External retaining rings, also called external circlips, snap rings, or shaft retaining rings, are mechanical fasteners designed to hold components securely on a shaft or in a groove. They are commonly used in machinery, automotive assemblies, gearboxes, pumps, motors, bearings, and industrial equipment.
Unlike nuts, bolts, and washers, an external retaining ring does not normally create a threaded connection. Instead, it fits into a machined groove around a shaft and prevents components from moving axially.
Choosing the correct external retaining ring size requires more than matching the shaft diameter. Important dimensions include shaft diameter, ring inside diameter, outside diameter, thickness, groove diameter, groove width, and free diameter.
This complete guide explains external retaining ring sizes, dimensions, standards, materials, groove requirements, installation methods, applications, and how to select the correct snap ring for your shaft.
What Is an External Retaining Ring?
An external retaining ring is a circular metal ring designed to fit into a groove cut around the outside of a shaft.
Once installed, the ring creates a shoulder that prevents a component from moving past the groove.
External retaining rings are commonly used to retain:
- Bearings
- Gears
- Pulleys
- Wheels
- Bushings
- Washers
- Spacers
- Collars
- Rotating components
- Mechanical assemblies
The ring usually has a small opening that allows it to expand or contract during installation.
Many external retaining rings have two small lugs or ears with holes. These holes allow snap-ring pliers to open the ring for installation or removal.
How Does an External Retaining Ring Work?
An external retaining ring is installed into a groove machined around a shaft.
The basic arrangement consists of:
Shaft → Groove → Retaining Ring → Retained Component
When the ring is installed correctly, part of the ring projects outward from the groove. A component positioned against the ring cannot move beyond this shoulder.
For example, a bearing may be positioned on a shaft with an external retaining ring installed in a groove next to it. The ring prevents the bearing from moving along the shaft.
The retaining ring therefore provides axial positioning rather than clamping the component in the same way as a threaded nut.
Important External Retaining Ring Dimensions
Several dimensions are important when selecting an external retaining ring.
Shaft Diameter
Shaft diameter is one of the primary dimensions used to identify an external retaining ring.
The ring must be designed for the actual shaft diameter and groove configuration.
For example, a retaining ring specified for a 20 mm shaft should not automatically be substituted with a ring intended for a 19 mm or 22 mm shaft.
Ring Thickness
Ring thickness is the thickness of the metal strip forming the retaining ring.
A thicker ring can provide a larger retaining shoulder and may be designed for higher loads, but thickness must match the groove.
Free Outside Diameter
The free outside diameter is the outside diameter of the ring before it is installed on the shaft.
Because the ring is designed to expand during installation, its free dimensions may differ from its installed dimensions.
Inside Diameter
The inside diameter refers to the opening through the center of the retaining ring.
The installed ring must grip the shaft groove correctly without interfering with the shaft outside the groove.
Groove Diameter
The groove diameter is the diameter at the bottom of the machined groove.
This dimension is critical because the retaining ring sits inside this groove.
Groove Width
The groove width must be compatible with the retaining ring thickness.
A groove that is too narrow can prevent proper seating, while an excessively wide groove can allow unwanted axial movement.
Radial Wall Height
Radial height refers to how far the ring extends outward from the shaft groove.
This creates the shoulder that retains the component.
How to Read an External Retaining Ring Size
External retaining rings may be specified according to the nominal shaft diameter or a manufacturer’s ring designation.
For example, a metric retaining ring may be associated with:
20 mm shaft
This does not mean every dimension of the ring is exactly 20 mm.
Instead, the 20 mm designation generally identifies the shaft size or application for which the ring is intended.
The complete specification may include:
- Nominal shaft diameter
- Ring thickness
- Groove diameter
- Groove width
- Free inside diameter
- Free outside diameter
- Material
- Standard
Always check the manufacturer’s dimensional drawing when an exact replacement is required.
