in6 UNC1.405021075526.9

    Reference Torque Values . B8 Class 2 (Strain-Hardened, 100 ksi YS ≤3/4 in)

    B8 Class 2 yield decreases at larger diameters per A193 Table 2: 100 ksi (≤3/4 in), 80 ksi (over 3/4 to 1 in), 65 ksi (over 1 to 1 1/4 in), 50 ksi (over 1 1/4 to 1 1/2 in). Adjust preload accordingly.

    Nominal DiameterThread PitchYield (ksi)Preload at 50% YS (lb)Reference Torque (lb-ft)
    1/4 in20 UNC10015906.6
    3/8 in16 UNC100387524.2
    1/2 in13 UNC100709559.1
    5/8 in11 UNC10011300117.7
    3/4 in10 UNC10016700208.8
    7/8 in9 UNC8018480269.5
    1 in8 UNC8024240404.0
    1 1/8 in7 UNC6524798464.9
    1 1/4 in7 UNC6531493656.1
    1 3/8 in6 UNC5028875661.4
    1 1/2 in6 UNC5035125878.1

    K-Factor Adjustment

    The reference torques assume K = 0.20. Real-world joints span a wide range:

    Thread / nut conditionTypical K-factorTorque adjustment vs K=0.20
    Plain stainless, no lubricant (galling risk)0.30 - 0.35+50% to +75% torque needed
    PTFE-coated nut, dry0.10 - 0.15-25% to -50% torque needed
    Stainless with nickel-based anti-seize (e.g., Never-Seez)0.15 - 0.20nominal
    Stainless with molybdenum-disulfide grease0.10 - 0.15-25% to -50% torque needed

    The galling risk on plain stainless threads is real; B8 austenitic stud bolts work-harden under thread friction, and without lubrication can seize and shear before reaching target preload. TorqBolt ships B8 stud bolts with a nickel-based anti-seize standard for this reason; the K-factor in the table assumes that application.

    ASME PCC-1 Joint Assembly Method

    For ASME B16.5 flange joints in pressure service, the correct procurement workflow does not use a generic torque chart. The steps in ASME PCC-1 Annex K are:

    1. Determine the gasket type, dimensions, and seating stress requirement.
    2. Calculate the bolt load required for gasket seating and for the design pressure operating condition. Take the maximum.
    3. Divide by the number of bolts to get target preload per bolt.
    4. Verify the bolt's allowable load (yield x area x safety factor). If insufficient, upsize bolt or change material.
    5. Apply the K-factor for the actual thread/lubricant condition to compute torque from target preload.
    6. Use a defined tightening pattern (PCC-1 Section 9), typically a 4-pass cross-pattern at 30%, 60%, 100%, and 100% (round).

    This page's reference values are useful for first-pass sizing checks. They are not a substitute for the PCC-1 calculation on any joint that carries pressure, sees thermal cycling, or has safety consequences from leakage. The cost of a flange leak in petrochemical service is large; the cost of a calculation is small.

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