NAS 1351 A286 Fine-Thread Socket Cap Screws (NASM 1351)
NAS 1351 is the National Aerospace Standard (now maintained as NASM 1351 / SAE-AS 1351) for fine-thread UNJF internally-wrenching socket-head cap screws cold-headed from A286 (UNS S66286). The fine UNJF thread series is specified wherever the bolted joint is repeatedly disassembled (maintenance access panels, accessory gearbox covers, instrument housings) and wherever precise preload control matters more than fastener cost. Diameter range 8-36 UNJF through 1-12 UNJF, with the J-profile root radius improving fatigue life over standard UN thread geometry. Specified on every aerospace assembly where torque-critical preload retention over thermal cycle drives the joint design.
Dimensional Reference (per NASM 1351 head profile, UNJF fine thread)
| Thread Size | Head Diameter (max) | Head Height (max) | Hex Socket | Body Diameter |
|---|---|---|---|---|
| 8-36 UNJF | 0.270 in (6.86 mm) | 0.164 in (4.17 mm) | 9/64 in | 0.164 in (4.17 mm) |
| 10-32 UNJF | 0.312 in (7.92 mm) | 0.190 in (4.83 mm) | 5/32 in | 0.190 in (4.83 mm) |
| 1/4-28 UNJF | 0.375 in (9.53 mm) | 0.250 in (6.35 mm) | 3/16 in | 0.250 in (6.35 mm) |
| 5/16-24 UNJF | 0.469 in (11.91 mm) | 0.312 in (7.92 mm) | 1/4 in | 0.312 in (7.92 mm) |
| 3/8-24 UNJF | 0.562 in (14.27 mm) | 0.375 in (9.53 mm) | 5/16 in | 0.375 in (9.53 mm) |
| 7/16-20 UNJF | 0.656 in (16.66 mm) | 0.438 in (11.13 mm) | 3/8 in | 0.438 in (11.13 mm) |
| 1/2-20 UNJF | 0.750 in (19.05 mm) | 0.500 in (12.70 mm) | 3/8 in | 0.500 in (12.70 mm) |
| 5/8-18 UNJF | 0.938 in (23.83 mm) | 0.625 in (15.88 mm) | 1/2 in | 0.625 in (15.88 mm) |
| 3/4-16 UNJF | 1.125 in (28.58 mm) | 0.750 in (19.05 mm) | 5/8 in | 0.750 in (19.05 mm) |
| 1-12 UNJF | 1.500 in (38.10 mm) | 1.000 in (25.40 mm) | 3/4 in | 1.000 in (25.40 mm) |
Dimensions per NASM 1351 head profile. UNJF thread per MIL-S-8879 specifies J-profile root radius for improved fatigue strength.
Why UNJF Fine Thread for Aerospace Joints
| Joint Characteristic | UNJF Fine (NAS 1351) | UNC Coarse (NAS 1352) |
|---|---|---|
| Preload Precision | Better (smaller torque change per turn) | Lower (coarser pitch) |
| Thread Engagement | More threads engaged per length | Fewer threads engaged |
| Fatigue Strength | Higher (J-profile root radius) | Lower (standard UN root) |
| Self-Loosening Resistance | Higher (finer helix angle) | Lower |
| Suitable for Repeated Assembly | Yes (no thread damage on disassembly) | Moderate |
| Suitable for Thin-Wall Components | Yes (more threads in same length) | Marginal |
Fine-thread UNJF per NAS 1351 is preferred over coarse-thread NAS 1352 wherever the joint is torque-critical, repeatedly assembled, or made into thin-wall A286 brackets.
Material Spec and Heat Treatment
| Property | NAS 1351 Material (A286 per AMS 5737) | Source |
|---|---|---|
| UNS Designation | S66286 | ASTM A453 |
| Bar Stock Spec | AMS 5737 (precision cold-heading bar) | SAE Aerospace |
| Tensile Strength (min) | 140 ksi (965 MPa) | ASTM A453 Class D |
| Yield Strength 0.2% (min) | 105 ksi (725 MPa) | ASTM A453 Class D |
| Service Temperature (max) | 700 degree C (1300 degree F) | Below eta-phase formation |
Same A286 / AMS 5737 material as NAS 1352, with the UNJF fine thread distinguishing it on the assembly drawing.
Typical Applications
NAS 1351 fine-thread screws are specified for aerospace torque-critical joints: gas-turbine compressor stator vane attachment screws, combustor liner mounting screws, accessory gearbox cover bolts (CFM56 / V2500 / GE9X), bleed-air valve mounting hardware, instrument-cluster screws on military aircraft, and any flange-bolted joint where the joint is opened and closed at every C-check or D-check inspection. The fine thread protects the joint against thread damage from repeated wrenching that would degrade a coarse-thread NAS 1352 over the service life.
Frequently Asked Questions
What does the J in UNJF stand for?
The J in UNJF designates a controlled-radius thread root profile per MIL-S-8879 (now SAE-AS 8879). The J-profile root has a larger radius than the standard UN thread root, eliminating the sharp stress-concentration corner at the thread root and dramatically improving fatigue life under cyclic loading. The 95 percent thread height carrying-load engagement remains the same as standard UN thread; only the unloaded portion of the root profile changes.
Is NAS 1351 interchangeable with NAS 1352?
Dimensionally yes (the head and overall envelope match), but functionally no. NAS 1351 uses UNJF fine thread (e.g. 1/4-28) and NAS 1352 uses UNC coarse thread (e.g. 1/4-20). You cannot substitute one for the other in an existing tapped hole because the thread pitch does not match. The decision between coarse and fine is made at design time based on preload precision, thermal cycling, and disassembly frequency requirements.
Why is A286 the standard material for NAS 1351?
A286 is the standard NAS 1351 material because it combines three properties that are otherwise mutually exclusive in commercial fastener alloys: 130 ksi minimum tensile strength after age hardening, corrosion resistance equivalent to 304 austenitic stainless without surface coatings, and serviceable strength to 700 degree C (1300 degree F). Alloy steel like 4140 or H-11 offers higher strength but requires cadmium plating that flashes above 230 degree C. Inconel 718 offers higher temperature capability but is over-specified (and over-cost) for this stress range. A286 sits in the engineering sweet spot.
What aerospace platforms use NAS 1351 fasteners?
Every commercial-aerospace and military-aerospace turbine-engine assembly drawing built since the 1970s references NAS 1351 for fine-thread socket cap screw applications. Platforms include CFM56 (737, A320 narrowbody fleet), V2500 (A320), GE90 (777), GEnx (787, 747-8), GE9X (777X), Trent 1000 (787), Trent XWB (A350), LEAP-1A and 1B (A320neo, 737 MAX), F135 (F-35), F119 (F-22), F100 (F-15, F-16), TF34 (A-10), T700 (Black Hawk). Industrial gas-turbine derivatives (LM2500, LM6000) also reference NAS 1351 on the airframe-derived hardware they retained.
Related A286 Products and Specifications
Cross-reference within the A286 fastener family: NAS 1352 A286 coarse-thread bolts, A286 NAS-series bolts hub, A286 socket head cap screws, A286 aerospace fasteners, A286 jet engine fasteners, AMS 5737 precision bar, ASTM A453 Grade 660.
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