Castle Nut Guide: DIN 935, Cotter Pin Install & Safety Rules

A castle nut (castellated nut) is a hex nut with six radial slots cut into the top, used with a cotter pin (split pin) through a cross-drilled bolt to mechanically lock the nut against rotation. Standard reference: DIN 935 / ISO 7035. Primary use: safety-critical automotive joints — ball joints, tie rod ends, drive axles — and any vibration-prone fastener that can't trust friction alone.

Castle Nut — Quick Reference

What Is a Castle Nut and How It Works

A castle nut looks like an ordinary hex nut with six rectangular slots cut into the top — the shape resembles the crenellated top of a castle wall, which is where the name comes from. The slots have one job: to line up with a transverse hole drilled through the threaded shank of the bolt or stud, so that a cotter pin (split pin) can be passed through both the nut slot and the bolt hole, mechanically locking the nut in place.

Here's how the lock works in practice. You torque the castle nut down to the specified value, exactly as you would any other nut. The cotter pin doesn't carry the clamping load — the bolt's tension still does that. What the cotter pin does is sit across one of the slots, so if the nut tries to rotate (under vibration, thermal cycling, or shock loading), the slot wall comes up against the cotter pin and stops. The nut physically can't back off because there's a piece of steel in the way.

This is a fundamentally different locking principle from a nyloc nut or a spring washer. Nylocs use friction from a nylon insert. Spring washers use spring tension and bite. Both can degrade — friction drops, springs flatten, nylon melts. The castle nut + cotter pin is purely mechanical: as long as the pin is in place and the bolt is still torqued, the nut cannot rotate. That's why it's the default specification for safety-critical joints where the consequences of the nut backing off include "loss of steering control".

The Cardinal Rule — Torque to Spec, Then Tighten to Align (Never Loosen)

This rule is so consistently violated that vehicle manuals often pre-empt it with explicit language. The typical instruction reads:

"Torque the castle nut to [spec]. If the slot does not align with the hole in the stud, tighten the nut to the next slot. Do not loosen the nut to achieve alignment."

If you've over-torqued and the joint feels tight beyond what the specification calls for, the correct move is to back off completely, inspect the stud and threads, and start again — not to ease back to the previous slot. Once a ball joint stud has been pulled into the taper, only forward (tightening) movement preserves the seat.

When you genuinely can't reach the next slot: the joint is telling you something is wrong. Common causes are a stretched or wrong-grade bolt, a wrong-thickness washer stack, a damaged thread, or the wrong nut for the job. Stop and investigate. Don't grind the nut down, don't shim with whatever's on the bench, and don't loosen back. Replace the suspect part.

Castle Nut Standards — DIN 935, ISO 7035, DIN 937, SAE

For metric castle nuts in Australia, the two standards that matter are DIN 935 (German) and its ISO equivalent ISO 7035. They're functionally identical and the terms are used interchangeably by Australian fastener suppliers. A separate but closely related standard, DIN 937 / ISO 7036, covers thinner "slotted nuts" — slots cut directly into the top of a standard-height hex, without a separate raised crown. Use DIN 935 for almost all automotive and engineering work; DIN 937 is for tight-clearance applications.

Size range under DIN 935: M5 through M64. Slot cut detail: up to M12, slots are cut on one side of the crown only; above M12, slots are cut all the way through the crown. Thread pitch follows ISO Metric Screw Thread.

Strength grades: the most common grades stocked in Australia are Class 8 (paired with Grade 8.8 bolts), Class 10 (Grade 10.9 bolts), and stainless A2-70 / A4-80. Always match — a Class 4 nut on a Grade 10.9 bolt will strip its threads under load long before the bolt yields.

How a Castle Nut Pairs with a Cotter Pin

The cotter pin is the active locking element. Without it, a castle nut is just an oddly-shaped hex nut that holds no better than any other unsecured nut. The bolt or stud must have a cross-drilled hole through the threaded section; the nut's slots align with that hole; the pin passes through both.

The pin diameter is sized to the cross-drill, not to the slot width. The slot is intentionally wider than the pin so that as you rotate the nut to align, you've got a reasonable chance of catching the hole. Typical pairings on Australian metric stock:

Always check the OEM workshop manual. Vehicle manufacturers sometimes call up a smaller pin than the hole would physically accept — this is intentional, to leave room for bending the legs over without binding.

