MS 21250 A286 12-Point External-Wrenching Bolts
MS 21250 is the US military specification for high-strength 12-point external-wrenching bolts used in commercial and military aerospace applications where the bolted joint demands both high preload (up to 220 ksi tensile in the alloy-steel variant) and corrosion resistance (the A286 variant for hot-section and chloride-exposed joints). The 12-point external head allows a thin-wall wrench to engage where a hex head cannot fit, making MS 21250 the standard fastener for engine casing flange joints, turbine hot-section attachments, and tightly-packaged accessory bolting. The A286 material variant per AMS 5737 ships in the precipitation-aged condition with 140 ksi tensile and serviceable temperature to 700 degree C, alongside the alternative H-11 tool-steel variant per AMS 6487 used where corrosion is not a concern and 220 ksi tensile is needed.
MS 21250 Dimensional Reference (per ASME B18.2.1 12-point head)
| Thread Size | Across Flats (12-point) | Head Height | Body Diameter |
|---|---|---|---|
| 1/4-28 UNJF | 0.375 in (9.53 mm) | 0.156 in (3.96 mm) | 0.250 in (6.35 mm) |
| 5/16-24 UNJF | 0.500 in (12.70 mm) | 0.196 in (4.98 mm) | 0.312 in (7.92 mm) |
| 3/8-24 UNJF | 0.562 in (14.27 mm) | 0.234 in (5.94 mm) | 0.375 in (9.53 mm) |
| 7/16-20 UNJF | 0.625 in (15.88 mm) | 0.273 in (6.93 mm) | 0.438 in (11.13 mm) |
| 1/2-20 UNJF | 0.750 in (19.05 mm) | 0.312 in (7.92 mm) | 0.500 in (12.70 mm) |
| 9/16-18 UNJF | 0.844 in (21.43 mm) | 0.352 in (8.94 mm) | 0.562 in (14.27 mm) |
| 5/8-18 UNJF | 0.938 in (23.83 mm) | 0.390 in (9.91 mm) | 0.625 in (15.88 mm) |
| 3/4-16 UNJF | 1.125 in (28.58 mm) | 0.468 in (11.89 mm) | 0.750 in (19.05 mm) |
12-point external head dimensions per ASME B18.2.1 series, body and thread per MS 21250 spec.
MS 21250 Material Options
| Material Variant | Spec | Tensile Min | Yield Min | Max Service Temp | Corrosion Resistance |
|---|---|---|---|---|---|
| A286 (UNS S66286) | AMS 5737 | 140 ksi (965 MPa) | 105 ksi (725 MPa) | 700 degree C | Excellent (304 SS equivalent) |
| H-11 Tool Steel | AMS 6487 | 220 ksi (1517 MPa) | 180 ksi (1241 MPa) | 540 degree C | Poor (requires Cd plating) |
A286 is the corrosion-resistant variant. H-11 is the high-strength variant. Choice between the two is driven by service environment and required ultimate strength.
When to Specify the A286 Variant of MS 21250
| Service Condition | A286 Suitable? | Why |
|---|---|---|
| Hot-section turbine bolting (550-700 C) | Yes | Strength retention through service temp |
| Marine / chloride-exposed | Yes | No surface coating required |
| Cold-section airframe (under 230 C) | Marginal | H-11 cheaper and stronger if no corrosion concern |
| Cryogenic (under -100 C) | Yes | Austenitic ductility retained |
| Repeated assembly | Yes | Surface does not degrade on disassembly |
| Hydrogen environment | Yes | Not subject to HSC like ferritic alloy steel |
Specify A286 MS 21250 wherever service temperature exceeds 230 degree C (cadmium plating limit on H-11), chloride exposure is anticipated, or repeated assembly cycles will degrade an alloy-steel fastener.
Typical Applications
MS 21250 A286 12-point bolts are specified on engine casing flange joints (where the 12-point head fits in the narrow casing channel that hex heads cannot reach), turbine combustor liner mounting flanges, hot-section gas-turbine bearing support bolts, exhaust nozzle attach hardware, and bleed-air valve high-temperature joints. The A286 material variant is the standard wherever the joint will see service above 230 degree C, where cadmium-plated H-11 would flash and fail. Naval aviation specifies the A286 variant universally because the alloy-steel variant requires chromate-passivated cadmium plating that is not compatible with shipboard environmental rules.
Frequently Asked Questions
What is the difference between MS 21250 A286 and MS 21250 H-11?
MS 21250 is the same dimensional spec for both materials; only the bar-stock and heat-treatment requirements change. The A286 variant per AMS 5737 delivers 140 ksi tensile with serviceable temperature to 700 degree C and corrosion resistance equivalent to 304 stainless. The H-11 tool-steel variant per AMS 6487 delivers 220 ksi tensile (significantly higher) but limited to 540 degree C service and requiring cadmium plating per QQ-P-416 for corrosion resistance. Choice is driven by required ultimate strength and the service environment.
Why use 12-point external heads instead of hex?
The 12-point head provides 12 wrench-engagement positions in 360 degrees instead of the 6 positions of a hex head, allowing a thin-wall wrench to engage at every 30 degrees of rotation. This matters in turbine engine assemblies where the casing geometry leaves only a narrow channel between bolts. A hex head requires 60 degrees of clear space to reposition the wrench; a 12-point head needs only 30. The 12-point also distributes wrenching force across more head metal, reducing local yielding under high torque.
What torque is specified for MS 21250 A286 bolts?
MS 21250 A286 torque is sized to achieve 75 percent of the bolt yield strength as joint preload. Per ASME PCC-1 Appendix A for A286 dry-thread torque: 1/4-28 UNJF = 10 ft-lb, 3/8-24 UNJF = 30 ft-lb, 1/2-20 UNJF = 75 ft-lb, 5/8-18 UNJF = 150 ft-lb, 3/4-16 UNJF = 270 ft-lb. Apply 70 percent of these values with anti-seize compound, since the reduced friction means the same preload is achieved at lower input torque. Engine assembly drawings typically specify a torque-then-angle protocol rather than a single torque value, which more reliably achieves the target preload.
Can MS 21250 A286 bolts be used in nuclear applications?
Yes, with the understanding that ASTM A453 Grade 660 (the bolting form of A286) is the more common nuclear material callout. MS 21250 A286 ships per AMS 5737 which is functionally equivalent to ASTM A453 Class D for the chemistry and mechanical properties, but the documentation chain is aerospace (AMS) rather than ASTM. For ASME Section III nuclear pressure-boundary applications, ASTM A453 Grade 660 Class B or D ordered to A453 (with stress-rupture per A453 Table 6) is the standard callout, not MS 21250.
Related A286 Products and Specifications
Cross-reference within the A286 high-strength fastener family: NAS 1352 A286 socket cap screws, NAS 1351 fine-thread variant, A286 aerospace fasteners, A286 gas turbine applications, AMS 5737 precision bar (the feedstock), ASTM A453 Grade 660 (pressure-vessel equivalent).
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