Castle Nut vs Slotted Nut vs Jam Nut
Castle, slotted, and jam nuts all help a joint resist loosening, but they do it in very different ways — and the names get mixed up constantly. Castle and slotted nuts use a cotter pin for a positive mechanical lock; a jam nut locks by wedging against a second nut. This guide explains the differences and when to use each.
Side-by-Side Comparison
| Feature | Castle Nut | Slotted Nut | Jam Nut |
|---|---|---|---|
| How it locks | Cotter pin through slotted crown | Cotter pin through slots in hex | Wedges against a second nut |
| Profile | Raised rounded crown above hex | Slots cut into standard hex height | Thin hex nut (about half height) |
| Needs a drilled bolt? | Yes (cross-drilled shank) | Yes (cross-drilled shank) | No |
| Lock type | Positive (mechanical) | Positive (mechanical) | Friction (double-nut) |
| Typical use | Wheel/axle nuts, aircraft (AN310/320) | General machinery, structural | Set position, back up a nut |
Castle vs Slotted: the Crown Difference
Both accept a cotter pin (split pin) through a cross-drilled hole in the bolt or shaft, giving a positive lock that cannot vibrate loose. The difference is the profile. A castle nut (castellated nut) has a rounded, cylindrical crown raised above the hex, with the slots cut into that crown — the style on automotive spindle and axle nuts and aircraft hardware (AN310, AN320, DIN 935). A slotted hex nut has slots cut directly into the top of a standard-height hex with no raised crown (ASME B18.2.2). If you are matching an existing part, the crown tells you which one you have.
How a Jam Nut Differs
A jam nut does not use a cotter pin at all. It is a thin hex nut used in the double-nut method — thread on the jam nut and a primary nut and tighten them against each other so they wedge on the threads. It is a friction lock, ideal for setting and holding a position on threaded rod or backing up a primary nut, but it does not give the positive, can-not-back-off security of a cotter-pinned castle or slotted nut.
Which Should You Use?
- Wheel bearings, spindle/axle nuts, ball joints, aircraft: castle nut with a cotter pin — positive lock, and the joint can stay free to rotate.
- General machinery / structural where a code calls for a cotter pin: slotted hex nut.
- Setting a position on rod, or locking a primary nut without a drilled bolt: jam nut.
- Friction locking for general vibration: a dedicated lock nut may be simpler — see our Lock Nut Comparison.
Installing a castle nut? See our step-by-step How to Install a Castle Nut with a Cotter Pin guide.
Frequently Asked Questions
What is the difference between a castle nut and a slotted nut?
A castle nut has a rounded crown raised above the hex body with the slots cut into that crown (DIN 935 / aircraft AN310-AN320 style). A slotted hex nut has slots cut directly into the top of a standard-height hex with no raised crown (ASME B18.2.2). Both accept a cotter pin and do the same job; the crown profile is how you match an existing part.
Do castle and slotted nuts need a cotter pin?
Yes. The slots exist to accept a cotter pin (or safety wire on some aircraft work) through a cross-drilled hole in the bolt or shaft. Without the pin you lose the positive lock. The bolt must be drilled for the pin.
Is a jam nut the same as a lock nut?
A jam nut is one way to lock a joint — the double-nut method — but it is not a dedicated lock nut. It is a thin nut used to set position or back up a primary nut. For friction locking in one part, use a nylon-insert or all-metal lock nut; for a positive mechanical lock, use a castle or slotted nut with a cotter pin.
Which is strongest against loosening?
A castle or slotted nut with a cotter pin gives a positive mechanical lock that cannot back off, which is why they are used on wheel bearings and aircraft. A jam nut relies on friction and is less secure under heavy vibration.
Need Castle, Slotted, or Jam Nuts?
Eugene Fastener stocks slotted and castle nuts, hex jam nuts, and matching cotter pins and drilled bolts — the whole joint from one source.