When to Use a Castle Nut

Castle nuts are specified — by OEMs, by standards bodies, and by site safety requirements — anywhere the consequences of a backed-off nut include personal injury, property damage, or equipment loss. The common Australian applications:

• Automotive suspension and steering — ball joints (upper and lower), tie rod ends, drag links, pitman arms, idler arms, sway bar end links on heavier vehicles. Vibration is constant, loads are reversing, failure mode is loss of steering. This is the largest single market for castle nuts in Australia.
• Drive axles and CV joints — axle stub nuts on solid rear axles, CV axle nuts on independent rear suspension. High torque, high cycle count.
• Trailer and caravan running gear — wheel bearing castle nuts (taper bearings), tow ball assemblies. Bearings need controlled preload and the cotter pin is what holds the preload in place.
• Agricultural equipment — PTO shafts, three-point linkage pins, header reel drives, harvester knife drives. Field vibration is brutal; serviceability matters.
• Marine — propeller shaft nuts, rudder pintles, anchor windlass clutches. Stainless A4/316 castle nuts are standard here.
• Heavy lifting and rigging — sheave pin nuts on lifting blocks (although shackles increasingly use safety bolts/nuts/pins as a complete assembly).
• Motorcycle and ATV — front and rear axle nuts on almost every motorcycle ever built, plus swingarm pivots and brake torque arms.

If the joint sees vibration, the connection is safety-critical, and the assembly will be serviced repeatedly over its life — a castle nut + cotter pin is the default specification.

Castle Nut vs Nyloc Nut vs Spring Washer — When Each Wins

This is a question that comes up constantly in workshops. The short answer: each has a job, and the cost of getting it wrong scales with how critical the joint is.

Mechanic forum consensus (Corvette Forum C4 Tech, BobIsTheOilGuy Engineering forum, Pilots of America): on a tapered-stud safety-critical joint, you replace nyloc with castle nut + pin and never the other way around. The nylon insert's reliance on friction is its weak point — it works the first time, often works the second time, and is unreliable thereafter. The cotter pin doesn't degrade, doesn't soften with heat, doesn't lose grip over time.

One important caveat: a castle nut without a cotter pin is worse than a nyloc, because it has no locking feature at all. This is the failure mode flagged in NHTSA recalls (2026 Roush/Nissan Frontier; 2023 Daimler Truck tie rod). Castle nut alone = unsecured. Always pin it.

Cotter Pin Sizing and Reuse

Cotter pins (or split pins, in Australian and British usage) are the active half of the lock. Get them right or the whole arrangement fails.

Single-use rule — every time

A cotter pin is a single-use safety fastener. The metallurgical reason: bending the legs to install, then unbending them for removal, work-hardens the steel at the fold point. The pin survives the first bend cleanly. The second bend (during reinstall) can crack the leg or leave a microscopic stress concentration that fatigues out during service vibration. Consensus across Yesterday's Tractors, Reddit r/Mechanics, ClassicBroncos.com and engineering practice texts is uniform: replace every time. A new split pin costs cents. A failed split pin on a tie rod end costs a car.

Sizing — match the bolt cross-drill, not the slot

The pin should pass through the cross-drilled hole in the bolt with no significant side play and no force needed to insert. If the pin is too small, it can fatigue and shear. Too large, and you'll deform the legs forcing it in, weakening it before you've even bent the lock over.

Material — plain steel, stainless, or brass

• Zinc-plated low-carbon steel — default for general automotive and engineering. Cheap, plenty strong for the job.
• Stainless steel (A2/304 or A4/316) — marine, food-grade, chemical exposure, or anything outdoors in coastal Australia.
• Brass / phosphor bronze — non-sparking applications (mining, fuel handling), non-magnetic requirements, decorative restorations.

Length — enough to bend, not so much you trim heavily

The pin should protrude past the bolt face by enough to bend both legs cleanly — typically one to one-and-a-half bolt diameters past the hole. Trim afterwards if needed so no leg protrudes more than the pin diameter once bent.