Metric External Retaining Ring Size Chart
The following chart provides a general reference for common external retaining ring shaft sizes.
| Nominal Shaft Size | Approx. Shaft Diameter | Common Application |
|---|---|---|
| 3 mm | 3 mm | Small mechanisms |
| 4 mm | 4 mm | Small equipment |
| 5 mm | 5 mm | Machinery |
| 6 mm | 6 mm | Small shafts |
| 8 mm | 8 mm | Motors and equipment |
| 10 mm | 10 mm | Machinery |
| 12 mm | 12 mm | Mechanical assemblies |
| 15 mm | 15 mm | Bearings and shafts |
| 16 mm | 16 mm | Industrial equipment |
| 18 mm | 18 mm | Mechanical assemblies |
| 20 mm | 20 mm | Bearings and machinery |
| 22 mm | 22 mm | Machinery |
| 25 mm | 25 mm | Shafts and gear assemblies |
| 30 mm | 30 mm | Industrial equipment |
| 35 mm | 35 mm | Heavy machinery |
| 40 mm | 40 mm | Large mechanical assemblies |
| 45 mm | 45 mm | Industrial machinery |
| 50 mm | 50 mm | Heavy-duty equipment |
Important: This is a nominal size reference rather than a universal dimensional standard. Exact ring thickness, groove dimensions, free diameter, and retaining capacity depend on the applicable standard and ring series.
Imperial External Retaining Ring Size Chart
External retaining rings are also commonly available for inch-based shafts.
| Nominal Shaft Size | Shaft Diameter | Typical Application |
|---|---|---|
| 1/8 in | 0.125 in | Small mechanisms |
| 3/16 in | 0.188 in | Small machinery |
| 1/4 in | 0.250 in | General equipment |
| 5/16 in | 0.313 in | Mechanical assemblies |
| 3/8 in | 0.375 in | Shafts and gears |
| 7/16 in | 0.438 in | Machinery |
| 1/2 in | 0.500 in | Bearings and shafts |
| 5/8 in | 0.625 in | Industrial equipment |
| 3/4 in | 0.750 in | Machinery |
| 7/8 in | 0.875 in | Mechanical assemblies |
| 1 in | 1.000 in | General machinery |
| 1-1/4 in | 1.250 in | Larger equipment |
| 1-1/2 in | 1.500 in | Industrial applications |
| 2 in | 2.000 in | Heavy machinery |
The exact ring dimensions should always be checked against the relevant retaining-ring standard.
External Retaining Ring vs Internal Retaining Ring
The biggest difference is where the ring is installed.
An external retaining ring fits around the outside of a shaft.
An internal retaining ring fits inside a bore or housing.
| Feature | External Retaining Ring | Internal Retaining Ring |
|---|---|---|
| Installation location | Shaft | Bore |
| Main purpose | Prevents axial movement on shaft | Prevents axial movement in housing |
| Groove location | Outside shaft | Inside bore |
| Common application | Bearings and gears | Bearings and housings |
| Installation action | Ring expands | Ring contracts |
The two types should not normally be substituted for each other.
External Retaining Ring vs E-Clip
An E-clip is another type of external shaft retaining ring, but its design differs from a conventional circlip.
A conventional external retaining ring is generally a nearly complete circular ring with installation holes or lugs.
An E-clip has an E-shaped profile and can often be pushed directly into a shaft groove without requiring traditional circlip pliers.
E-clips are commonly used on:
- Small shafts
- Motors
- Appliances
- Automotive components
- Small machinery
- Consumer products
The appropriate type depends on the shaft design and groove.
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External Retaining Ring Standards
Several standards and ring series are used in retaining-ring manufacturing.
Common standards and specifications include:
- ANSI retaining-ring standards
- ASME specifications
- DIN standards
- Manufacturer-specific series
- Metric retaining-ring standards
- Inch retaining-ring standards
A retaining ring’s nominal shaft size does not by itself define every dimension.
When precision is important, identify the standard, series, and manufacturer specification in addition to the nominal diameter.
How to Measure an External Retaining Ring
If you need to identify an existing ring, measure several dimensions.
Step 1: Measure the Shaft
Use a caliper to measure the shaft diameter.
Measure an unworn section of the shaft if possible.
Step 2: Measure Ring Thickness
Measure the thickness of the ring with a micrometer or caliper.
Step 3: Measure Ring Outside Diameter
Measure the ring’s outside diameter in its free condition if it has been removed.
