ASME B18.21.1 Internal Tooth Lock Washers: Selection, Applications & Sourcing

ASME B18.21.1 internal tooth lock washers are toothed washers designed with serrations projecting inward around the washer's inside diameter. 

They are commonly used beneath screw heads, bolt heads, or nuts where a compact toothed interface is required.

Also called internal star washers, internal toothed lock washers, or internal serrated washers, these components differ from external tooth lock washers primarily in the location of the teeth.

With an internal tooth design, the serrations remain largely within the fastener's bearing area. 

This makes the configuration useful where engineers require a relatively compact washer outside diameter or prefer the teeth to remain less exposed around the fastener head.

Typical applications can include:

• Electrical equipment

• Control cabinets

• Instrumentation

• Appliances

• Automotive electrical assemblies

• Small motors

• Industrial automation

• Electronic equipment

• Mechanical brackets

• General industrial assemblies

However, an internal tooth lock washer should not automatically be treated as a universal solution for vibration-induced fastener loosening.

 Its performance depends on the complete bolted joint, including preload, materials, surface hardness, coatings, friction and external loading.

What Is an ASME B18.21.1 Internal Tooth Lock Washer?

ASME B18.21.1 is an important reference for lock washers used with inch-series fasteners in North American industrial applications.

For engineers and procurement teams, identifying the applicable standard is important because the description "internal tooth washer" alone may not sufficiently define the required component.

An RFQ or purchasing specification should identify the required standard, size, material and finish rather than relying only on a photograph or generic washer description.

This becomes particularly important when qualifying a second source for an existing OEM assembly.

How Does an Internal Tooth Lock Washer Work?

An internal tooth lock washer contains multiple inward-facing teeth around the inside diameter.

During tightening, the washer is compressed beneath the bearing surface of the fastener.

Depending on the joint materials and surface conditions, the teeth may:

• Deform during assembly

• Create localized contact pressure

• Engage the fastener bearing surface

• Engage the mating surface

• Increase resistance to rotational movement

• Penetrate certain surface films or coatings

• Create localized metal contact in suitable electrical assemblies

The result is different from the behavior of a conventional flat washer.

A flat washer primarily provides bearing-area and load-distribution functions. A toothed washer introduces localized tooth engagement at the joint interface.

That distinction matters when engineers are deciding which washer belongs in an assembly.

Internal Tooth vs External Tooth Lock Washers

One of the most common engineering searches is:

Internal tooth lock washer vs external tooth lock washer — what is the difference?

Both use serrated teeth, but the geometry changes how the washer interacts with the available bearing area.

Internal Tooth Lock Washer

The teeth project inward.

This configuration may be useful when:

• A compact external washer diameter is preferred

• The fastener head provides sufficient coverage

• The teeth should remain largely underneath the fastener head or nut

• The surrounding assembly has limited radial clearance

• A visually cleaner outside profile is desirable

External Tooth Lock Washer

The teeth extend outward around the outside diameter.

This configuration provides tooth contact at a larger effective diameter and can be useful where a broader toothed contact pattern is appropriate.

External tooth washers are also frequently considered in electrical bonding and grounding assemblies where multiple exposed contact points are desirable.

Neither configuration is universally better.

The correct choice depends on:

• Fastener head diameter

• Washer outside diameter

• Available installation space

• Mating material

• Surface hardness

• Coating

• Electrical requirements

• Joint loading

• Required contact pattern

For additional comparison, engineers should also review our External Tooth Lock Washer Solutions.

Why Internal Tooth Washers Are Useful in Compact Assemblies

The primary geometric advantage of an internal tooth washer is not simply "better locking."

It is where the teeth are located.

Because the teeth point inward, the external profile of the washer can remain relatively compact compared with designs where teeth project outward.

This can matter in assemblies involving:

• Small screw heads

• Closely spaced fasteners

• Electrical terminals

• Instrument housings

• Compact brackets

• Control components

• Small motors

• Electronic equipment

In these applications, available radial space may be as important as the nominal screw size.

This is why washer selection should consider the entire fastener envelope rather than only the thread diameter.

ASME B18.21.1 and Inch-Series Fastener Sourcing

For North American OEM and industrial supply chains, the distinction between metric and inch-series hardware is commercially important.

A purchasing request may specify an inch-series screw together with an ASME B18.21.1 internal tooth lock washer.