Materials and Grades

Castle nuts are specified by material grade matched to the bolt they secure. Get the match right and the joint behaves predictably; get it wrong and the nut yields before the bolt does, dropping clamp force without obvious warning.

• Zinc-plated carbon steel, Class 8 (DIN 935-1) — the workhorse. Matches Grade 8.8 (8.8 = ~800 MPa tensile, ~640 MPa yield) bolts. Used for the vast majority of automotive ball joint, tie rod and axle applications.
• Zinc-plated carbon steel, Class 10 — for Grade 10.9 bolts. Less common in everyday workshop work but specified on high-load suspension components, truck and trailer axles, agricultural equipment.
• Stainless A2 / 304 — corrosion resistance for general outdoor, freshwater, mildly aggressive environments. Lower tensile than carbon Class 8 (typical A2-70 ≈ 700 MPa) — verify the spec matches the load.
• Stainless A4 / 316 (marine grade) — saltwater, coastal Australia, chemical processing. Even lower tensile than A2 in some grades — A4-80 is the higher-strength variant.
• Grade 5 / Grade 8 (imperial) — for SAE-spec assemblies. Grade 5 ≈ Class 8, Grade 8 ≈ Class 10. Used on older / imported / aftermarket US-pattern hardware.

The rule: the nut should always be at least as strong as the bolt. Use a stronger nut if available; never go weaker.

How to Install a Castle Nut Correctly — Step by Step

1. Inspect first. Threads on the bolt clean, no damage, no stretched or necked-down sections. The cross-drilled hole clean and burr-free. Nut threads run freely onto the bolt by hand. New cotter pin on the bench.
2. Check the OEM torque spec. For automotive ball joints, tie rods and axles, this is non-negotiable. Workshop manual, online OEM database, or supplier technical bulletin. Don't guess.
3. Run the nut down by hand until it contacts the joint. Use a clean torque wrench from this point — clicking or beam, doesn't matter, just make sure it's calibrated.
4. Torque to the manufacturer's specification in a smooth, controlled motion. Don't snatch the wrench. Click and stop.
5. Check slot alignment with the cross-drilled hole. Sight through the slot from the side. If the slot lines up with the hole — perfect, go to step 7.
6. If the slot doesn't align — tighten further to the next slot. Never loosen. Small additional rotation; do not exceed the maximum torque allowed by the OEM spec if one is given. If you can't reach the next slot within the maximum torque window, stop and investigate — see the cardinal rule callout above.
7. Insert a new cotter pin through the slot and the cross-drill. Push it home so the looped head sits flush against the side of the nut.
8. Bend the legs to lock. Classic bend: fold the longer leg back over the top of the bolt towards the centre, and bend the shorter leg flat against the side of the nut. Aviation/motorcycle bend: wrap both legs around the bolt and nut so neither sticks out — used where snagging is a concern.
9. Trim excess length with side cutters if any leg protrudes more than one pin diameter once bent. Sharp edges go on the inboard side, away from boots and brake hardware.
10. Final inspection. Pin can't be wiggled out. Nut can't be rotated by hand pressure on a spanner. No interference with adjacent components, dust boots, brake lines, or the wheel.

For ball joints specifically: after install, lift the vehicle so the suspension droops, then re-check the cotter pin is secure. The taper sometimes "sets" further into the knuckle in the first few load cycles; a final inspection after a short test drive is good practice on safety-critical assemblies.

AIMS Industrial Castle Nut and Cotter Pin Range

AIMS stocks DIN 935 / ISO 7035 castle nuts plus matched split pins (cotter pins) for automotive, engineering, marine and industrial applications across Australia. Materials range from zinc-plated carbon steel (Class 8 and Class 10) through to A2/304 and A4/316 marine stainless. Standard sizes M5 through M24 in stock, larger sizes available to order.