Step 4: Measure the Groove
Measure:
- Groove diameter
- Groove width
- Groove location
Step 5: Check the Ring Profile
Determine whether it is:
- Standard circlip
- E-clip
- Spiral retaining ring
- Heavy-duty retaining ring
- Other specialized design
Step 6: Identify the Material
Look for markings or manufacturer information if available.
External Retaining Ring Groove Dimensions
The groove is just as important as the ring itself.
A properly designed groove should have the correct:
- Diameter
- Width
- Depth
- Edge geometry
- Location
The groove must provide enough space for the ring to seat completely.
If the groove is too shallow, the ring may not fully engage.
If the groove is too deep, the retaining ring may sit too far below the shaft surface and have insufficient retaining shoulder.
Why Groove Diameter Matters
The groove diameter determines how the retaining ring interacts with the shaft.
The ring is designed to sit securely in the groove while remaining sufficiently exposed to retain the component.
Incorrect groove diameter can cause:
- Ring movement
- Excessive play
- Ring failure
- Component movement
- Difficulty installing the ring
For this reason, the groove should be machined according to the retaining-ring manufacturer’s specifications.
How to Choose the Correct External Retaining Ring
Selecting the right ring involves more than matching the shaft diameter.
1. Measure the Shaft
Determine the nominal shaft diameter.
2. Identify the Groove
Check whether the shaft already has a groove and measure its dimensions.
3. Determine the Load
Consider the axial force the ring must resist.
4. Choose the Ring Series
Select a suitable standard or manufacturer series.
5. Check Ring Thickness
Make sure the ring thickness matches the groove width.
6. Check Groove Diameter
Verify the groove diameter against the ring specification.
7. Check Component Clearance
Make sure the retained component can properly contact the ring.
8. Select the Material
Choose a suitable material and finish for the environment.
External Retaining Ring Load Capacity
Retaining ring capacity depends on the entire assembly rather than the ring alone.
Important factors include:
- Ring material
- Ring thickness
- Shaft material
- Groove geometry
- Groove depth
- Shaft diameter
- Component geometry
- Axial loading
- Shock loading
- Rotation
- Operating temperature
A ring may fail because of deformation, fracture, groove failure, or shaft damage.
For high-load applications, use the manufacturer’s published load ratings and engineering data.
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External Retaining Rings for Bearings
One of the most common applications is retaining a bearing on a shaft.
A typical arrangement may be:
Shaft → Bearing → Spacer → External retaining ring
The retaining ring provides a mechanical stop that prevents the bearing from moving axially.
When selecting a ring for a bearing application, consider:
- Shaft diameter
- Bearing width
- Groove location
- Axial load
- Rotation
- Temperature
- Ring material
- Groove dimensions
The retaining ring should be designed so that the bearing properly contacts the retaining shoulder.
External Retaining Rings for Gears and Pulleys
Retaining rings are frequently used to position gears, pulleys, wheels, and other rotating components.
They can provide a compact alternative to a threaded nut and washer arrangement when the shaft has a suitable retaining groove.
Applications include:
- Gearboxes
- Electric motors
- Conveyors
- Power transmission equipment
- Pumps
- Mechanical drives
The ring must be suitable for the rotational speed and axial loading of the assembly.
External Retaining Ring Materials
Retaining rings are manufactured from materials selected for spring properties, strength, and environmental resistance.
Carbon Spring Steel
Spring steel is commonly used because it provides the elasticity required for installation and retention.
Stainless Steel
Stainless steel retaining rings are useful where corrosion resistance is important.
Applications may include:
- Outdoor machinery
- Marine equipment
- Food-processing equipment
- Moist environments
- Corrosive conditions
Heat-Treated Steel
Heat-treated steel rings can provide improved mechanical properties for demanding applications.
The correct material should be selected based on the manufacturer’s specifications and operating environment.
External Retaining Ring Finishes
Retaining rings may have different finishes or surface treatments.
Common options include:
- Black oxide
- Phosphate
- Zinc coating
- Stainless steel finish
- Other protective treatments
A corrosion-resistant finish may be important when the ring is exposed to moisture, chemicals, outdoor conditions, or elevated humidity.
How to Install an External Retaining Ring
Proper installation is essential to prevent ring failure.
