Slotted Nuts

Slotted nuts and castle nuts (castellated nuts) are hex nuts with a slotted top that accepts a cotter pin through a cross-drilled hole in the bolt or shaft, locking the nut positively against rotation. They’re the go-to fastener anywhere vibration, rotation, or safety makes a self-loosening joint unacceptable: wheel bearings, spindle and axle nuts, ball joints, tie-rod ends, machinery shafts, and heavy equipment. Eugene Fastener & Supply stocks slotted hex nuts and castle nuts across inch sizes 5/16″ through 2″ and metric M6 through M16, in steel zinc-plated and plain — with 18-8 and 316 stainless, brass, and silicon bronze available to order.

The two styles do the same job and live together here: true castle nuts have a raised, rounded crown (DIN 935 and aircraft AN310/AN320 patterns), while slotted hex nuts have slots cut into a standard hex height (ASME B18.2.2). Whether you need a single AN spindle nut for a restoration or a production quantity for an equipment line, we sell by the piece or in bulk, cross-reference specialty and legacy part numbers, and ship the matching drilled bolts and cotter pins from the same order. Same-day shipping on orders placed before 2pm Pacific.

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Slotted & Castle Nuts — Steel, Stainless, Brass & Silicon Bronze

Need castle nuts for a production run, an AN/MS spec callout, or a hard-to-find size?

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Include the thread size, material, and standard (DIN 935, AN310, ASME B18.2.2) plus quantity for fastest quoting.

Castle Nut vs. Slotted Nut — What’s the Difference?

The two terms are often used interchangeably, but there is a real distinction:

  • Castle nut (castellated nut): Has a rounded, cylindrical crown raised above the hex body, with slots cut into that crown. The raised crown keeps the slot bearing surface above the load-carrying threads. This is the style used on automotive spindle and axle nuts and on aircraft hardware (AN310, AN320, DIN 935).
  • Slotted hex nut: A standard-height hex nut with slots cut directly into the top of the hex — no raised crown. Covered by ASME B18.2.2. Common in general machinery and structural work where a cotter pin or safety wire is required but the aircraft-style crown isn’t needed.

Both do the same job: they let a cotter pin (split pin) pass through a cross-drilled hole in the bolt or shaft and through the slots, mechanically preventing the nut from backing off. If you’re matching an existing part, the crown profile tells you which one you need.

What Castle & Slotted Nuts Are Used For

  • Automotive & truck: Wheel bearing/spindle nuts, axle nuts, ball joints, tie-rod ends, castle-and-cotter steering and suspension joints.
  • Machinery & equipment: Rotating shafts, clevis and pin joints, sheave and pulley retention — anywhere a positively-locked but field-serviceable nut is required.
  • Agricultural & heavy equipment: Implement pins, linkage joints, and high-vibration assemblies.
  • Aircraft & aerospace (AN/MS): AN310 (full-height) and AN320 (thin/shear) castle nuts to military spec, secured with cotter pins or safety wire.
  • Marine: Silicon bronze and 316 stainless castle nuts for steering gear, rudder fittings, and deck hardware exposed to salt water.

Sizes, Threads & Standards

  • Inch sizes: 5/16″ through 2″, in UNC (coarse) and UNF (fine) threads — smaller sizes available to order.
  • Metric sizes: M6 through M16, DIN 935 hexagon castle nuts, coarse and fine pitch.
  • Standards: ASME B18.2.2 (slotted hex & heavy hex slotted nuts), DIN 935 (metric castle nuts), AN310 / AN320 and MS35690 / MS35691 (aircraft castle nuts).
  • Cross-drilled bolts: We also supply drilled-shank hex bolts and the matching cotter pins so the whole castle-and-pin joint ships from one source.

Materials & Finishes — When to Use Each

MaterialBest ForTrade-Offs
Steel, Zinc Plated / PlainGeneral automotive, machinery, and equipment — the default, best strength-per-dollarCorrodes in wet/marine service; not for long-term salt exposure
18-8 StainlessOutdoor, food, and washdown environments needing corrosion resistanceHigher cost; lower torque-out resistance than plated steel; can gall on install
316 StainlessMarine and chloride-exposed service — superior pitting resistance vs 18-8Highest stainless cost
BrassElectrical, decorative, and non-magnetic applicationsLower strength; not for structural loads
Silicon BronzeMarine and architectural — outstanding salt-water corrosion resistance and galvanic compatibility with bronze hardwarePremium price; specialty item we drop-ship in the full size range

How to Install a Castle or Slotted Nut

  1. Torque to spec. Tighten the nut to the joint’s specified torque (or, for free-rotating joints like wheel bearings, to the seating torque, then back off to the specified running clearance).
  2. Align a slot with the cotter-pin hole. If a slot doesn’t line up with the cross-drilled hole in the bolt or shaft, advance (or, where allowed, back off) the nut to the nearest slot. Never over-torque just to reach a slot.
  3. Insert the cotter pin through the slot and the hole, then bend the legs around the nut/bolt to lock it. Use a fresh cotter pin on every install — they are single-use.

