Internal Tooth Lock Washers: Working Principle, Selection and Industrial Applications

An internal tooth lock washer is a thin mechanical locking washer with multiple teeth arranged around its inside diameter. 

It is installed beneath a screw head, bolt head or nut so that the teeth elastically deform and bite into the opposing contact surfaces during tightening.

Internal tooth lock washers are also called internal star washers, internal tooth washers, internal toothed lock washers or Type J tooth lock washers.

Unlike a flat washer, an internal tooth washer is not primarily intended to distribute load over a large area. 

Its main function is to increase resistance to rotation by creating localized tooth engagement beneath the fastener.

Internal tooth lock washers are commonly used with smaller screw heads and in assemblies where the outer edge of the washer should remain relatively smooth. 

They can be manufactured from hardened spring steel, stainless spring material, copper alloys and other application-specific materials.

JUXIN FASTENERS supplies metric and inch internal tooth lock washers for industrial OEMs,

 electrical-equipment manufacturers, automotive suppliers, machinery producers and assembly operations.

 Standard and drawing-specific requirements can be evaluated according to dimensions, tooth geometry, material, hardness, finish, corrosion performance and production volume.

How Does an Internal Tooth Lock Washer Work?

An internal tooth lock washer does not work by filling a gap or by being pressed into a hole.

It is installed as part of a threaded fastener assembly.

During tightening:

1. The underside of the screw head, bolt head or nut compresses the washer.

2. The internal teeth elastically deflect.

3. The tooth edges create localized contact pressure.

4. The teeth indent or bite into the fastener’s bearing surface and the mating component.

5. The resulting surface engagement increases resistance to reverse rotation.

The locking effect depends on controlled interaction between:

• Tooth hardness

• Tooth shape

• Fastener-head geometry

• Mating-surface hardness

• Coating thickness

• Installation preload

• Surface condition

• Direction and severity of dynamic loading

The washer does not produce a positive lock in the same way as a cotter pin, safety wire or bent tab washer. It increases rotational resistance but does not make loosening physically impossible.

What Do the Internal Teeth Contact?

For an internal tooth washer to work, its teeth must be compressed between suitable contact surfaces.

Depending on the assembly, the teeth may engage:

• The underside of a screw or bolt head

• The bearing face of a nut

• The upper surface of the clamped component

• A conductive metal surface in an electrical bonding assembly

The fastener head or nut must provide sufficient bearing coverage to compress the teeth correctly.

If the head is too small, incorrectly shaped or supported by an incompatible radius, the tooth washer may not seat properly.

If the bearing face does not fully engage the tooth region, the washer can tilt, deform unevenly or fail to provide the intended resistance.

Internal Tooth Versus External Tooth Lock Washers

Internal and external tooth washers use a similar tooth-engagement principle, but the location and effective radius of the teeth differ.

External teeth act at a larger radius and may provide greater resistance to rotation when the fastener head is large enough to engage the washer correctly.

Internal tooth washers are often selected when:

• The screw head is relatively small

• The washer must remain inside the head outline

• Exposed external teeth are undesirable

• The outside edge should not catch adjacent components

• The application needs localized contact near the screw shank

See the JUXIN FASTENERS guide to external tooth lock washers for a detailed comparison.

Toothed Versus Serrated Internal Lock Washers

“Toothed” and “serrated” are sometimes used interchangeably, but DIN product terminology historically distinguished different geometries.

DIN 6797 J Toothed Lock Washers

DIN 6797 J identifies a legacy internal-tooth configuration. The teeth are formed around the inside diameter and elastically engage the contact surfaces.

DIN 6797 has been withdrawn, but the designation remains common in catalogs, legacy drawings and replacement programs.

DIN 6798 J Serrated Lock Washers

DIN 6798 J identifies a legacy washer with internal serrations. The serration geometry and contact behavior differ from the DIN 6797 toothed form.

DIN 6798 has also been withdrawn, but the designation remains commercially relevant.