Procurement teams should therefore verify:

• Applicable standard

• Washer style

• Nominal fastener size

• Material

• Finish

• Dimensional requirements

• Customer drawing requirements

A washer that appears visually similar may not necessarily meet the required dimensional specification.

For supplier qualification, the standard designation should be treated as part of the technical requirement rather than merely a product name.

Does an Internal Tooth Lock Washer Prevent Fastener Loosening?

It can increase rotational resistance in suitable assemblies, but its actual effectiveness depends on the joint.

Fastener loosening can be influenced by:

• Initial preload

• Transverse joint movement

• Joint stiffness

• Fastener stiffness

• Surface friction

• Lubrication

• Bearing-surface hardness

• Coating

• Temperature cycling

• Vibration amplitude

• External loading

The washer's teeth can provide localized mechanical engagement, but this should not be interpreted as a guarantee against self-loosening in every application.

For safety-critical assemblies or severe transverse-vibration conditions, engineers should evaluate the complete locking strategy and validate the joint under representative operating conditions.

Alternative or additional locking methods may need to be considered depending on the application.

Why Surface Hardness Affects Tooth Engagement

The teeth must interact with the surfaces around them.

That makes surface hardness an important design consideration.

A relatively soft mating surface may allow greater tooth penetration.

A harder surface may produce different engagement behavior.

Engineers should therefore evaluate the relationship between:

Washer Teeth → Fastener Bearing Surface → Mating Surface

Changes to any of these elements can change joint behavior.

For example, replacing a plated carbon steel component with a hardened stainless component may change how the teeth interact with the assembly.

This is why visual similarity alone is not enough when approving a replacement washer.

Why Surface Coatings Matter

Coatings influence both friction and tooth engagement.

Common surface conditions can include:

• Zinc plating

• Zinc-nickel coating

• Zinc-flake coating

• Black oxide

• Phosphate-based finishes

• Paint

• Powder coating

• Conversion coatings

• Natural oxide layers

A tooth washer may penetrate certain coatings under sufficient assembly pressure, but the result depends on the coating system.

Coating thickness and mechanical properties can affect:

• Tooth engagement

• Friction

• Surface damage

• Corrosion protection

• Electrical contact

For this reason, a coating should not be specified only for appearance.

It is part of the functional joint design.

Internal Tooth Lock Washers in Electrical Assemblies

Internal star washers are frequently associated with electrical and electromechanical assemblies because their teeth can create localized contact points.

Potential applications include:

• Electrical cabinets

• Control panels

• Instrumentation

• Terminal assemblies

• Electrical enclosures

• Appliances

• Small motors

• Power supplies

• Automation equipment

Where electrical continuity or bonding is required, engineers must evaluate whether the washer can establish and maintain the required conductive path.

Factors include:

• Washer material

• Fastener material

• Mating material

• Surface coating

• Contact pressure

• Corrosion

• Environmental exposure

• Assembly torque

A toothed washer by itself does not guarantee an acceptable grounding or bonding connection.

Where electrical safety or compliance depends on the connection, the completed assembly should be tested according to the applicable requirements.

Internal Tooth Washers for Control Cabinets and Automation Equipment

Control cabinets and industrial automation equipment contain numerous small mechanical and electrical connections.

Typical assemblies can involve:

• Controllers

• Relays

• Terminal equipment

• Sensors

• Power supplies

• Mounting brackets

• Instrumentation

• Electrical enclosures

An internal tooth washer can be useful where a compact toothed interface is required beneath a screw head or nut.

For OEM production, however, consistency becomes important.

Variation in:

• Tooth geometry

• Washer thickness

• Material hardness

• Coating thickness

• Flatness

• Overall dimensions

can influence how the washer behaves during automated or manual assembly.

Procurement should therefore treat small washers as controlled engineering components when their function is important to the finished equipment.

Automotive and EV Electrical Applications

Modern vehicles contain increasing numbers of electronic modules, sensors, actuators, control units and electrical connections.

Potential internal tooth washer applications can include:

• Electrical modules

• Small motor assemblies

• Instrumentation

• Brackets

• Interior mechanisms

• Auxiliary electrical equipment

• Enclosures

• Control assemblies

EV and hybrid platforms also contain extensive electrical and power-electronics systems.

Where a toothed washer is specified, its suitability should be based on the OEM or Tier supplier drawing, validation requirements and approved joint design.

