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    12-Point Bolts (Bi-Hex / Double-Hex), Aerospace and Motorsport

    Twelve-point aerospace bolts on a fastener QC bench, each head stamped with the tB ligature and alloy grade marking

    12-point bolts (also written 12 point bolts, bi-hex bolts, double-hex bolts, 12-spline bolts) are high-strength external-drive fasteners with twelve splines arranged every 30 degrees around the head perimeter. The 12-point head is widely used in aerospace, motorsport and high-performance diesel applications because it provides 2x the wrench engagement angles of a standard hex head, which allows higher applied torque to be reached for the same head size with reduced cam-off risk under heavy preload.

    12-point bolts are also known for their slightly lower head height compared to a hex bolt, which makes them very popular in tight engine bays and aircraft structural pockets where access is restricted. The 12-point geometry is itself material-agnostic: the same head shape is produced in a wide range of alloys, and the alloy choice (along with the heat treatment cycle) is what sets the final tensile and yield strength.

    TorqBolt manufactures 12-point bolts to a wide range of aerospace, military and industrial specifications, such as MS21250 (160 ksi mil-spec aerospace bolt), NAS1351 (close-tolerance NAS), AS3216 (180 ksi superalloy turbine), NAS6303 through NAS6320 (NAS 6300-series tension hex / 12-point), MIL-DTL-7838 (legacy MIL spec), DIN 12-point (German metric), and the related ISO 14579 internal-hexalobular standard. Each lot is supplied with a mill test report (MTC 3.1 standard, MTC 3.2 critical with third-party witness inspection) and is manufactured under AS9100D Rev D and ISO 9001:2015 quality systems, with NORSOK M-630 and NACE MR0175 / ISO 15156 qualifications for offshore and sour-service applications.

    Each 12-point bolt head also carries a struck tB manufacturer ligature plus the alloy grade marking (such as A286, MP35N, INCO 718, Ti-6Al-4V, 8740 or L19) so that traceability survives separation from the original packaging. Heat treatment, stamping, dimensional inspection and final QC happen in-house at TorqBolt facilities in Mumbai (head office and QA-QC) and Rajkot (production).

    Why a 12-point head geometry

    • 12 splines = 2x wrench engagement angles vs hex (every 30 degrees vs every 60)
    • Higher applied torque for the same head size before cam-off
    • Lower head height than hex for tight clearances
    • Reduced cam-off risk at high preload
    • Flat top face takes manufacturer + grade stamps cleanly
    • Drives with a 12-point socket wrench (matched to bolt)
    • A 6-point socket fits but engages only half the splines
    • External 12-point is different from internal hexalobular (ISO 14579)
    • Material-agnostic geometry; alloy choice sets strength
    • Aerospace, motorsport and diesel-truck primary markets

    Aerospace specifications we produce

    12-point bolt geometry is governed by several aerospace and military standards. TorqBolt produces bolts to all of the following, with full UNJ-3A external thread per SAE AS8879 (controlled-root-radius) and acceptance per ANSI Z1.4:

    SpecificationTensile (room)Common alloysTypical use
    MS21250160 ksiA286, AISI 8740, Inconel 718Mil-spec aerospace structural
    NAS1351160 ksiA286Close-tolerance 12-point external-wrenching
    AS3216180 ksiA286, Inconel 718, WaspaloySuperalloy 12-point for turbine section
    NAS6303 to NAS6320160 ksiA286NAS 6300-series tension hex / 12-point
    MIL-DTL-7838variesalloy steelLegacy mil-spec, replaced by NAS series
    DIN 12-point10.9, 12.9 classalloy steelEuropean metric 12-point
    ISO 14579(internal hexalobular)variesDisambiguation: NOT external 12-point

    Materials

    12-point bolt geometry is material-agnostic. Alloy choice is set by the spec, the service temperature, the corrosion environment, and the strength target.