A buyer should not assume that DIN 6797 J and DIN 6798 J are interchangeable. Confirm:

• Inside diameter

• Outside diameter

• Thickness

• Tooth or serration count

• Tooth angle

• Free height

• Material

• Hardness

• Required contact behavior

For DIN 6798 J products, see JUXIN FASTENERS’ internal serrated lock washers.

Applicable International Standards

ASME B18.21.1

ASME B18.21.1 covers inch-series helical spring-lock, tooth-lock and plain washers. It includes dimensional requirements, physical properties and related test methods for tooth-lock washers.

For an inch-series RFQ, the purchaser should identify:

• ASME B18.21.1

• Nominal screw or bolt size

• Internal-tooth configuration

• Material

• Finish

• Any supplementary requirements

DIN 6797 J

DIN 6797 J remains a common metric legacy designation for internal toothed lock washers, although the DIN 6797 standard has been withdrawn.

Where a drawing specifies DIN 6797 J, confirm the edition and whether the customer requires exact historical conformity or an approved commercial equivalent.

DIN 6798 J

DIN 6798 J remains a common legacy designation for internally serrated lock washers. DIN 6798 is also withdrawn.

The washer should be sourced against a controlled drawing or dimensional table rather than only the abbreviated standard number.

Customer and OEM Specifications

Automotive, electrical, aerospace, railway and industrial OEMs may use proprietary washer specifications with different requirements for:

• Dimensions

• Tooth geometry

• Hardness

• Coating

• Electrical performance

• Torque resistance

• Vibration testing

• Traceability

• Quality documentation

The customer drawing must take priority when it differs from a general catalog standard.

Do Internal Tooth Washers Prevent Vibration Loosening?

Internal tooth lock washers can increase resistance to fastener rotation, but they do not guarantee preload retention under every vibration condition.

Their performance depends on whether the teeth maintain effective engagement.

Locking performance may be reduced when:

• The mating surface is too hard for the teeth to indent

• The surface coating is too thick or too soft

• The washer is reused after the teeth have flattened

• The fastener preload is too low

• The clamped parts slip transversely

• The joint settles or creeps

• The screw head does not cover the tooth region

• The assembly is exposed to severe cyclic shear

• The contact surface fractures or flakes

• Lubrication changes the achieved preload

For severe transverse vibration, the engineer should compare internal tooth washers with:

• Wedge-lock washer pairs

• Prevailing-torque nuts

• All-metal locknuts

• Nylon-insert locknuts

• Thread-locking adhesive

• Serrated flange fasteners

• Tab washers

• Safety wire

• Cotter pins

• Customer-approved positive locking devices

See the broader guide to thread-locking fastener solutions.

Why Mating-Surface Hardness Matters

The washer teeth must be hard enough to maintain their shape while still being capable of engaging the mating surfaces.

Three conditions are possible.

Mating Surface Is Too Hard

If the component or fastener bearing surface is significantly harder than the washer teeth, the teeth may slide, polish or flatten instead of biting into the surface.

Mating Surface Is Too Soft

If the component is soft plastic, thin aluminum, soft copper or an easily deformable coating, the teeth may embed excessively.

This can cause:

• Surface damage

• Loss of preload

• Washer settlement

• Material cracking

• Reduced locking effectiveness

Surface Hardness Is Compatible

When washer hardness, tooth geometry and mating surfaces are compatible, the teeth create controlled indentation and increased resistance to reverse rotation.

The correct relationship must be verified for the complete assembly rather than assumed from washer material alone.

Internal Tooth Washers on Painted or Coated Surfaces

Toothed washers are often used because their teeth can penetrate paint, oxide or a surface film. This can support mechanical engagement and, in some cases, electrical contact.

However, penetration of the coating creates tradeoffs:

• The protective finish may be damaged

• Bare metal may be exposed

• Local corrosion can begin

• Paint debris may contaminate the assembly

• Contact resistance may change over time

• Preload can decrease as a soft coating settles

If the coating is important for corrosion protection, the assembly requires an approved repair, sealing or corrosion-control strategy.