A generic internal star washer should not be assumed to meet automotive requirements simply because the geometry appears similar.

Industrial Machinery Applications

Internal tooth washers may also be used in industrial machinery for compact mechanical or electrical fastening points.

Examples can include:

• Motors

• Instrument housings

• Sensors

• Control components

• Machine enclosures

• Electrical boxes

• Small brackets

• Auxiliary equipment

Where equipment is exposed to continuous vibration, the locking method should be validated according to the actual service condition.

Electronics and Instrumentation

Compact geometry can make internal tooth washers particularly relevant to smaller assemblies.

Potential uses include:

• Instrument panels

• Measuring equipment

• Electronic housings

• Power supplies

• Connector assemblies

• Small electromechanical devices

The relatively contained tooth profile can be useful where external radial clearance is limited.

For sensitive electronics, engineers should also evaluate whether tooth penetration, metallic debris, coating damage or surface marking is acceptable.

Materials for Internal Tooth Lock Washers

Material selection should be based on mechanical, environmental and electrical requirements.

Carbon or Spring Steel

Steel internal tooth washers are commonly used where strength, tooth resilience and cost efficiency are important.

Depending on the application, a protective surface finish may be applied for corrosion resistance.

Potential applications include:

• Industrial equipment

• Electrical assemblies

• Appliances

• Automotive components

• General machinery

Stainless Steel

Stainless steel may be selected where increased corrosion resistance is required.

Potential environments include:

• Outdoor equipment

• Food-service equipment

• Medical equipment

• Marine-related systems

• HVAC equipment

• Humid industrial environments

The required stainless grade should be specified rather than relying on the generic term "stainless steel."

Specialty Materials

Some applications may require alternative materials because of:

• Electrical conductivity

• Corrosion environment

• Temperature

• Magnetic requirements

• Customer specifications

Where materials such as copper alloys or nickel-based alloys are required, the exact grade and functional requirement should be stated on the drawing or RFQ.

Surface Finish Selection

Possible surface treatments depend on washer material and customer requirements.

Options may include:

• Zinc-based coatings

• Zinc-nickel systems

• Zinc-flake systems

• Black oxide

• Phosphate-based finishes

• Passivation for appropriate stainless materials

• Customer-specified coatings

Selection should consider:

• Corrosion resistance

• Electrical conductivity

• Coating thickness

• Tooth geometry

• Fastener coating

• Mating material

• Friction

• Environmental exposure

• Hydrogen-embrittlement risk where applicable

If the washer forms part of an electrical contact path, coating selection deserves particular attention.

Are Internal Tooth Lock Washers Reusable?

They should not automatically be specified as reusable components.

During installation, the teeth can deform and create localized impressions in the mating surfaces.

After disassembly:

• Tooth geometry may have changed

• Original contact locations may be disturbed

• Coatings may have been penetrated

• Friction conditions may change

• Electrical contact behavior may change

For critical joints, replacing the washer after disassembly may be appropriate unless reuse has been specifically validated.

This is particularly relevant for controlled electrical or mechanical connections.

Can Internal Tooth Washers Damage the Surface?

Yes.

The tooth geometry is specifically intended to interact with the surfaces around it.

Possible effects include:

• Surface impressions

• Scratching

• Coating penetration

• Localized marking

This may be desirable when tooth engagement or electrical contact is required.

It may be undesirable on:

• Decorative panels

• Painted visible surfaces

• Soft materials

• Precision cosmetic components

• Surfaces where coating integrity must remain intact

The engineer should therefore determine whether controlled surface penetration is acceptable.

Internal Tooth Washer vs Flat Washer

A flat washer and an internal tooth washer should not be treated as equivalent components.

A flat washer is primarily used to:

• Increase bearing area

• Distribute load

• Protect the mating surface

• Reduce localized indentation

An internal tooth washer is selected primarily for:

• Tooth engagement

• Increased rotational resistance

• Compact serrated contact

• Potential electrical contact in appropriate designs

In some joints, both load distribution and locking functions may be required.

The appropriate washer arrangement should be determined from the complete joint design.

Internal Tooth Washer vs Split Lock Washer

Split helical lock washers and internal tooth washers use different geometries and interact with the joint differently.