• Cotter Pins (Split Pins) — 29 SKUs covering common metric and imperial sizes for automotive and industrial use.
• Nuts (full range) — 264 SKUs including castle nuts, hex nuts, nyloc nuts and specialty fasteners.
• Hex Nuts — 119 SKUs for general engineering and plain-finish requirements.
• Nylon Lock Nuts (Nyloc) — 55 SKUs for non-safety-critical vibration-locking applications. See the comparison table above for when nyloc is and isn't appropriate.
• Lock Nuts — 24 SKUs covering serrated flange, all-metal lock and prevailing-torque variants.
• Spring Washers — for low-criticality applications where the engineering case for them is established.
• Torque Wrenches — 127 SKUs. Don't install castle nuts on suspension or steering without one.
• Pliers and Side Cutters — for installing and trimming cotter pins.
• Loctite — when you need chemical thread locking on plain nuts (always check whether OEM spec calls for mechanical locking instead).
• All Fasteners — full AIMS fastener range, 1,485 SKUs.

Trade accounts available for workshops, fleet operators, agricultural contractors and engineering firms — call our Milperra warehouse on (02) 9773 0122 or use our online account application. Same-day dispatch on in-stock items ordered before midday Sydney time.

Related guides

• Split Pin / Cotter Pin Guide — sizing, materials and install detail.
• Clevis Pin Guide — the sister fastener for shear-load applications.
• Metric Bolt Torque Chart — for working out the right torque on a Grade 8.8 / 10.9 castle nut joint.
• Bolt Grade Chart — matching nut grade to bolt grade.
• Screw Head Types Guide — companion reference.
• Roll Pin Guide — another pin family.
• Loctite 243 Guide — for the chemical-locking alternative on non-tapered joints.

Castle Nut Guide — FAQ

What is a castle nut?

A castle nut (also called a castellated or slotted nut) is a hex nut with six radial slots cut into the top face that resemble the crenellations of a castle wall. The slots accept a cotter pin (split pin) that passes through a transverse hole drilled in the bolt or stud, mechanically locking the nut against rotation. Castle nuts are specified for safety-critical joints in automotive suspension, steering, drive axles, agricultural equipment and marine applications — anywhere a nyloc or spring washer can't be trusted to hold under vibration and shock loading. The standard reference for metric castle nuts in Australia is DIN 935 / ISO 7035.

Should you tighten or loosen a castle nut to align with the cotter pin hole?

Tighten. Never loosen. This is the single most-violated castle nut rule in DIY automotive work. Forum consensus across Team Chevelle, Corvette Forum, the Mopar forums and MOOG Parts technical bulletins is unanimous: torque the nut to the manufacturer's specification, and if the castellation slot doesn't line up with the cotter pin hole, tighten further to the next slot. Loosening a tapered stud nut (ball joint, tie rod end) can let the taper slip, drop clamping force, and cause the joint to separate in service. Tightening past spec by a few degrees to reach the next slot is the accepted compromise.

What is DIN 935 and what sizes are available?

DIN 935 is the German industrial standard for hexagon slotted (castle) nuts; the equivalent ISO standard is ISO 7035. Metric castle nuts under DIN 935 are typically available from M5 through M64, in zinc-plated carbon steel (most common), A2/304 stainless and A4/316 marine-grade stainless. Up to and including M12 the slots are cut on one side only; above M12 the slots are cut through the crown. Thread pitch follows ISO Metric Screw Thread standards. Grade 8 (DIN 935-1) and lower strength variants are common — always match the nut grade to the bolt grade for the joint.

Can you reuse a cotter pin?

No. Cotter pins are single-use safety fasteners. The reason is metal fatigue: when you bend the legs to install, then unbend them to remove, the steel work-hardens at the bend points. It bends fine once, but the second time you fold those legs over they're brittle and can snap during install — or worse, fatigue and shear in service. Mechanic consensus on Yesterday's Tractors, ClassicBroncos and Reddit r/Mechanics is uniform: replace every time. They cost a few cents each and they're the last line of defence on a ball joint.

What size cotter pin goes with what castle nut?

Match the cotter pin diameter to the cross-drilled hole in the bolt or stud — not to the slot width. As a rule of thumb: M8 stud uses a 2.0–2.5mm pin; M10 stud uses 3.0mm; M12 uses 3.0–3.5mm; M14 uses 3.5mm; M16 uses 4.0mm; M20 uses 4.0–5.0mm; M24 uses 5.0–6.0mm. The pin should pass through the hole snugly with no significant side play. Always check the OEM workshop manual for the exact spec — vehicle manufacturers sometimes call up a smaller pin than the hole would accept, deliberately, to leave room for bending the legs.