A toothed washer should not be installed over a painted surface simply because it appears capable of cutting through the paint.

Electrical Bonding and Grounding Applications

Internal tooth lock washers are frequently used in electrical enclosures, terminal assemblies and grounding connections because the teeth may penetrate surface films and establish metal-to-metal contact.

Potential applications include:

• Electrical control cabinets

• Chassis bonding

• Ground terminals

• Panel assemblies

• Switchgear

• Power supplies

• Electronic equipment

• Communication equipment

• Lighting systems

The washer itself does not guarantee a compliant electrical bond.

Electrical performance depends on:

• Base metal

• Coating

• Contact pressure

• Washer material

• Tooth engagement

• Corrosion

• Current level

• Thermal cycling

• Environmental exposure

• Applicable electrical standard

• Long-term contact resistance

Grounding and bonding connections should be tested and approved according to the equipment manufacturer’s requirements and applicable safety standards.

Load Distribution and Surface Protection

An internal tooth lock washer does not distribute pressure as evenly as a correctly sized flat washer.

The teeth intentionally create concentrated contact pressure.

This makes the washer unsuitable for applications where the primary requirement is to protect:

• Soft plastic

• Decorative surfaces

• Fragile coatings

• Thin unsupported sheet

• Soft aluminum

• Composite materials

If both load distribution and locking are required, the assembly may need:

• A flange-head fastener

• A hardened flat washer

• A supported joint surface

• A captive washer assembly

• A serrated flange component

• Another approved locking system

Adding a flat washer between the tooth washer and the mating surface changes the engagement interface and should not be done without engineering review.

Suitable Fastener-Head Types

Internal tooth washers are often used beneath:

• Pan head screws

• Cheese head screws

• Fillister head screws

• Machine screws

• Small hex heads

• Nuts with suitable bearing faces

• Captive screw-and-washer assemblies

Compatibility depends on actual bearing-face diameter rather than the head name alone.

The fastener bearing surface must:

• Cover the intended tooth region

• Avoid interference with the washer’s inside edge

• Seat without contacting an under-head radius

• Apply reasonably uniform compression

• Remain compatible with the washer hardness

Countersunk screws normally require a washer specifically designed for the countersunk geometry. A conventional flat internal tooth washer should not be forced beneath a conical head.

Materials

Hardened Carbon Spring Steel

Hardened spring steel is widely used because it provides:

• Elastic tooth deflection

• Wear resistance

• Tooth strength

• Economical high-volume stamping

• Compatibility with multiple coatings

The finished washer requires controlled stamping, forming, heat treatment and hardness.

Stainless Spring Material

Stainless steel internal tooth washers may be selected for corrosion-sensitive environments.

The material must still provide suitable spring properties. A generic stainless designation does not automatically define:

• Hardness

• Tooth strength

• Elastic recovery

• Galling behavior

• Corrosion resistance in the actual medium

EN 1.4310 may be considered for stainless spring components. A4-type stainless material may be evaluated for more demanding corrosion exposure, subject to spring-performance requirements.

Copper Alloys and Spring Bronze

Copper-alloy washers may be used in selected electrical or non-ferrous assemblies.

Material selection should account for:

• Electrical conductivity

• Spring properties

• Relaxation

• Corrosion behavior

• Temperature

• Galvanic compatibility

Brass, bronze, steel and stainless steel versions should not be treated as mechanically interchangeable without validation.

Surface Treatments

Available finishes may include:

• Black oxide

• Zinc plating

• Mechanical zinc

• Zinc-flake coating

• Nickel plating

• Phosphate-based finishes

• Plain stainless steel

• Customer-specified coatings

The finish must be selected according to:

• Corrosion environment

• Coating thickness

• Friction requirement

• Electrical conductivity

• Contact resistance

• Appearance

• Hydrogen-embrittlement risk

• RoHS and REACH requirements

• Customer approval

For hardened spring-steel washers, electroplating processes require appropriate hydrogen-embrittlement controls.

