•   0091-22-66157017
  •   info@torqbolt.com
    • Home

    12-Point Head Bolts

    12-Point Head Bolts, TorqBolt 12-point fastener with tB ligature and alloy grade marking

    12-point head bolts are high-strength fasteners used to clamp the engine cylinder head to the block, where the head is gasketed against combustion pressure (often 100-200 bar peak) and thermal cycling. The 12-point geometry allows the higher torque-to-yield needed to develop full preload without head cam-off.

    TorqBolt manufactures 12-point head bolts in AISI 8740 chromoly heat-treated to 220 ksi, AISI 4340 alloy steel, A286 (UNS S66286), MP35N (UNS R30035), and Custom 465 stainless. Each bolt head carries the struck tB ligature plus the alloy grade marking (8740, 4340, A286, MP35N).

    Engine Applications

    Head Bolt vs Head Stud

    A head bolt threads directly into the block as a single piece. A head stud installs as two pieces: the stud threads into the block first (lighter installation torque), then the head is dropped over the studs and a nut clamps the head down. For high-boost diesel and forced-induction race builds, head studs allow more accurate clamping load and easier head removal between rebuilds. See Head Studs.

    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.

    What to include in your RFQ

    A useful 12-point bolt RFQ packs the engineering data into one email so we can quote in 24 working hours instead of an iteration loop. Include: (1) drawing or spec callout (NAS / MS / AS / MIL / AMS / DIN number), (2) thread size (metric M-designation or inch UNJ designation), (3) length under-head, (4) alloy preference (or "per spec"), (5) lot quantity, (6) MTC level (3.1 standard or 3.2 critical-service witness inspection), (7) need-by date, (8) end-use industry (aerospace, motorsport, diesel, marine), and (9) any coating or surface treatment requirement.

    Drawings can be 2D PDF, 3D STEP, or photograph of an existing bolt with caliper-measured dimensions. We accept all formats. For repeat-buy programs we file the drawing in the customer master and the second order quotes off the on-file drawing.

    Lead time and mill test report

    Stock alloys (A286, AISI 8740 chromoly, AISI 4340, 17-4 PH, 316 stainless) ship in 4 to 6 weeks. Specialty alloys (Inconel 718, MP35N, Custom 465, Ti-6Al-4V, L19 race-grade) take 8 to 12 weeks depending on raw-bar availability. Critical-service orders requiring MTC 3.2 third-party witness inspection (Lloyd's, DNV, ABS, BV) add 1 to 2 weeks for the witness scheduling at the customer's preferred witness body.

    Mill test report MTC 3.1 is standard at no additional cost and covers chemistry, mechanical properties (tensile, yield, elongation, hardness), microstructure note, and heat-treatment cycle. MTC 3.2 adds the third-party witness signature on every test event from raw bar inspection through final lot acceptance, and is required by most aerospace prime customers and offshore oil-gas operators for tier-1 critical-service joints.

    Request a Quote

    RFQ to info@torqbolt.com, call +91-22-66157017, or WhatsApp +91-22-66157017.

    Questions? Contact us.
    Showcase of our Microsites.

    Satisfaction Survey

    Customer satisfaction survey

    TÜV Rheinland certified

    DIN Mitglied (DIN member)

    © TorqBolt Private Limited · Built with in India.