Castle Nut
Nuts · slotted nut · castellated nut
Definition: A castle nut is a hex nut with radial slots cut into its top, through which a cotter pin passes to positively lock the nut to a drilled bolt or shaft.
Anatomy
■ Body■ Internal thread
Governing standards: ASME B18.2.2 · DIN 935 · ISO 7035
Common Sizes
| Inch | Metric | Head width | Head height |
|---|---|---|---|
| 1/4-20 | M6 | 7/16 | 5/16 |
| 3/8-16 | M10 | 9/16 | 7/16 |
| 1/2-13 | M12 | 3/4 | 9/16 |
Representative sizes. Full dimensional data lives in the standards listed below.
Grades & Strength
| Grade | Material | Tensile (psi) | Marking |
|---|---|---|---|
| Grade 5 | medium carbon steel, Q&T | 120,000 | 3 radial lines |
| Grade 8 | alloy steel, Q&T | 150,000 | 6 radial lines |
Materials
- Zinc-plated steel
- Stainless steel
Pros & Cons
Pros
- Positive mechanical lock with a cotter pin
- Cannot back off when pinned
- Visually inspectable at a glance
Cons
- Needs a drilled bolt or shaft
- Bulkier than a nyloc
- Torque-then-align-slot adds a step
Typical Uses
- Wheel bearings and spindles
- Suspension and steering joints
- Axle and hub retention
Alternatives & Related
- Lock Nut — Simpler anti-loosening where a drilled shaft is not available
- Hex Nut — Standard nut for non-critical joints
Where to Buy
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FAQ
Castle nut vs nyloc?
A castle nut locks positively with a cotter pin through a drilled shaft, and survives heat a nylon insert cannot. A nyloc is simpler but is a friction lock and heat-limited.
How do you torque a castle nut?
Torque to spec, then tighten only enough further to align the nearest slot with the hole — never back off to align. Insert the cotter pin and bend the legs.