The castle nut’s value is the positive mechanical lock — it does not depend on friction, so it holds even in high-vibration joints and on connections that must remain free to rotate.

Castle & Slotted Nut FAQ

What is a castle nut?

A castle nut (also called a castellated nut) is a hex nut with a slotted crown on top. The slots line up with a cross-drilled hole in the bolt or shaft so a cotter pin can pass through both, locking the nut against rotation. It’s used where vibration or safety makes a self-loosening joint unacceptable — wheel bearings, axle and spindle nuts, ball joints, and machinery shafts.

What’s 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 (the DIN 935 and 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.

What is a castle nut used for?

Castle nuts secure joints that can’t be allowed to vibrate loose, and joints that must stay serviceable. The most common uses are automotive wheel-bearing, spindle, and axle nuts, ball joints and tie-rod ends, machinery and equipment shafts, agricultural linkage pins, and aircraft hardware. Marine versions in silicon bronze or 316 stainless are used on steering gear and deck hardware.

Do castle nuts need a cotter pin?

Yes — the cotter pin is what makes a castle nut a castle nut. The slotted crown exists specifically to accept a cotter pin (or, on some aircraft applications, safety wire) through a cross-drilled hole in the bolt or shaft. Without the pin you lose the positive mechanical lock and the nut can back off. Always install a fresh cotter pin; they’re single-use.

How do you install and torque a castle nut?

Torque the nut to the joint’s specification. If a crown slot doesn’t align with the cotter-pin hole, advance the nut to the nearest slot — never over-torque just to reach one. For free-rotating joints like wheel bearings, follow the procedure of seating then backing off to the specified clearance. Insert the cotter pin through the slot and hole and bend the legs to lock it.

What sizes and threads do castle and slotted nuts come in?

We stock inch sizes from 5/16″ through 2″ in UNC and UNF, and metric sizes M6 through M16 (DIN 935) in coarse and fine pitch. Smaller and aircraft sizes are available to order — contact us with the size, pitch, or AN/MS pattern you need.

What materials and finishes are available?

Steel zinc-plated and plain are stocked. 18-8 stainless, 316 stainless for marine/chloride service, brass for electrical and non-magnetic work, and silicon bronze for marine and architectural applications are available to order — we drop-ship the full silicon-bronze size range even on items not stocked locally.

What standards do your castle and slotted nuts meet?

Slotted hex and heavy hex slotted nuts conform to ASME B18.2.2. Metric castle nuts are made to DIN 935. Aircraft castle nuts are available to AN310 (full height) and AN320 (thin/shear), MS35690/MS35691. Tell us the callout and we’ll match it.

Can you supply the drilled bolt and cotter pin too?

Yes. We stock cross-drilled (drilled-shank) hex bolts and the matching cotter pins, so the complete castle-and-pin joint ships from one source. Add them to your order or ask us to spec the matched set.

Do you sell by the piece, and can you cross-reference part numbers?

Both by the piece and in bulk. Send us any AN, MS, DIN, or manufacturer part number and we’ll match it to a stocked equivalent — usually same business day — with pricing and lead time.

Why Buy Castle & Slotted Nuts from Eugene Fastener

  • Full material range — steel zinc and plain stocked; 18-8 and 316 stainless, brass, and silicon bronze to order, including drop-ship on specialty bronze sizes.
  • By the piece or in bulk — one AN nut for a restoration or production quantities for an equipment line.
  • Matched joint hardware — drilled-shank bolts and cotter pins from the same source.
  • Cross-reference assistance — send any AN, MS, or DIN number and we’ll match it, usually same day.
  • Since 1969 — Eugene Fastener & Supply has served Pacific Northwest manufacturers, contractors, and equipment makers for over 55 years.

Building the whole joint? Pair slotted and castle nuts with our cotter and clevis pins and drilled-shank hex bolts. Need a different locking style? See our lock nuts (nylon-insert, all-metal, and prevailing-torque) and hex jam nuts. For high-load joints, see heavy hex nuts and structural heavy hex nuts. Eugene Fastener & Supply has supplied Pacific Northwest manufacturers, contractors, and equipment makers with specialty nuts and fasteners since 1969 — contact our sales team for specifications, cross-references, or bulk pricing.

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