Castle nuts and cotter pins are a safety system. Get the pairing wrong and the nut backs off. This guide covers which nut goes with which bolt, which cotter pin pairs with each nut, the torque procedure, and the key FAA AC 43.13 references.
AN310 vs AN320 — Which One and When
This is the first decision. They are not interchangeable for all applications.
AN310 — Standard Castle Nut (Tension Applications)
- Full-height nut with deep castellations for cotter pin
- Used on bolts under tension loads — structural joints where the bolt is being pulled
- Used with drilled-shank hex head bolts, eye bolts, clevis bolts in tension
- Tensile rating: approximately 125,000 psi (matches alloy steel AN bolt)
- Corrosion-resistant version: AN310C (stainless)
AN320 — Castellated Shear Nut (Shear Applications Only)
- Thinner profile — roughly half the height of AN310
- Half the tensile strength of AN310
- Used with clevis bolts subject to shear stress only — control linkage attachment points where bolt is loaded laterally, not axially
- Corrosion-resistant version: AN320C
The rule: If the bolt is under tension (being pulled through), use AN310. If the bolt is loaded in shear (transverse to the bolt axis, like a clevis pin through a rod end), use AN320. When in doubt, AN310.
Castle Nut to Bolt Size Pairing
| Bolt | Thread | AN310 Part # | AN320 Part # |
|---|---|---|---|
| AN3 | 10-32 | AN310-3 | AN320-3 |
| AN4 | 1/4-28 | AN310-4 | AN320-4 |
| AN5 | 5/16-24 | AN310-5 | AN320-5 |
| AN6 | 3/8-24 | AN310-6 | AN320-6 |
| AN7 | 7/16-20 | AN310-7 | AN320-7 |
| AN8 | 1/2-20 | AN310-8 | AN320-8 |
| AN10 | 5/8-18 | AN310-10 | AN320-10 |
Cotter Pin Selection (MS24665 Series)
Cotter pins for aircraft use are MS24665 series. Do not use hardware-store cotter pins — the material spec and diameter tolerances are not the same. The cotter pin passes through the castle nut slots and the drilled hole in the bolt shank.
| MS Part # | Diameter | Length | Typical Application |
|---|---|---|---|
| MS24665-132 | 1/16" | 3/4" | AN3 bolts, small clevis pins |
| MS24665-151 | 3/32" | 1" | AN3–AN4 bolts (stainless, preferred for corrosion exposure) |
| MS24665-208 | 1/8" | 1-1/4" | AN4–AN5 bolts |
| MS24665-262 | 5/32" | 1-1/2" | AN5–AN6 bolts |
| MS24665-315 | 3/16" | 1-3/4" | AN6–AN8 bolts |
| MS24665-369 | 1/4" | 2" | AN8–AN10 bolts |
The Torque-and-Back-Off Procedure
Per FAA AC 43.13-1B, Chapter 7:
- Torque to the minimum value in the AC table for that bolt size and nut combination.
- Check cotter pin alignment. If a castellation aligns with the bolt hole, install the cotter pin and stop.
- If alignment is not achieved at minimum torque, continue tightening until the next alignment position — up to the maximum torque value.
- If maximum torque is reached without alignment, back off slightly to the nearest alignment position below maximum, install cotter pin.
- Do not back off below minimum torque to achieve alignment.
AC 43.13-1B Torque Reference Table (Selected Sizes):
| Bolt | Nut | Recommended Torque | Maximum |
|---|---|---|---|
| AN3 (10-32) | AN310-3 | 20–25 in-lb | 40 in-lb |
| AN3 (10-32) | AN320-3 | 12–15 in-lb | 25 in-lb |
| AN4 (1/4-28) | AN310-4 | 50–70 in-lb | 90 in-lb |
| AN4 (1/4-28) | AN320-4 | 30–40 in-lb | 60 in-lb |
| AN5 (5/16-24) | AN310-5 | 100–140 in-lb | 200 in-lb |
| AN6 (3/8-24) | AN310-6 | 160–190 in-lb | 300 in-lb |
| AN8 (1/2-20) | AN310-8 | 450–500 in-lb | 700 in-lb |
Source: AC 43.13-1B CHG 1, Table 7-1. Always verify against the current version of the AC and your aircraft's maintenance manual where specified.
Cotter Pin Installation
- Insert the cotter pin through the slot and bolt hole.
- Bend the top leg: wrap it over the bolt end to at least the centerline of the bolt.
- Trim the bottom leg: cut just short of flush with the bottom surface of the castle nut.
- Ensure the bottom leg does not contact adjacent structure or foul any moving parts.
- Do not reuse cotter pins — they are single-use hardware.
When NOT to Use a Castle Nut
Castle nuts require a drilled bolt shank. If your plans call for a self-locking nut (AN365, MS21044), using a castle nut is not a direct substitute. Self-locking nuts maintain preload through the nylon or metal insert. High-temperature locations (above 250°F, nylon melts) require all-metal lock nuts or castle/cotter pin arrangements.
Follow your plans. If the plans say AN365, use AN365. If they say AN310 with cotter, use AN310 with cotter. See the AN3–AN10 Sizing Reference for the full hardware pairing table by bolt size. The AN Bolt Grip Length Chart covers how to select the correct dash number for your stack.