Split pins, or cotter pins, hold parts in place and stop movement. They are used in machines, vehicles, and tools. Sometimes, people use alternatives if pins are missing or don’t fit. This guide explains safe options, proper use, and when not to replace a split pin. It also covers materials, sizes, standards, and key safety tips.
What Is a Split Pin?
A split pin, also known as a cotter pin, is a two-pronged metal fastener that locks parts in place after insertion. It is placed through a pre-drilled hole in a bolt, shaft, or clevis pin. After insertion, the two prongs are bent in opposite directions to prevent the pin from backing out during operation.
Split pins serve as secondary locking devices. They are not designed to carry mechanical loads. Instead, they act as a fail-safe to prevent components like castle nuts, hinged linkages, or pivot pins from rotating or slipping off during use. Once deformed, a split pin becomes work-hardened and must be discarded after removal.
Technical Specifications
Common Materials and Their Properties
| Material | Key Properties | Typical Applications |
|---|---|---|
| Mild Steel | Soft, easily bent, low-cost | General-purpose mechanical assembly |
| Stainless Steel (A2/A4) | Corrosion-resistant, high durability | Marine, chemical plants, outdoor equipment |
| Brass | Corrosion-resistant, non-magnetic, soft | Low-load, electrical, decorative assemblies |
⚠️ Bronze is not commonly used in split pin manufacturing. Most standard applications specify mild or stainless steel due to availability and strength characteristics.
Size Ranges and Dimensional Standards
- Metric Sizes: M1 to M18 (diameter in millimeters)
- Imperial Sizes: 1/32″ to 1/2″ (diameter)
- Lengths: Range from 6 mm to 120 mm
- Tolerances: Based on shaft hole clearance; pins should fit snugly without excessive force
Relevant Standards
| Standard | Region / Scope | Description |
| DIN EN ISO 1234 | Global / European Standard | Current ISO-compliant design for cotter pins |
| BS 7774 | British Standard (UK) | Replaces outdated BS 1574 |
These standards define dimensions, tolerances, and material specifications for both metric and imperial cotter pins. Always use certified components in regulated industries.
Load and Shear Capacity Clarification
Split pins do not carry primary loads. Their function is to prevent movement in threaded or slotted systems after initial clamping forces are applied by other fasteners (e.g., castle nuts).
- Shear Strength: Not rated. Manufacturers do not specify shear loads for split pins.
- Fatigue Considerations: Bending causes plastic deformation and introduces micro-cracks. Reuse increases the risk of failure.
Conclusion: Always use split pins as secondary locks only, and replace them after each removal to maintain mechanical integrity.
Types of Split Pins
| Type | Characteristics | Common Applications |
| Standard | Two equal tines, straight design | General-purpose uses, widely available |
| Extended Prong | One leg longer for easier bending and grip | Tight spaces or assembly-line setups |
| Hammer Lock | Designed for installation by hammer strike | High-speed production, pre-aligned holes |
| Half-Round Section | One flat, one curved leg for added tension | Rotating machinery or equipment under vibration |
| Tapered End | Narrow tip eases insertion into close-tolerance holes | Automotive hubs, shaft retaining components |
Some manufacturers also produce grooved or slotted variants for specialized equipment.
Installation and Removal Techniques
Installation Steps
- Select the correct diameter and length based on hole and component thickness.
- Align the holes in the components to ensure smooth insertion.
- Insert the split pin fully through the hole.
- Using pliers, bend both prongs in opposite directions, usually at least 90 degrees.
- Trim the ends if necessary to prevent interference with moving parts.
Proper installation ensures that the split pin stays in place during operation and avoids wear or noise from vibration.
Recommended Tools
- Needle-nose Pliers: For bending and fine control
- Cotter Pin Puller: For clean removal without damaging the hole
- Side Cutters: For trimming excess pin length
- Lubricant: For removing corroded or seized pins
Removal Methods
- Straighten both prongs using pliers or a flat screwdriver.
- Pull pin out with cotter pin puller or strong grip pliers.
- For corroded pins, apply penetrating oil and wait before pulling.
Never reuse removed pins. Internal stress from bending weakens the metal.
Typical Applications of Split Pins
| Assembly Component | Function of the Split Pin |
| Castle Nuts | Prevents nut rotation via slot and pin interlock |
| Clevis Pins | Holds linkage shafts in place |
| Wheel Bearings | Prevents axial movement of axle nuts |
| Marine Hinges | Corrosion-resistant securement of moving joints |
| Farm Machinery | Locks large pins in rotating or vibrating environments |
Split pins are standard in automotive, aerospace, marine, construction, HVAC, and agricultural industries. Failure to use correct pins in these applications can lead to serious equipment malfunction or accidents.
