Slotted Hex Nuts

Plain Steel Slotted Hex Nuts

What Are Slotted Hex Nuts?

Slotted hex nuts—also called castle nuts—are hex nuts with slots cut into the top face. A cotter pin passes through a slot and the bolt’s cross-hole to create a positive mechanical lock that resists rotation in high-vibration or safety-critical assemblies.

Key Benefits of Slotted Hex Nuts

Mechanical locking: Cotter pin through the slot prevents loosening without chemical threadlocker.
Vibration resistance: Ideal for dynamic loads, impact, or cyclic motion.
Serviceable design: Easy visual inspection; replace the cotter pin during maintenance.
Material options: Zinc-plated steel for general use; 304/316 stainless for outdoor, marine, or corrosive environments; brass for non-sparking or decorative needs.
Standards compliance: Available to common inch and metric specs for consistent fit and performance.

Common Applications

Automotive suspensions, steering linkages, and brake hardware
Heavy equipment and machinery assemblies
Marine and coastal hardware
Structural and industrial fabrications subject to vibration
Aerospace, agricultural, and off-road equipment

How to Install a Slotted Hex Nut

Torque the nut to the joint’s specified value.
Advance (do not back off) the nut just enough to align a slot with the bolt’s cross-hole.
Insert the cotter pin fully through the hole and slot.
Bend the cotter pin legs around the nut to secure; trim excess if required.

Tip: Follow your drawing/standard for permissible additional rotation after reaching torque to achieve slot alignment.

Maintenance Tips

Always replace cotter pins after removal; never reuse a bent or fatigued pin.
Inspect slots and threads for damage or deformation before reassembly.
Match materials to limit galvanic corrosion (e.g., stainless nut with stainless bolt in wet environments).

Materials & Finishes

304 Stainless Steel: General outdoor corrosion resistance.
316 Stainless Steel: Superior pitting resistance for marine/chloride exposure.
Zinc-Plated Steel: Cost-effective protection for dry, indoor, or sheltered settings.
Brass: Non-sparking, decorative, or low-magnetic applications.

Sizing & Standards

Available in inch and metric sizes in both coarse and fine thread pitches. Select the nut to match the bolt’s diameter and thread pitch; ensure the bolt has a cross-hole for the cotter pin and sufficient thread engagement (commonly at least one bolt-diameter, unless otherwise specified by your standard).

Slotted Hex Nuts FAQs

Answers to the most-asked questions about castle/slotted hex nuts.

What’s the difference between a slotted hex nut and a hex jam nut?

Slotted (castle) nuts lock with a cotter pin through the nut’s slot and the bolt’s cross-hole. Jam nuts are thin nuts that lock by tightening against a full-height nut—no pin or hole required.

How tight should I torque a slotted hex nut?

Torque to the joint spec first, then advance just enough to align a slot with the cross-hole (follow your drawing/standard for permitted adjustment).

Do I still need threadlocker if I use a cotter pin?

Usually no, the cotter pin is the mechanical lock. If your spec calls for it in extreme vibration, use a removable grade in addition to the pin.

Which material should I choose for outdoor or marine use?

304 stainless for general outdoor exposure; 316 stainless for marine or chloride environments due to superior pitting resistance.

Can I mix stainless nuts with zinc-plated bolts?

You can, but mixing dissimilar metals outdoors can promote galvanic corrosion. Match materials when possible in wet or coastal settings.

What size cotter pin should I use?

Match the bolt’s cross-hole diameter and choose a length that lets you bend both legs securely around the nut. Follow your BOM or governing standard.

How do I align the slot without under-torquing the joint?

Reach required torque, then advance the nut slightly to the next slot alignment within allowed tolerance, avoid backing off below spec clamp load.

Are slotted hex nuts reusable?

Yes, if threads and slots are undamaged. Always replace the cotter pin after removal.

What thread engagement do I need?

A common rule is at least one full bolt-diameter of engaged threads (or per your governing standard) for strength and reliable slot/hole alignment.

Do slotted hex nuts come in coarse and fine threads?

Yes, both coarse (UNC/metric coarse) and fine (UNF/metric fine). Match the nut to the fastener’s thread form and pitch.