Installation Guidelines

Select the Correct Washer Type

Confirm whether the application requires:

• Internal toothed

• Internal serrated

• External toothed

• External serrated

• Internal/external tooth

• Conical serrated

• Countersunk tooth

These designs are not automatically interchangeable.

Match the Washer to the Fastener

Verify the inside diameter, outside diameter and tooth region against the screw or bolt.

Clean the Contact Surfaces

Remove uncontrolled contamination that could prevent seating or produce an unstable interface.

A specified lubricant or assembly coating should not be removed without approval.

Position the Washer Beneath the Rotating Fastener

Install the washer beneath the screw head, bolt head or nut specified by the joint design.

A conventional internal tooth washer is not a press-fit retainer and should not be forced into a hole.

Apply the Approved Tightening Procedure

Use the torque or preload specification established for the:

• Fastener size

• Property class

• Material

• Coating

• Lubrication

• Joint stiffness

• Washer arrangement

Inspect Tooth Engagement

Where appropriate, production validation may include inspection of:

• Washer seating

• Tooth deformation

• Surface indentation

• Head coverage

• Coating damage

• Installed torque

• Electrical resistance

Can Internal Tooth Washers Be Reused?

Reuse is generally not recommended unless specifically permitted and validated.

During the first installation:

• Teeth deform

• Contact surfaces become indented

• Coatings may be penetrated

• Tooth edges may flatten

• The washer may lose free height

A reused washer may not engage a new surface with the same force or geometry.

For controlled OEM production, maintenance-critical equipment and electrical bonding connections, a new washer is usually the more reliable choice.

Common Failure and Misapplication Modes

Washer Installed as a Press-Fit Component

An internal tooth lock washer is normally used beneath a threaded fastener. It should not be confused with a push-on retaining washer or star-lock retainer.

Teeth Do Not Engage

The mating surface may be too hard, too smooth or covered with an incompatible coating.

Teeth Flatten During Tightening

Possible causes include insufficient hardness, incorrect material, excessive load or incompatible fastener geometry.

Washer Damages a Soft Component

The concentrated tooth pressure may cut into plastic, aluminum, thin sheet or decorative finishes.

Fastener Still Loosens

The joint may be experiencing transverse slip, insufficient preload, settlement or vibration beyond the washer’s capability.

Electrical Resistance Increases

Corrosion, coating damage, contamination or reduced contact pressure can degrade an electrical bond.

Corrosion Begins at Tooth Marks

The teeth can penetrate the protective coating and expose the base material.

Washer Spins With the Fastener

If tooth engagement is inadequate, the washer may rotate during tightening and score the surface without producing reliable locking.

Industrial Applications

Electrical and Electronic Equipment

Internal tooth washers are widely used in:

• Control cabinets

• Power-distribution equipment

• Terminal assemblies

• Electronic enclosures

• Lighting equipment

• Communication equipment

• Power supplies

• Instrumentation

Electrical-bonding performance must be validated separately from mechanical locking performance.

Industrial Machinery

Potential applications include:

• Machine covers

• Control mechanisms

• Sensors

• Guards

• Small brackets

• Instrument panels

• Serviceable assemblies

They are best suited to light- and moderate-duty connections with compatible bearing surfaces.

Automotive Auxiliary Assemblies

Internal tooth washers may be used in approved:

• Electrical modules

• Interior systems

• Lighting assemblies

• Sensors

• Small brackets

• Non-structural accessories

They should not be presented as a universal solution for steering, braking, suspension, restraint or structural battery joints.

Appliances and Commercial Equipment

Applications may include:

• Commercial kitchen equipment

• HVAC equipment

• Domestic appliances

• Pumps

• Fans

• Heating equipment

• Access panels

Material and coating selection should reflect moisture, heat, cleaning chemicals and expected service life.