Split Pin Alternatives
In emergency or non-critical applications, temporary substitutes may be used. However, these should never replace split pins in safety-critical or regulatory environments.
1. Steel Nails
- Choose nails close in diameter to the pin hole.
- Insert and bend as with a split pin.
- Only suitable for low-vibration, non-rotating components.
2. Steel Wire (Not Copper)
- Use high-tensile steel wire (≥ 1.5 mm).
- Loop through the hole and twist ends tightly.
- Avoid using electrical or soft wires.
3. Bicycle Wheel Spoke
- Strong and rust-resistant.
- Can be cut and bent to shape with hand tools.
- Works in emergency roadside or field repair.
4. Umbrella Rib (Spring Steel)
- Lightweight and flexible.
- Provides temporary hold but limited long-term durability.
5. Locking Nuts (Nyloc / Jam Nut)
- Only usable with threaded bolts.
- Nyloc nuts resist vibration using a nylon collar.
- Jam nuts work by friction between stacked nuts.
These do not lock shafts or clevis pins. Use only where threads are available.
Check it out – How to Lock a Nut on a Threaded Rod
Unsafe Substitutes to Avoid
| Substitute | Risks |
| Cable Ties | Plastic degrades, breaks under stress |
| Electric Wire | Too soft, poor holding capacity |
| Hair Pins | Weak, easily displaced |
| Welding | Irreversible, complicates future disassembly |
Where Split Pin Alternatives Should NOT Be Used
Avoid substitutes in:
- Rotating Machinery: Flywheels, crankshafts, or gearboxes
- High-Vibration Areas: Engines, driveshafts, or transportation equipment
- Critical Safety Systems: Brakes, suspensions, lifting arms
- Extreme Weather/Corrosion: Marine decks, acid environments, high heat
Use only certified split pins that match standard specifications for the given industry.
Troubleshooting and Maintenance
| Problem | Solution |
|---|---|
| Split pin stuck | Apply penetrating oil and gently extract |
| Hole too large/damaged | Replace mating part or insert larger diameter pin |
| Corroded or rusted pin | Switch to stainless steel (A4 grade preferred) |
| Cannot bend prongs | Trim or reposition with appropriate pliers |
Always inspect the fastened area for wear after removal. Reinstall only after ensuring the components are not damaged.
Cost Comparison: Split Pins vs. Substitutes
| Fastener Type | Average Unit Cost | Reusable? | Long-Term Safety | Best Use Case |
|---|---|---|---|---|
| Split Pin | $0.02–$0.15 | ❌ | ✅ | Any safety or mechanical component |
| Nail | $0.01–$0.10 | ❌ | ❌ | Temporary fixes in static locations |
| Steel Wire | $0.01–$0.05/ft | ❌ | ❌ | Light-duty backups |
| Bicycle Spoke | Free | ❌ | ❌ | DIY emergency repairs |
| Locking Nut | $0.05–$0.30 | ✅ | ✅ | Threaded components only |
Legal, Warranty, and Safety Regulations
| Aspect | Details |
|---|---|
| Standards Compliance | Use DIN EN ISO 1234 or BS 7774 certified pins |
| Warranty Validity | Use of non-compliant substitutes may void warranties |
| Insurance Coverage | Damage caused by improper fasteners may lead to denied claims |
| Safety Inspections | Substitutes may cause a failure in ISO/CE/FAA safety audits |
Always document the use of correct fasteners during maintenance for audit and compliance.
Historical Note
The modern split pin was invented by Ira J. Young in 1912 in St. Louis, Missouri. It was designed to provide a cheap and efficient method to mechanically secure parts under vibration. Over 100 years later, it remains a staple in mechanical and structural assemblies worldwide.
Conclusion
Split pins are small but crucial components. They prevent catastrophic failure by stopping bolts and pins from shifting out of place. While alternatives can provide temporary or low-risk fixes, they are never a full replacement in load-bearing or critical safety applications.
- Always match the pin to the hole diameter and material requirements.
- Never reuse a split pin.
- Avoid substitutes in high-risk environments.
- Follow standards (DIN EN ISO 1234, BS 7774) to ensure reliability and compliance.
Questions or feedback? Share them below, and make sure to use the right fastener for the right job. Your safety depends on it.
Read Also – How to Use a Hitch Pin