Step 1: Inspect the Groove
Make sure the groove is clean and free from burrs.
Step 2: Check the Ring
Inspect the ring for:
- Cracks
- Distortion
- Corrosion
- Damage
- Incorrect dimensions
Step 3: Use Correct Pliers
For rings with installation holes, use external retaining-ring pliers.
Step 4: Expand the Ring Carefully
Open the ring only enough to pass over the shaft.
Do not over-expand it.
Step 5: Position the Ring
Align the ring with the shaft groove.
Step 6: Release the Ring
Allow the ring to contract into the groove.
Step 7: Verify Seating
Rotate or inspect the ring to ensure it is completely seated around the groove.
Never assume that the ring is installed correctly simply because it appears to be in the groove.
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Common External Retaining Ring Installation Mistakes
Over-Expanding the Ring
Excessive expansion can permanently deform the ring and reduce its retaining force.
Using the Wrong Pliers
Incorrect pliers can damage the ring or cause it to slip during installation.
Ignoring the Groove
A damaged or incorrectly sized groove can cause premature failure.
Installing a Damaged Ring
Retaining rings should not be reused if they are cracked, distorted, excessively corroded, or otherwise damaged.
Using the Wrong Size
A ring that does not match the shaft and groove dimensions may not seat correctly.
External Retaining Ring Removal
Removal should also be performed carefully.
For a conventional external circlip:
- Place external retaining-ring pliers into the ring holes.
- Expand the ring only enough to clear the groove.
- Carefully remove the ring from the shaft.
- Inspect the ring and groove.
Some rings can be difficult to remove because of corrosion, contamination, or limited access.
Avoid using excessive force or improvised tools that can damage the shaft groove.
Can External Retaining Rings Be Reused?
Whether a retaining ring can be reused depends on its design and the manufacturer’s recommendations.
During removal, the ring may be expanded beyond its intended operating condition.
Repeated expansion can change its dimensions or spring properties.
For critical applications, replacing the retaining ring after removal is often the safer approach, particularly if the manufacturer specifies single-use installation.
Always inspect the ring before reuse.
Common External Retaining Ring Problems
Ring Pops Out of the Groove
Possible causes include:
- Incorrect ring size
- Incorrect groove dimensions
- Excessive axial load
- Incomplete seating
- Ring deformation
Ring Breaks
Possible causes include:
- Excessive stress
- Corrosion
- Incorrect installation
- Wrong material
- Over-expansion
Component Has Excessive Axial Movement
Possible causes include:
- Incorrect groove width
- Incorrect ring thickness
- Improper component fit
- Worn shaft or groove
Ring Is Difficult to Install
Possible causes include:
- Incorrect ring size
- Burrs in the groove
- Damaged ring
- Wrong installation tool
External Retaining Ring Applications
External retaining rings are used across many industries.
Common applications include:
- Automotive systems
- Electric motors
- Gearboxes
- Pumps
- Hydraulic equipment
- Industrial machinery
- Agricultural equipment
- Power tools
- Bearings
- Shafts
- Gears
- Pulleys
- Wheels
- Conveyors
- Manufacturing equipment
Their compact design makes them useful where space is limited and a threaded retaining nut would be inconvenient.
External Retaining Ring Size Chart Quick Reference
| Nominal Shaft | Common System | Typical Application |
|---|---|---|
| 3 mm | Metric | Small mechanisms |
| 4 mm | Metric | Small equipment |
| 5 mm | Metric | Machinery |
| 6 mm | Metric | Small shafts |
| 8 mm | Metric | Motors |
| 10 mm | Metric | Machinery |
| 12 mm | Metric | Mechanical assemblies |
| 15 mm | Metric | Bearing assemblies |
| 16 mm | Metric | Industrial equipment |
| 18 mm | Metric | Mechanical assemblies |
| 20 mm | Metric | Bearings and machinery |
| 25 mm | Metric | Gear assemblies |
| 30 mm | Metric | Industrial machinery |
| 40 mm | Metric | Large assemblies |
| 50 mm | Metric | Heavy equipment |
| 1/4 in | Imperial | General equipment |
| 3/8 in | Imperial | Shafts and gears |
| 1/2 in | Imperial | Bearings |
| 5/8 in | Imperial | Industrial equipment |
| 3/4 in | Imperial | Machinery |
| 1 in | Imperial | General machinery |
| 1-1/4 in | Imperial | Large equipment |
| 1-1/2 in | Imperial | Industrial applications |
| 2 in | Imperial | Heavy machinery |
Note: The table represents nominal shaft sizes commonly associated with external retaining rings. It does not provide universal ring thickness, groove depth, groove width, outside diameter, or load ratings. Those dimensions must be selected according to the specific retaining-ring standard and manufacturer.