Selection should consider:

• Fastener size

• Bearing surface

• Available radial space

• Surface hardness

• Mating material

• Required contact behavior

• Assembly process

• Service conditions

Neither should automatically be treated as universally superior.

Internal Tooth Washer vs Serrated Flange Fastener

A serrated flange nut or serrated flange screw integrates the serrations into the fastener itself.

An internal tooth washer is a separate component.

Integrated serrations may reduce component count.

A separate washer may be preferred when:

• Existing fasteners must remain unchanged

• Different washer materials are required

• Service replacement is needed

• Customer drawings specify a separate washer

• Electrical contact requirements influence washer selection

The decision should be made at assembly level.

Internal Tooth Washer vs Thread-Locking Adhesive

These technologies act at different interfaces.

A thread-locking adhesive acts primarily within the threaded connection.

An internal tooth washer acts primarily at the bearing surfaces.

Selection can depend on:

• Assembly speed

• Cleanliness requirements

• Temperature

• Maintenance

• Disassembly

• Production automation

• Storage requirements

• Joint validation

For some applications, engineers may evaluate multiple locking strategies before choosing the production solution.

How to Select an ASME B18.21.1 Internal Tooth Lock Washer

A structured selection process reduces sourcing errors.

1. Confirm the Standard

Identify whether the application requires:

• ASME B18.21.1

• Another recognized standard

• Customer-specific drawing requirements

Do not substitute standards based only on nominal fastener size.

2. Confirm the Fastener Size

Specify the exact inch-series fastener size and verify compatibility with:

• Washer inside diameter

• Fastener shank

• Head diameter

• Nut bearing surface

3. Check the Installation Envelope

Evaluate:

• Maximum allowable washer OD

• Adjacent components

• Recesses

• Counterbores

• Housing geometry

This is particularly important for compact assemblies.

4. Define Material

Specify the required:

• Material type

• Grade where applicable

• Mechanical requirements

• Corrosion requirements

5. Define Surface Finish

State:

• Coating type

• Finish requirements

• Corrosion requirement

• Electrical requirements where relevant

6. Evaluate the Mating Surface

Provide information about:

• Material

• Hardness

• Coating

• Paint

• Surface finish

7. Validate Functional Performance

If anti-loosening or electrical performance is critical, validate the completed joint under representative service conditions.

What Should Be Specified on the Drawing?

For an ASME B18.21.1 internal tooth lock washer, engineering documentation may need to identify:

• Applicable standard

• Washer style

• Nominal fastener size

• Inside diameter

• Outside diameter

• Thickness

• Tooth configuration

• Material

• Hardness where required

• Surface finish

• Coating requirement

• Corrosion requirement

• Dimensional tolerances

• Electrical requirements where applicable

• Inspection requirements

For custom designs, the customer drawing should control the required geometry.

Standard vs Custom Internal Tooth Washers

Standard ASME washers can be appropriate where the assembly is designed around established dimensions.

Custom internal tooth washers may be required when the project involves:

• Non-standard inside diameter

• Restricted outside diameter

• Special tooth geometry

• Special thickness

• Unique material

• Customer-specific hardness

• Special coating

• Electrical contact requirements

• Proprietary assembly geometry

This distinction is important for OEM sourcing.

If the existing component does not conform exactly to a standard, forcing it into a standard designation can create dimensional or functional problems.

A drawing-controlled custom washer may be the more appropriate sourcing route.

What Procurement Teams Should Verify

For purchasing and supplier development teams, the product description alone is not enough.

A supplier evaluation should consider:

• Can the supplier manufacture to the specified ASME requirement?

• Is the material controlled?

• Is hardness controlled where required?

• Is tooth geometry consistent?

• Are ID, OD and thickness controlled?

• Is the specified finish available?

• Can corrosion requirements be supported?

• Are coating requirements understood?

• Can electrical requirements be reviewed?

• Can samples be provided for qualification?

• Can production lots be supplied against an approved specification?

These questions become increasingly important for:

• OEM production

• Tier suppliers

• Automotive programs

• Electrical equipment

• Industrial automation

• Long-term supply agreements

Second-Source Qualification for Existing Washers

Internal tooth lock washers are often sourced as replacement or second-source components.

A useful qualification process is:

Existing Drawing → Standard Verification → Sample Comparison → Material Review → Dimensional Inspection → Finish Review → Assembly Testing → Production Approval

If a drawing is unavailable, an existing sample can help identify the geometry, but critical specifications should still be confirmed before production.

