Aerospace Nuts: Self-Locking, Castellated and Anchor Nuts
Aerospace nuts are ordered by standard part number. Self-locking nuts are procured to NASM25027, which covers 250, 450 and 800 °F nuts, and the familiar MS21042, MS21043 and MS21044 sheets now live on as NASM numbers. Castellated AN310 and plain AN315 nuts need a cotter pin or other locking device.
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Nut types and where they are used
The first choice is how the nut stays put: a locking element built into the nut, or a plain nut secured by a cotter pin, safety wire or check nut. FAA AC 43.13-1B describes two kinds of self-locking nut in service, the all-metal type and the fiber or nylon insert type. The table uses the titles in AC 43.13-1B Table 7-13 and the cited standards.
| Part family | Description | Where it is used |
|---|---|---|
| MS21042 (NASM21042) | Self-locking, 450 °F, reduced hexagon, reduced height, ring base, non-corrosion resistant steel | General structure where space and weight are tight |
| MS21043 (NASM21043) | Self-locking, 800 °F, reduced hexagon, reduced height, ring base, corrosion resistant steel | High temperature locations up to the 800 °F rating |
| AN365, MS21044 (NASM21044) | Self-locking, 250 °F, fiber or nylon insert; distributors cross-reference AN365 numbers to MS21044 | Airframe joints below the insert's temperature limit |
| MS17829, MS17830 | Self-locking, nylon insert, 250 °F, regular height, steel and corrosion resistant steel | General use below 250 °F |
| NAS1291 | Hexagonal self-locking nut, low height | Tight clearances |
| AN310 (NASM310) | Plain castellated nut | Drilled shank bolts, clevis bolts and studs in tension, with cotter pin or safety wire |
| AN320 | Castellated shear nut | Hardware loaded in shear only |
| AN315 | Plain hex nut | Large tension loads with a check nut or safety wire; limited use on structure |
| MS21047, MS21059, NAS1068 | Anchor (plate) nuts: two lug fixed low height; two lug floating low height; miniature two lug floating | Riveted to structure behind panels and blind locations |
| NAS1034 through NAS1041 | Self-locking gang channel nut assemblies | Rows of panel fasteners with few rivets |
The specifications that govern nuts
NASM25027 is the procurement specification for self-locking nuts, and the MS and AN sheets define individual part geometries. Most of the MS sheets were cancelled in the 1990s and superseded by NASM sheets that kept the same part numbers.
| Spec | What it covers | Status as verified |
|---|---|---|
| NASM25027 | Nut, self-locking, 250 °F, 450 °F and 800 °F | Active; adopted for DoD use 14 December 1999; supersedes MIL-DTL-25027 Rev. H Amendment 1 |
| MIL-DTL-25027 | Earlier self-locking nut specification | Superseded by NASM25027; it had itself superseded MIL-N-25027 |
| MS21042 | 450 °F reduced hex, reduced height nut | Rev. M (6 January 1995) cancelled, superseded by NASM21042; part numbers identical |
| MS21043 | 800 °F reduced hex, reduced height CRES nut | Cancelled, superseded by NASM21043 |
| MS21044 | Self-locking nut, nylon insert in distributor listings | Rev. H (21 September 1993) cancelled, superseded by NASM21044; part numbers identical |
| AN310 | Nut, plain, castellated, airframe | Made inactive for new design, later cancelled and superseded by NASM310; part numbers identical |
| MS21047 | Two lug anchor nut | A published hardware guide names NASM21047 as the current spec, still using MS21047 part numbers; confirm on ASSIST |
AN365 and MS20365 have a tangled history: distributor cross-references point AN365 numbers to MS21044, while EverySpec records MS20365 revisions as superseded by MS21042. Order the exact number on the drawing or parts list, and get engineering approval before substituting across families, because the temperature rating and locking element differ.
Have a part like this?Send it in under a minute. A person replies within 1 business day.How the part number is built
Anchor nuts are a good example because one letter changes the hardware. A published hardware guide decodes MS21047L3 this way:
- MS21047: self-locking, two lug, low height, fixed anchor nut.
- L: dry film lubricated, the most common version. A plain dash in that position means not lubricated.
- K (when present): a version for dimpled or countersunk rivet holes.
- 3: the thread code, here 10-32. The guide lists the common range as 6-32 through 3/8-24.