Castle nut versus nyloc nut — which is better?

It depends on the application. Castle nut with cotter pin: mechanical lock, holds independently of friction, performs under heat, vibration and repeated loosening cycles, takes longer to install. Nyloc: faster install, but the nylon insert relies on friction — it degrades above ~120°C, loses grip after one or two install cycles, and is rated single-use for safety-critical joints. For steering, suspension, ball joints, drive axles, agricultural PTO and marine prop shafts, castle nut wins on safety grounds. For one-time assembly on lower-criticality joints (panel work, brackets, mid-vibration applications), nyloc is acceptable. It's almost always safe to replace a nyloc with a castle nut on a tapered-stud joint; the reverse is often not safe.

Why does the cotter pin go through the nut and the bolt — what stops the nut?

It's the bolt or stud that does the holding. The cotter pin passes through a hole drilled across the threaded shank of the bolt, with one castellation slot of the nut bridging the pin on each side. If the nut tries to rotate, the slot wall hits the pin and the rotation stops. The pin doesn't carry the joint clamp load — that's still the nut's job. The pin only stops the nut backing off once tension is established by torque.

Can a castle nut be torqued to a specific value, or do you just 'tighten and pin'?

Both, depending on the joint. Vehicle ball joints, tie rod ends and drive axles have specified torque values in the workshop manual (commonly 50–80 lb-ft / 68–108 Nm for passenger car ball joints, much higher for heavy axles). The procedure is: torque to spec, then if the slot doesn't align with the pin hole, tighten further to the next slot — never back off. For lower-criticality joints (some farm and trailer applications), 'tighten until snug then pin' is acceptable, but always check the OEM spec first.

What grade of castle nut do I use for a Grade 8.8 bolt?

Match the strength. For a Grade 8.8 metric bolt, use a Class 8 DIN 935 castle nut (often marked '8' on the flat face). For a Grade 10.9 bolt, use a Class 10 nut. Mismatching — particularly a low-grade nut on a high-strength bolt — can fail the threads in the nut under load, stripping out and dropping clamp force. The nut should always be at least as strong as the bolt.

What's the difference between a castle nut and a slotted nut?

Often used interchangeably, but technically there is a difference under the German DIN system. DIN 935 castle nut has a crown (raised hub) above the hex with the slots cut into the crown — taller nut, deeper engagement. DIN 937 'slotted nut' is shorter; the slots are cut directly into the top of the hex with no separate crown — thinner profile, used where height is constrained. ISO 7035 corresponds to the DIN 935 castle form; ISO 7036 corresponds to the DIN 937 slotted form. Most automotive applications specify DIN 935.

Can I use a castle nut without a cotter pin if I torque it correctly?

No, and this is exactly what NHTSA recalls keep flagging. A 2026 recall on Roush-modified Nissan Frontier Pro4X R vehicles, and a 2023 Daimler Truck recall on tie rod ends, both stemmed from missing cotter pins in the castle nut. Even at correct torque, the castle nut alone cannot survive sustained vibration cycles on a steering or suspension joint — that's literally the reason the nut has slots in it. The cotter pin is the safety device. No pin, no drive.

How do I install a cotter pin correctly — which legs bend where?

Insert the pin so the looped head sits flush against the side of the nut, with both legs protruding through the bolt hole on the other side. Then either: (a) the 'classic' bend — fold the longer leg back over the top of the bolt towards the centre, and bend the shorter leg flat against the side of the nut; or (b) the 'aviation' bend — wrap both legs around the bolt and nut so neither sticks out (used in aircraft and motorcycle racing to avoid snagging). Trim excess length so no leg projects more than the pin diameter. Never leave a sharp pin end pointing outward — it can catch on boots, clothing or wheel components.

Where do I find castle nuts and cotter pins in Australia?

AIMS Industrial stocks DIN 935 castle nuts (M5–M24+) in zinc-plated steel and 304/316 stainless, plus matched split pins (cotter pins) across the same size range — see our Cotter Pins collection and the broader Nuts range. For automotive workshop use we also stock Grade 8 and Grade 10 castle nuts for suspension and steering applications. Bulk and trade-account pricing available; same-day dispatch from our Milperra warehouse on in-stock items.