Three MS Nut Types, One Purpose
MS nuts are the standard retention hardware for AN bolts in airframe applications. Every certificated aircraft in the US fleet uses them. There are three main types — each with a specific use case.
MS21042
Self-locking hexagonal nut. All-metal locking insert. Single-use by standard practice.
MS21083
Prevailing torque hexagonal nut. Nylon insert. Most common MS nut in general aviation.
MS20365
All-metal castle nut. Drilled for cotter pin. Reusable if hole aligns.
MS21042 — Self-Locking Hex Nut
MS21042 is an all-metal self-locking hex nut with a controlled-collapse locking feature built into the nut body itself. Because it's all-metal, it maintains its locking capability at elevated temperatures — making it the preferred choice for engine compartment and high-heat applications.
MS21042 nuts are identified by a dash number matching the thread size: MS21042-04 = 1/4"-28 thread, MS21042-05 = 5/16"-24, MS21042-06 = 3/8"-24, etc.
- Single-use (self-locking feature deforms on first installation)
- Temperature rated to 425°F (MS21042C rated to 1,300°F with A-286 material)
- Non-corrosive in most environments; cadmium plate standard finish
MS21083 — Prevailing Torque Hex Nut
MS21083 is the most common MS nut in general aviation aircraft. It uses a nylon insert in the top threads to create prevailing torque — the friction that prevents the nut from self-loosening under vibration. Unlike MS21042, the locking feature is non-metallic and degrades with temperature and re-use.
Standard practice: MS21083 nuts are single-use. If an MS21083 nut is removed during maintenance, replace it. Don't re-install it.
MS20365 — Castle Nut
MS20365 is a hexagonal castle nut — a hex body with slots (the "castle") cut into one face to accept a cotter pin. Unlike self-locking nuts, it requires a mechanical positive retention method (the cotter pin) to prevent loosening. This makes it the preferred choice for critical structural and control surface applications where you need positive, visual confirmation of proper installation.
MS20365 castle nuts are used with AN bolts in applications where the maintenance manual explicitly requires them, or where: (a) vibration is severe, (b) access for inspection is required, or (c) positive retention is specified by engineering.
Reuse is permitted — unlike MS21042/MS21083, MS20365 does not have a single-use locking feature. Reuse is acceptable when: the nut surface is in good condition, the threads are not damaged, and the cotter pin hole aligns without modification.
MS Nut Selection Table
| MS Spec | Locking Method | Reuse | Temp Rating | Standard Finish | Typical Application |
|---|---|---|---|---|---|
| MS21042 | All-metal controlled collapse | Single-use | 425°F (1,300°F C-suffix) | Cadmium plate | High-temp, engine mounts |
| MS21083 | Nylon insert (prevailing torque) | Single-use | 250°F max | Cadmium plate | General airframe, primary structures |
| MS20365 | Cotter pin (mechanical positive) | Reusable if in good condition | Thread material rating | Cadmium plate | Control surfaces, critical joints, landing gear |
Common Sizes in Stock
| Part Number | Thread | For AN Bolt | Status |
|---|---|---|---|
| MS21042-04 | 1/4"-28 | AN4 | In stock |
| MS21083-04 | 1/4"-28 | AN4 | In stock |
| MS21042-05 | 5/16"-24 | AN5 | In stock |
| MS21083-05 | 5/16"-24 | AN5 | In stock |
| MS21042-06 | 3/8"-24 | AN6 | In stock |
| MS21083-06 | 3/8"-24 | AN6 | Sourcing available |
| MS20365-06 | 3/8"-24 | AN6 | Sourcing available |
Browse our full MS nut inventory for sizes and availability. For bulk quantities or non-standard sizes, submit an RFQ.
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Pairing with Cotter Pins
MS20365 castle nuts pair with cotter pins (AN380, AN381) through the bolt's drilled shank. The cotter pin goes through the castle slots and the bolt hole, then its ends are bent back against the nut face. See our full castle nut and cotter pin pairing guide.
- AN380 — 1/32" × 1" flat head cotter pin. Most common.
- AN381 — 1/32" × 1-1/2" flat head cotter pin. For extra clearance.
- Never use a cotter pin that's too small — it must fill the hole to provide positive retention.
Frequently Asked Questions
MS21042 is a hexagonal self-locking nut with an all-metal insert. MS21083 is a hexagonal prevailing torque nut (non-metal insert). MS20365 is a castle nut with drilled holes for a cotter pin. Each serves different structural requirements — MS21083 is most common for airframe applications, MS20365 for control surface and gear applications where positive retention is required.
Self-locking MS nuts (MS21042, MS21083) are single-use by standard practice — the locking element deforms under first installation. MS20365 castle nuts (retained by cotter pin) can be reused as long as the nut and mating bolt are in good condition and the cotter pin hole aligns properly after removal.
Prevailing torque is the friction value that must be overcome to rotate the nut on the bolt — it indicates that the locking feature is engaging. MS21083 nuts have a nylon insert that provides the prevailing torque. The torque spec for MS nuts is the same as a standard hex nut of the same size — the locking feature does not change the torque requirement.
An AN4 (1/4"-28) bolt uses a MS21042-04 or MS21083-04 nut — both are 7/16" hex, 1/4"-28 thread. Check the applicable maintenance manual for your specific application. Some manuals specify MS21042 where MS21083 is also acceptable. The MS number specifies both the thread size and the style.
No. AN/MS/NAS hardware is manufactured to MIL-SPEC with documented traceability. Commercial grade hardware (ASTM, SAE) may have the same dimensions but is not approved for installation on type-certificated aircraft without explicit engineering approval. Only use MS nuts on AN/MS/NAS bolts in airframe applications.
Find the right MS nuts for your aircraft
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