The same guide notes that K1000 is the Kaynar number for a similar anchor nut, which is the kind of cross-reference a buyer should treat as an engineering question, not a purchasing decision. For hex locknuts, the useful rule is that the NASM part number is the old MS number: an MS21042 nut ordered today is identified the same way, with the NASM sheet as the controlling document. Distributor data also shows AN365-820A cross-referenced to MS21044N8, but confirm any cross-reference against the drawing before you buy.
Temperature limits and finishes
Temperature is the main reason a nut gets rejected in design review. NASM25027 is organized around three ratings, 250, 450 and 800 °F, and the locking element and finish set which one a nut meets.
- Fiber or nylon insert nuts: AC 43.13-1B says they are not installed where temperatures exceed 250 °F. The insert is unthreaded and smaller than the bolt, which produces the locking friction.
- All-metal nuts: the locking section is out of round or slotted and pinched in, and the locking action depends on the resilience of the metal. The AC says metal locknuts are primarily used in high temperature areas.
- Cadmium plating: limited to 450 °F. Above that, the AC warns that cadmium diffuses into the base metal and makes it brittle.
- Silver plating: used on locknuts for higher temperatures; the AC credits it with resistance to 1,400 °F and good anti-galling behavior. The MS21043 800 °F nut is a silver plated corrosion resistant steel nut in distributor listings.
- Molybdenum disulfide dry film: applied over cadmium or silver to prevent seizing and galling, which is what the L in many anchor nut numbers calls for.
Material choice also brings in galvanic and specialty metal questions. Steel and corrosion resistant steel nuts on DoD hardware fall under DFARS specialty metals rules; see DFARS specialty metals. Plating detail is on the surface finishing page.
Where self-locking nuts may not be used, and reuse rules
AC 43.13-1B paragraph 7-64 sets the limits most maintenance and design manuals repeat:
- Do not use self-locking nuts on parts subject to rotation. Self-locking castellated nuts with cotter pins or lockwire may be used in any system.
- Do not use them on turbine powered aircraft where a loose nut, bolt, washer or screw could fall or be drawn into the engine intake.
- Do not use them to attach access panels or doors, or on parts routinely removed before or after each flight. They may be used with anti-friction bearings and control pulleys if the bearing inner race is clamped to the structure by the nut and bolt.
- After tightening, at least one thread of the bolt or stud should show past the nut.
On reuse, the AC says a fiber or nylon locknut must not be reused if it cannot meet the minimum prevailing torque in its Table 7-2, for example 8 inch-pounds for 7/16-20 and 10 inch-pounds for 1/2-20. The table starts at 7/16 in, so smaller nuts need values from the manufacturer or the maintenance manual. NTSB Safety Alert SA-028 adds that locknuts wear with each use, that a nut which can be turned on the thread by hand should be discarded, and that binding in the assembly can give a false reading of locking torque.
Plain and castellated nuts need their own locking: AC 43.13-1B says to lock or safety every bolt or nut except self-locking nuts and never to reuse cotter pins or safety wire.
Anchor, floating and barrel nuts
Anchor nuts, also called plate nuts, are riveted to structure so a panel or bracket can be bolted from one side. Fixed versions such as MS21047 hold the thread in one place. Floating versions such as MS21059 and the miniature NAS1068 let the threaded element move slightly, which absorbs small hole misalignment between the panel and the structure. Boeing Distribution describes the MS21059L4 as a two lug floating anchor nut in heat treated, cadmium plated steel with dry film lubricant. Gang channel nuts carry many floating nuts on a strip attached with only a few rivets, per AC 43.13-1B paragraph 7-64g.
Barrel nuts and other special nuts are usually drawing-controlled or proprietary part numbers rather than common MS sheets. For those, the right purchase is the exact manufacturer part number or source controlled drawing, with the cert requirements written into the PO.
Ordering checklist
- Full part number with thread code and every letter (lubricant, countersunk rivet holes, material or finish codes), plus the controlling standard, for example NASM21042 for MS21042 numbers.
- Procurement spec where one applies: NASM25027 for self-locking nuts, with the revision your program accepts.
- Temperature rating and finish, stated rather than assumed, especially if an alternate is offered.
- Manufacturer and lot traceability, certificate of conformance and any test reports the program requires.
- DFARS 252.225-7009 flow down on DoD work for steel and corrosion resistant steel nuts.
- Packaging by lot, with shelf life noted for nylon insert nuts if your program controls it.
