Thread form and thread class are two separate specifications that are both encoded in an aerospace fastener part number. The thread form (UN vs UNJ) determines the root geometry and fatigue resistance. The thread class (1A–3A for external, 1B–3B for internal) determines the dimensional tolerance. Getting either one wrong when selecting a replacement fastener is an engineering substitution — not a procurement shortcut. This guide explains both systems and when the distinction actually matters in practice.
The Two Thread Systems You Will Encounter
All aerospace hardware threads fall into one of two families:
- Unified National Thread (UN): Standard commercial thread form per ASME B1.1. The thread profile has a flat or slightly rounded root. This is the thread form on all standard AN hardware — AN3 through AN20, all MS equivalents.
- Unified National Thread with Modified Root (UNJ): Aerospace thread form per MIL-S-8879 (superseded by SAE AS8879). The defining characteristic is a controlled-radius root: r = 0.15P to 0.18P where P is the thread pitch. The larger root radius eliminates the stress concentration at the thread root that is the primary fatigue crack initiation site in cyclic-load applications. NAS close-tolerance bolts and most high-performance aerospace hardware use UNJF (UNJ fine pitch).
Fine pitch (UNF / UNJF) is used throughout AN and NAS bolt series because fine pitch provides more threads per inch of engagement, better resistance to self-loosening in thin materials, and higher tensile stress area for the same nominal diameter compared to coarse pitch.
Thread Form Comparison
| Thread Form | Root Profile | Fatigue Resistance | Applications | Interchangeable? |
|---|---|---|---|---|
| UNF (Unified Fine) | Flat root | Baseline | AN bolts, standard hardware | External UNF mates with internal UNF only |
| UNJF (UNJ Fine) | Rounded root (r = 0.15–0.18P) | +30% fatigue life | NAS bolts, high-cycle applications | External UNJF mates with UNJ or UNF internal |
| UNC (Unified Coarse) | Flat root | Baseline | Structural inserts, platenuts | Not interchangeable with UNF |
| UNJC (UNJ Coarse) | Rounded root | +30% fatigue life | Heavy structural inserts | External UNJC mates with UNC internal |
The interchangeability rule is critical: an external UNJF bolt will mate correctly with either an internal UNJ or an internal UNF nut, because the larger root radius of the UNJ external thread does not interfere with the standard internal thread crest. However, a UNF bolt will not correctly engage a UNJF nut — the flat root of the UNF bolt may not achieve full contact at the nut thread roots under preload. Do not substitute UNF bolts into UNJF nut assemblies.
Thread Classes — What 3A/3B Means
Thread class defines the dimensional tolerance applied to the thread form. There are three classes, each designated with a letter suffix: A for external (bolt) threads, B for internal (nut) threads.
- Class 1A / 1B: Loose fit, generous clearance, easy assembly in dirty or damaged conditions. Not used in aircraft structural fasteners.
- Class 2A / 2B: Standard commercial tolerance. The default for commercial-grade hardware not intended for precision applications. Fasteners from a hardware store are typically Class 2A/2B.
- Class 3A / 3B: Tight tolerance, zero allowance (no intentional clearance between pitch diameters at maximum material condition). All aerospace-grade AN, MS, and NAS bolts are Class 3A external, 3B internal. The tighter tolerance provides more consistent clamp load for a given torque value and reduces thread fatigue under vibration.
Never substitute Class 2A bolts in aircraft applications calling for Class 3A hardware. The looser tolerance changes the torque-clamp load relationship, reduces fatigue resistance, and introduces more variability into joint preload. "It threads in" is not the same as "it meets the design specification."
Thread Class Tolerances — UNF 1/4-28 Example
| Class | External (Bolt) Pitch Dia Range | Internal (Nut) Pitch Dia Range |
|---|---|---|
| 2A / 2B | 0.2127" – 0.2172" | 0.2218" – 0.2278" |
| 3A / 3B | 0.2140" – 0.2172" | 0.2218" – 0.2258" |
The difference between Class 2A and 3A at 1/4-28 is 0.0013" on the lower bound of the bolt pitch diameter — a tolerance tightening of just over 1 thousandth of an inch. That sounds small. Under vibration loading, the difference in joint stiffness and preload consistency is measurable and is why the specification exists.
AN Bolt Threads — The Standard Series
Every standard AN bolt uses UNF threads at Class 3A. The thread designation follows the bolt size directly:
- AN3: 10-32 UNF-3A (No. 10 diameter, 32 threads per inch, fine pitch, Class 3A)
- AN4: 1/4-28 UNF-3A (1/4" diameter, 28 TPI)
- AN5: 5/16-24 UNF-3A (5/16" diameter, 24 TPI)
- AN6: 3/8-24 UNF-3A (3/8" diameter, 24 TPI)
- AN7: 7/16-20 UNF-3A (7/16" diameter, 20 TPI)
- AN8: 1/2-20 UNF-3A (1/2" diameter, 20 TPI)
- AN10: 5/8-18 UNF-3A (5/8" diameter, 18 TPI)
- AN12: 3/4-16 UNF-3A (3/4" diameter, 16 TPI)
All AN nuts in the AN310/AN365/MS20365 series are 3B internal threads, matched to the 3A bolt series. Mixing a Class 2B commercial nut onto a Class 3A AN bolt is legal from an assembly standpoint — the 3A bolt fits in the 2B nut — but it changes the as-designed preload characteristics.
NAS Bolts and UNJF Threads
NAS1303 through NAS1320 close-tolerance bolts use UNJF threads per MIL-S-8879 (SAE AS8879). The rounded root radius (r = 0.15P to 0.18P) is the key differentiation. Testing has shown that UNJF threads in cyclic fatigue produce approximately 30% higher fatigue life than UNF threads of the same diameter and pitch, because the stress concentration at the thread root — the primary initiation site for fatigue cracks — is reduced by the radius.
- UNJF bolt mates correctly with UNF or UNJF internal threads. External UNJ is compatible with standard AN365 or MS20365 nuts that have UNF internal threads — this is the designed compatibility.
- Do not install UNF bolts into UNJF-specified nut assemblies: the flat root of the UNF bolt may leave insufficient contact at the nut thread root under preload, reducing the effective thread engagement area.
- High-cycle applications — control surface hinges, primary flight control attach fittings, engine mount bolts on high-vibration installations — specify UNJF by design. The fatigue improvement is the reason for the specification. Do not substitute UNF in these locations.
Lock Wire Holes and Thread Integrity
Two details about drilled bolts and thread runout that affect replacement selection:
- Drilled bolts (AN4H suffix, or NAS with "D" suffix) have a cross-drilled hole through the unthreaded shank portion. Verify the hole is located in the grip region, not in the thread runout zone. A hole in the thread runout reduces thread engagement area and can be a fatigue initiation site. Reputable aerospace fastener manufacturers locate the hole well clear of the thread runout region — but this is a quality issue to verify on unfamiliar surplus lots.
- Thread runout is the transition zone between full thread and the unthreaded shank. Nut engagement that extends into the runout zone reduces effective thread engagement and can cause thread stripping under load. Correct grip length selection prevents this — the nut should seat on full thread engagement, not in the runout. See our AN bolt grip length chart for grip selection guidance.