    AlloyTensile (room)Service envelope
    A286 (UNS S66286)160 ksiup to 700 °C, oxidation resistant
    Inconel 718 (UNS N07718)180 ksiup to 700 °C, jet-engine service
    Ti-6Al-4V (Grade 5)130 ksiup to 350 °C, 56% lighter than steel
    MP35N (UNS R30035)260 ksicobalt-nickel, sour service
    Custom 465220 ksi (H950)precipitation-hardening stainless
    AISI 8740 chromolyup to 220 ksi (heat-treated)standard race + general high-strength
    AISI 4340 alloy steelup to 220 ksigeneral high-strength fastener stock
    17-4 PH stainless180 ksi (H1025)corrosion-resistant precipitation-hardening
    L19 race-grade270 ksitop-end race connecting rod bolts
    H-11 tool steel240 ksiextreme-duty fastening

    Industries served

    Browse the 12-point catalog

    By head geometry

    By aerospace spec

    Tool reference

    UNJ-3A External Thread Dimensions (Common 12-Point Bolt Sizes)

    Per SAE AS8879 (controlled-root-radius UNJ external thread). 12-point aerospace bolts under MS21250, NAS1351, AS3216, NAS6303 to NAS6320 are produced to UNJ-3A external thread tolerances. Root radius 0.150 P to 0.180 P (where P = pitch) for fatigue resistance, this is the difference vs standard UN-3A and the reason aerospace specifies UNJ.

    NominalTPIPitch P (mm)Major Dia Min (mm)Major Dia Max (mm)Pitch Dia Min (mm)Pitch Dia Max (mm)Minor Dia Min (mm)Minor Dia Max (mm)
    1/4-28 UNJF-3A280.9076.2976.3505.7065.7705.1565.296
    5/16-24 UNJF-3A241.0587.8757.9387.1857.2496.5536.711
    3/8-24 UNJF-3A241.0589.4629.5258.7738.8408.1398.298
    7/16-20 UNJF-3A201.27011.05111.11310.22710.2979.4679.652
    1/2-20 UNJF-3A201.27012.63812.70011.81511.88711.05111.243
    9/16-18 UNJF-3A181.41114.22514.28813.31013.38512.46612.673
    5/8-18 UNJF-3A181.41115.81215.87514.89714.97314.05414.262
    3/4-16 UNJF-3A161.58818.99119.05017.96118.04017.01817.246
    7/8-14 UNJF-3A141.81422.16622.22521.00121.08519.92620.181
    1-12 UNJF-3A122.11725.33625.40023.97424.06422.71722.985
    1 1/4-12 UNJF-3A122.11731.68731.75030.32830.41829.07229.343
    1 1/2-12 UNJF-3A122.11738.03738.10036.67836.76835.42235.693
    1 15/16-12 UNJF-3A122.11749.14949.21347.79147.88646.53546.812
    2-12 UNJF-3A122.11750.73750.80049.37849.47348.12248.399

    Internal thread (UNJ-3B, mating nut) tolerances and metric M-J equivalents available on request. Send drawing or spec callout to info@torqbolt.com.

    Request a Quote

    Send a drawing, spec, or BOM. Quotes are returned within 24 hours during the working week. Mill test reports (MTC 3.1 standard, MTC 3.2 critical) ship with every lot.

    Frequently asked questions

    What is a 12-point bolt?

    A 12-point bolt has a head with twelve external splines arranged every 30 degrees around the perimeter, also called a bi-hex or double-hex bolt. The 12 splines provide twice the tool engagement angles of a standard hex head, allowing higher applied torque for the same head size and reduced wrench-fit clearance, which is why aerospace and motorsport applications adopt the geometry.

    Which alloys does TorqBolt manufacture 12-point bolts in?

    TorqBolt produces 12-point bolts in A286 (UNS S66286), Inconel 718, Ti-6Al-4V, MP35N, Custom 465, AISI 8740 chromoly heat-treated, AISI 4340, 17-4 PH H1025 condition, H-11 tool steel, and L19 race-grade alloy steel. Alloy choice depends on tensile strength target, service temperature, corrosion environment, and the governing aerospace or motorsport spec.

    What is the difference between MS21250, NAS1351 and AS3216?

    MS21250 covers 160 ksi tensile 12-point external-wrenching aerospace bolts, commonly produced in A286 or AISI 8740. NAS1351 covers close-tolerance 12-point bolts under the National Aerospace Standard, used where shear-pin tolerance is required. AS3216 covers 180 ksi superalloy 12-point bolts, typically in A286 or Inconel 718, for turbine-section service.

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