Reverse engineering only the visible dimensions may overlook:

• Material

• Hardness

• Tooth forming condition

• Coating

• Functional requirements

For OEM supply, these details should be resolved before mass production.

RFQ Checklist for ASME B18.21.1 Internal Tooth Lock Washers

For a faster and more accurate quotation, provide:

• ASME B18.21.1 designation

• Washer style

• Fastener size

• Drawing, if available

• Inside diameter

• Outside diameter

• Thickness

• Material

• Hardness requirement

• Surface finish

• Coating requirement

• Corrosion requirement

• Application

• Mating material

• Electrical requirement, if applicable

• Prototype quantity

• Production quantity

• Estimated annual volume

• Documentation requirements

• Packaging requirements

For a second-source project, supplying an existing approved sample together with the drawing can improve technical review.

Engineering Search vs Procurement Search

Engineering teams may search for:

• ASME B18.21.1 internal tooth lock washer

• internal tooth lock washer dimensions

• internal star washer

• internal tooth vs external tooth washer

• how does an internal tooth lock washer work

• internal tooth washer for grounding

• internal tooth washer material

• internal tooth lock washer application

Procurement and supplier-development teams may search for:

• ASME B18.21.1 washer manufacturer

• internal tooth lock washer supplier

• internal star washer manufacturer

• stainless steel internal tooth washer supplier

• inch internal tooth washer supplier

• custom internal tooth lock washer

• OEM toothed washer manufacturer

• lock washer second source

These are different search journeys.

Engineering is trying to determine whether the washer fits the joint.

Procurement is trying to determine whether the supplier can consistently manufacture and document the required washer.

A useful product page must answer both questions.

Related Washer and Fastening Solutions

For outward-facing serrations and a larger tooth-contact diameter, review our External Tooth Lock Washer Solutions.

For general bearing-load distribution without toothed engagement, review our Flat Washer Solutions.

For elastic axial preload and tolerance compensation, review our Wave Spring Washer Solutions.

For curved elastic preload applications, review our Curved Spring Washer Solutions.

For angular compensation in misaligned bolted joints, review our Spherical Washer Solutions.

For applications requiring an integrated serrated bearing surface, review our Serrated Flange Nut and Serrated Flange Fastener Solutions.

These internal links should be connected according to the engineering problem being solved rather than simply grouping every washer page together.

ASME B18.21.1 Internal Tooth Lock Washer Manufacturing and OEM Supply

JUXIN FASTENERS supports industrial sourcing projects involving:

• ASME B18.21.1 internal tooth lock washers

• Internal star washers

• Internal toothed washers

• Carbon steel lock washers

• Stainless steel internal tooth washers

• Inch-series lock washers

• Electrical contact washers

• Custom serrated washers

• Drawing-based washer components

Projects can be reviewed from:

• Standard designations

• 2D drawings

• Existing samples

• Material specifications

• Surface-finish requirements

• Application information

For standard products, confirming the applicable specification helps reduce ambiguity.

For custom products, a drawing provides the clearest basis for reviewing geometry, material and surface requirements.

From Engineering Requirement to Production RFQ

For a new design, the sourcing path can be structured as:

Joint Requirement → ASME/Customer Specification → Fastener Size → Washer Geometry → Material → Hardness → Surface Finish → Prototype → Assembly Validation → Production

For an existing second-source project:

Existing Part + Drawing → Specification Review → Dimensional Comparison → Material/Finish Review → Samples → Customer Validation → Production Approval

This approach is more reliable than selecting an internal star washer from appearance alone.

If you are sourcing ASME B18.21.1 internal tooth lock washers, internal star washers, stainless steel toothed washers,

 inch-series lock washers or custom internal tooth washers, send your standard reference, drawing, existing sample or application requirements to JUXIN FASTENERS.

For an efficient RFQ, include:

• Standard or drawing

• Fastener size

• Material

• Hardness, if specified

• Surface treatment

• Mating material

• Application

• Electrical requirements, if applicable

• Corrosion requirement

• Prototype quantity

• Production quantity

• Estimated annual demand

• Inspection and documentation requirements

Email: info@juxinfasteners.com

JUXIN FASTENERS can review the engineering and commercial requirements and evaluate a standard or custom internal tooth lock washer solution for OEM and industrial production.