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- NACE MR0175/ISO 15156-2
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MIL-DTL-7838 legacy 12-point aerospace bolts
MIL-DTL-7838 is a legacy US Military Detail Specification for 12-point external-drive aerospace bolts. It was widely used through the 1960s and 1970s on military airframe structural fastening and engine attachment, and it has since been largely superseded by the NAS6303 through NAS6320 series and MS21250 for new designs. MIL-DTL-7838 callouts still appear in legacy aircraft maintenance documents, repair manuals, and overhaul kits for older military airframes such as the C-130, F-4, and KC-135 platforms, which is why TorqBolt continues to produce the spec for sustainment and depot-level repair programmes.
TorqBolt manufactures MIL-DTL-7838 bolts in A286 (UNS S66286), AISI 8740 chromoly heat-treated, AISI 4340, and 17-4 PH H1025 to AS9100D Rev D and ISO 9001:2015 quality systems. Each bolt head carries the struck tB manufacturer ligature plus the alloy grade marking, and every lot ships with a mill test report (MTC 3.1 standard, MTC 3.2 critical with third-party witness inspection of chemistry, hardness, tensile and dimensional records). For new-design programmes, the equivalent active spec is MS21250 (160 ksi) or AS3216 (180 ksi superalloy).
Common Alloys for MIL-DTL-7838
| Alloy | Tensile | Service envelope |
|---|---|---|
| A286 (UNS S66286) | 160 ksi | up to 700 °C |
| Inconel 718 (UNS N07718) | 180 ksi | up to 700 °C, jet-engine |
| AISI 8740 chromoly | up to 220 ksi (HT) | standard high-strength |
| Ti-6Al-4V (Grade 5) | 130 ksi | 56% lighter than steel |
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.
| Nominal | TPI | Pitch 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-3A | 28 | 0.907 | 6.297 | 6.350 | 5.706 | 5.770 | 5.156 | 5.296 |
| 5/16-24 UNJF-3A | 24 | 1.058 | 7.875 | 7.938 | 7.185 | 7.249 | 6.553 | 6.711 |
| 3/8-24 UNJF-3A | 24 | 1.058 | 9.462 | 9.525 | 8.773 | 8.840 | 8.139 | 8.298 |
| 7/16-20 UNJF-3A | 20 | 1.270 | 11.051 | 11.113 | 10.227 | 10.297 | 9.467 | 9.652 |
| 1/2-20 UNJF-3A | 20 | 1.270 | 12.638 | 12.700 | 11.815 | 11.887 | 11.051 | 11.243 |
| 9/16-18 UNJF-3A | 18 | 1.411 | 14.225 | 14.288 | 13.310 | 13.385 | 12.466 | 12.673 |
| 5/8-18 UNJF-3A | 18 | 1.411 | 15.812 | 15.875 | 14.897 | 14.973 | 14.054 | 14.262 |
| 3/4-16 UNJF-3A | 16 | 1.588 | 18.991 | 19.050 | 17.961 | 18.040 | 17.018 | 17.246 |
| 7/8-14 UNJF-3A | 14 | 1.814 | 22.166 | 22.225 | 21.001 | 21.085 | 19.926 | 20.181 |
| 1-12 UNJF-3A | 12 | 2.117 | 25.336 | 25.400 | 23.974 | 24.064 | 22.717 | 22.985 |
| 1 1/4-12 UNJF-3A | 12 | 2.117 | 31.687 | 31.750 | 30.328 | 30.418 | 29.072 | 29.343 |
| 1 1/2-12 UNJF-3A | 12 | 2.117 | 38.037 | 38.100 | 36.678 | 36.768 | 35.422 | 35.693 |
| 1 15/16-12 UNJF-3A | 12 | 2.117 | 49.149 | 49.213 | 47.791 | 47.886 | 46.535 | 46.812 |
| 2-12 UNJF-3A | 12 | 2.117 | 50.737 | 50.800 | 49.378 | 49.473 | 48.122 | 48.399 |
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 drawing or PO line referencing MIL-DTL-7838 to info@torqbolt.com or call +91-22-66157017.
Related aerospace specs
For broader 12-point aerospace specification context, see also the parent aerospace fasteners hub and the full 12-point catalog, plus sibling specs:
- NAS1351, close-tolerance 12-point external-wrenching
- MS21250, 160 ksi 12-point external-drive aerospace bolt
- AS3216, 180 ksi superalloy 12-point for turbine section
- NAS6303, 3/16" body NAS 6300-series tension cap-screw
- NAS6304, 1/4" body NAS 6300-series tension cap-screw
- NAS6305, 5/16" body NAS 6300-series tension cap-screw
- NAS6308, 1/2" body NAS 6300-series tension cap-screw
- NAS6310, 5/8" body NAS 6300-series tension cap-screw
- NAS6312, 3/4" body NAS 6300-series tension cap-screw
- NAS6315, 15/16" body NAS 6300-series tension cap-screw
- NAS6320, 1-1/4" body NAS 6300-series tension cap-screw
- DIN 12-Point, European metric 12-point M5 to M30, grade 12.9
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