Counterfeit and quality risks, and how to receive nuts
The common failures are commercial nylon insert nuts sold as aerospace parts, wrong temperature class, missing dry film lubricant, mixed lots and plating that does not match the part number. Receive nuts by checking the full part number on the bag against the PO, matching lot numbers to the CoC and the manufacturer's paperwork, looking for the correct locking feature and finish, and checking a sample for thread fit and locking torque against the manufacturer's or standard values. A nut that spins freely by hand is a reject. Where the supply chain is long or the part is obsolete, use the controls on the counterfeit parts avoidance page and the receiving inspection guide.
How we source nuts
We do not hold stock or publish prices. We source the exact NASM, MS, AN or NAS number from authorized and franchised distributors or the manufacturer, flow down your spec revision, temperature class, finish and specialty metals clauses, and check the paperwork before it ships. Nuts can ride on the same PO as the machined parts and bolts and screws they go with, or be kitted by assembly with the right washers. Discontinued anchor nuts and legacy AN numbers are handled as obsolete parts under counterfeit avoidance controls. Pin and collar fasteners are on the collars and lockbolts page, and the full range is on fasteners.
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Questions
What replaced MIL-N-25027 for self-locking nuts?
NASM25027. MIL-N-25027G was superseded by MIL-DTL-25027H, and the initial release of NASM25027 superseded MIL-DTL-25027 revision H, amendment 1. NASM25027 was adopted for Department of Defense use on 14 December 1999 and is listed as active. It covers self-locking nuts rated for 250, 450 and 800 °F. Drawings that still call the MIL number should be read as NASM25027 unless the contract says otherwise.
Is MS21042 still a valid part number?
Yes. MS21042 revision M, dated January 1995, was cancelled and superseded by NASM21042, and the cancellation states that the part numbers are identical. You can still order an MS21042 part number; the controlling document is now NASM21042. The nut is a 450 °F, reduced hexagon, reduced height, ring base, non-corrosion resistant steel self-locking nut. For 800 °F service in corrosion resistant steel, the matching family is MS21043, now NASM21043.
What is the temperature limit for nylon insert locknuts?
FAA AC 43.13-1B says fiber or nylon self-locking nuts are not installed where temperatures exceed 250 °F. Above that, use an all-metal locknut rated for the temperature, such as a 450 °F or 800 °F nut under NASM25027. Also check the finish: cadmium plated nuts are limited to 450 °F because cadmium diffuses into the steel above that, while silver plated nuts are used for higher temperatures.
Can self-locking nuts be reused?
Sometimes. AC 43.13-1B says a fiber or nylon locknut must not be reused if it cannot meet the minimum prevailing torque in Table 7-2, such as 8 inch-pounds for a 7/16-20 nut. NTSB Safety Alert SA-028 adds that a nut that turns on the bolt by hand should be thrown away. The aircraft or equipment maintenance manual takes priority where it gives a rule, and many programs simply require new nuts.
What is the difference between a fixed and a floating anchor nut?
Both are riveted to structure so a bolt can be installed from one side. In a fixed anchor nut such as MS21047, the threaded element cannot move. In a floating anchor nut such as MS21059 or the miniature NAS1068, the nut can shift slightly inside its cage, which tolerates small misalignment between the panel hole and the structure. Floating nuts are common on removable panels where hole positions vary from part to part.
Where are self-locking nuts not allowed?
FAA AC 43.13-1B lists several cases: parts subject to rotation, locations on turbine powered aircraft where a loose nut or bolt could be drawn into the engine intake, and access panels, doors or parts removed before or after every flight. They may be used on anti-friction bearings and control pulleys when the bearing inner race is clamped by the nut and bolt. A self-locking castellated nut with a cotter pin or lockwire may be used anywhere.
Related
Sources
- EverySpec: NASM25027 adoption notice, self-locking nuts 250, 450 and 800 F
- EverySpec: MS21042M, cancelled and superseded by NASM21042
- EverySpec: MS21043E, cancelled and superseded by NASM21043
- EverySpec: MS21044H Notice 1, superseded by NASM21044
- EverySpec: AN310 Rev. 10 Notice 2, superseded by NASM310
- FAA AC 43.13-1B, Chapter 7: aircraft hardware, nuts, washers and tables 7-12 to 7-14
- NTSB Safety Alert SA-028: self-locking nut reuse
- Standard parts hardware guide: anchor nut identification
- Boeing Distribution: MS21059L4 floating anchor nut
- DFARS 252.225-7009 (JAN 2023), specialty metals, COTS and commercial fastener exceptions
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