Specifications
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175/ISO 15156-2
- NORSOK M-650
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
12-Point Bolt Head Dimensions
Reference dimensions for 12-point external-drive bolt heads. Three primary measurements:
- Across-flats (AF): distance between two parallel flats on opposite splines. Sets the socket size required.
- Across-corners (AC): distance between two opposite corner points (between splines). Larger than AF by ~15% for a 12-point head.
- Head height (H): distance from under-head bearing face to top of head. 12-point heads are typically lower-profile than equivalent hex heads.
- Drive depth: depth of socket engagement on the head. Most 12-point socket designs engage at least 60% of the head height for full torque capacity.
12-Point vs Hex Head Comparison
For a given thread size, the 12-point head has a smaller across-flats than the equivalent hex head, allowing the bolt to sit closer to neighbouring features. The trade-off is socket-fit precision: a 12-point head requires a 12-point socket; a 6-point socket fits but engages only half the splines.
Per-Spec Dimensions
Detailed dimensions are specified per the governing aerospace or industrial standard:
- MS21250 mil-spec 160 ksi 12-point bolt
- NAS1351 close-tolerance 12-point
- AS3216 superalloy 180 ksi 12-point
- NAS6303 through NAS6320 tension series
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.
Common Confusion Patterns and How to Resolve Them
Three confusion patterns dominate the 12-point bolt search space and are worth disambiguating up front:
- External 12-point vs internal hexalobular (ISO 14579): both involve twelve splines. External 12-point has 12 splines on the OUTSIDE of the bolt head, driven by a 12-point socket wrench. ISO 14579 (often confused) has 6 lobes (12 contact points) RECESSED INSIDE the cap-screw head, driven by a Torx-style internal driver. Different tool, different market.
- 12-point socket vs 12-point bolt: searchers often type "12-point socket" looking for the wrench tool that drives the bolt. We make the bolt. For the tool, retail brands like Snap-on, Craftsman, GearWrench, and Sunex carry 12-point socket sets in standard metric and imperial sizes.
- 12-point vs hex (6-point): same bolt body, different head geometry. The 12-point head has 12 external splines around the perimeter. The hex head has 6 flats. Mechanical performance under tension is identical (set by alloy + heat treatment); the difference is wrench-engagement quality and preload-control accuracy. Hex is cheaper and more universal; 12-point is preferred for aerospace and high-end motorsport where preload accuracy matters.
How to order to this spec
Send the spec callout from your drawing exactly as it appears: full series + dash-number + alloy code (where applicable). For example: NAS6304-12-A286 reads as series NAS6304, grip 0.75 inch (12 sixteenths), A286 alloy. We file the spec at order placement and ship to the active revision unless the drawing locks a specific revision.
Acceptance test procedures (ATP) are issued as a one-page summary with each lot: tensile test result, hardness sweep, magnetic-particle or fluorescent-penetrant inspection result, dimensional inspection (across-flats, across-corners, head-height, thread-pitch, drive-depth), and lot-acceptance review per ANSI Z1.4 (or AQL specified by buyer). Mill test report is MTC 3.1 standard or MTC 3.2 with third-party witness inspector at customer request.
Alloy options for this spec
Most aerospace 12-point specs accept multiple alloys depending on the dash-suffix or alloy code. The common alloy menu: A286 (UNS S66286, AMS 5731 / 5732 / 5737, the workhorse for 700 deg C service), AISI 8740 chromoly heat-treated to 160 ksi (cost-effective alloy steel for room-temperature structural service), Inconel 718 (UNS N07718, jet engine and turbine section to 700 deg C), Ti-6Al-4V Grade 5 (where mass matters more than tensile), MP35N (UNS R30035, sour-service and 220+ ksi tensile applications). Specify in the RFQ; the right alloy is set by service temperature, corrosion environment, and tensile target, not by the spec number alone.
Frequently asked questions
What is a 12-point bolt?
A 12-point bolt has 12 splines arranged every 30 degrees on the OUTSIDE of the bolt head, also called a bi-hex or double-hex bolt. The 12 external splines provide twice the wrench-engagement angles of a standard hex head, allowing higher applied torque before cam-off, which is why aerospace and motorsport applications adopt the geometry.
Are 12-point bolts the same as ISO 14579?
No. ISO 14579 specifies an INTERNAL hexalobular socket head (similar to Torx) recessed inside a cap screw. External 12-point bolts under MS21250 / NAS1351 / AS3216 use an EXTERNAL bi-hex head driven by a 12-point socket wrench. Both involve 12 features but on different sides of the bolt head and require different drivers.
Which alloys does TorqBolt manufacture 12-point bolts in?
TorqBolt produces 12-point bolts in A286 (UNS S66286), Inconel 718 (UNS N07718), Ti-6Al-4V Grade 5, MP35N (UNS R30035), Custom 465 H1025, AISI 8740 chromoly heat-treated, AISI 4340, 17-4 PH, H-11 tool steel, and L19 race-grade alloy steel. Choice depends on tensile target, service temperature, and corrosion environment.
Is TorqBolt AS9100D certified?
Yes. TorqBolt operates under AS9100D Rev D and ISO 9001:2015 quality management systems, audited annually by the registrar. NORSOK M-630 and NACE MR0175 / ISO 15156 qualifications cover offshore and sour-service applications. Mill test reports MTC 3.1 standard and MTC 3.2 critical-service witness are issued with every lot.
What lead time should I expect?
Stock alloys (A286, AISI 8740, AISI 4340, 17-4 PH, 316 stainless) ship in 4 to 6 weeks. Specialty alloys (Inconel 718, MP35N, Custom 465, Ti-6Al-4V, L19) ship in 8 to 12 weeks depending on raw-bar lead time at the alloy producer. Critical-service orders requiring third-party witness inspection (MTC 3.2) add 1 to 2 weeks for witness scheduling.
Request a Quote
For specific drawing-callout dimensions, send the spec reference to info@torqbolt.com.
Satisfaction Survey