Frequently Asked Questions About External Retaining Ring Sizes
What is an external retaining ring?
An external retaining ring is a mechanical fastener installed in a groove around a shaft to prevent components from moving axially. It is commonly used to retain bearings, gears, pulleys, wheels, spacers, and other mechanical components.
What are the common external retaining ring sizes?
External retaining rings are available for many metric and imperial shaft diameters. Common metric sizes include 5, 6, 8, 10, 12, 15, 16, 20, 25, 30, and 40 mm. Common imperial sizes include 1/4, 3/8, 1/2, 3/4, and 1 inch.
How do I measure an external retaining ring?
Measure the shaft diameter, ring thickness, ring inside and outside diameters, groove diameter, and groove width. If the ring has been removed, measuring its free dimensions can help identify the correct size, but the original shaft and groove dimensions should also be checked.
What is the difference between an external and internal retaining ring?
An external retaining ring fits around a shaft, while an internal retaining ring fits inside a bore. External rings retain components on shafts, whereas internal rings retain components inside housings or bores.
What size retaining ring do I need for a 20 mm shaft?
For a 20 mm shaft, you generally need an external retaining ring specified for a 20 mm shaft, but the shaft groove must also match the ring’s required diameter, width, and depth. Always verify the manufacturer’s dimensional drawing before selecting a replacement.
Can an external retaining ring be reused?
Some retaining rings may technically be reusable, but removal can alter the ring’s spring properties or shape. For important mechanical applications, follow the manufacturer’s reuse recommendations and replace rings that are damaged, distorted, corroded, or excessively expanded.
What tools are used to install external retaining rings?
External retaining rings with installation holes are normally installed using external retaining-ring pliers. The pliers expand the ring so it can pass over the shaft and then allow it to contract into the groove.
Why does an external retaining ring keep coming out?
A retaining ring can come out because of an incorrect ring size, incorrect groove dimensions, incomplete seating, excessive axial loading, ring deformation, or installation damage. Inspect both the ring and groove to identify the underlying problem.
Do retaining rings have load ratings?
Yes, retaining rings can have rated capacities, but the allowable load depends on the ring design, material, shaft, groove geometry, component, and loading conditions. Manufacturer engineering data should be used when the retaining ring is exposed to significant axial forces.
Are external retaining rings used on bearings?
Yes. External retaining rings are commonly used to position bearings on shafts and prevent axial movement. The ring and shaft groove must be properly designed for the bearing load, operating conditions, and required axial retention.
Final Thoughts
An external retaining ring size chart provides a convenient starting point when selecting a snap ring for a shaft, but the nominal shaft diameter is only one part of the complete specification.
The most important dimensions include shaft diameter, ring thickness, inside diameter, outside diameter, groove diameter, groove width, and groove depth. Material, load, operating temperature, corrosion exposure, and installation method are also important.
Common metric retaining-ring sizes range from small shafts such as 3 mm and 4 mm to larger industrial shafts of 40 mm, 50 mm, and beyond. Imperial retaining rings are similarly available in sizes ranging from small fractional shafts to large industrial diameters.
For the best results, always match the retaining ring to the shaft groove and applicable standard, rather than choosing a ring based solely on its nominal diameter. Proper groove dimensions and complete seating are essential for reliable axial retention.
For high-load, high-speed, safety-critical, or industrial applications, use the manufacturer’s engineering specifications for ring capacity, groove dimensions, allowable loads, installation procedures, and material selection.
Engineer Hassan is a Mechanical Engineer with 16+ years of hands-on experience in mechanical hardware, fasteners, and workshop tools. He focuses on size selection, thread identification, and measurement, helping users choose the correct components with accuracy and safety.