Automation and Instrumentation

Compact internal tooth washers may support:

• Sensor mounting

• Controller assemblies

• Small motors

• Measurement equipment

• Robotic auxiliary systems

• Precision enclosures

Where accurate positioning or clean surfaces are required, the effect of tooth indentation must be considered.

Quality and Inspection Requirements

A professional purchasing specification may include:

• Applicable ASME, legacy DIN or customer standard

• Internal tooth or internal serrated form

• Nominal size

• Inside diameter

• Outside diameter

• Thickness

• Tooth count

• Tooth height

• Tooth angle

• Free height

• Material

• Heat-treatment condition

• Hardness

• Surface finish

• Coating thickness

• Corrosion requirement

• Hydrogen-embrittlement controls

• Burr and edge requirements

• Crack inspection

• Dimensional inspection

• Electrical-resistance requirement

• Torque-resistance test

• Lot traceability

• Material certificate

• Packaging and labeling

Where locking performance is critical, the customer should define an assembly-level test rather than approving the washer only by dimensions.

RFQ Checklist

To receive an accurate internal tooth lock washer quotation, provide:

1. Metric or inch fastener size

2. Applicable standard or customer drawing

3. Internal toothed or internal serrated form

4. Washer inside diameter

5. Washer outside diameter

6. Thickness

7. Tooth count and geometry

8. Fastener-head type and bearing diameter

9. Washer material

10. Required hardness

11. Surface finish

12. Coating thickness

13. Mating-surface material

14. Mating-surface hardness

15. Paint or coating on the joint surface

16. Corrosion requirement

17. Electrical bonding requirement

18. Vibration or torque-resistance requirement

19. Inspection and documentation requirements

20. Prototype quantity

21. Annual production volume

22. Packaging and delivery requirements

Frequently Asked Questions

What is an internal tooth lock washer?

It is a mechanical washer with teeth around the inside diameter. The teeth deform and engage the fastener bearing face and mating surface during tightening.

Does it fill the gap around a bolt?

No. Its locking action comes from tooth engagement and localized surface indentation, not from filling a gap.

Is an internal tooth washer press-fitted?

No. A conventional internal tooth lock washer is installed beneath a screw head, bolt head or nut. It should not be confused with a push-on star-lock retainer.

Are DIN 6797 J and DIN 6798 J current standards?

No. Both DIN 6797 and DIN 6798 have been withdrawn, although their designations remain common for legacy parts and replacement sourcing.

Which standard applies to inch internal tooth washers?

ASME B18.21.1 covers inch-series tooth-lock washers, along with other washer types included in that standard.

Is an internal tooth washer better than an external tooth washer?

Neither type is universally better. Internal teeth suit compact heads and a smooth outer profile. External teeth act at a larger radius and may provide greater rotational resistance when correctly engaged.

Can it be used for electrical grounding?

It may help create metal-to-metal contact, but the completed grounding or bonding connection must be tested and approved according to the equipment requirements.

Can it be installed on painted surfaces?

The teeth may penetrate paint, but this can damage corrosion protection and expose bare metal. The complete joint requires engineering review.

Can it be reused?

Reuse is generally not recommended because the teeth and contact surfaces deform during the first installation.

Can stainless steel internal tooth washers be used outdoors?

Potentially, but the exact stainless spring material, chloride exposure, mating materials and corrosion requirements must be confirmed.

Do internal tooth washers work under severe vibration?

They can increase rotational resistance, but severe transverse vibration may require a more effective locking system validated for the actual joint.

Can JUXIN FASTENERS produce custom internal tooth washers?

Yes. JUXIN FASTENERS can evaluate customer drawings for non-standard dimensions, tooth geometries, materials, finishes and inspection requirements.

Why Source Internal Tooth Lock Washers from JUXIN FASTENERS?

JUXIN FASTENERS supports standard and drawing-specific internal tooth washer requirements for industrial OEM production.

Supply and development options can include:

• Metric internal tooth lock washers

• Inch internal tooth lock washers

• DIN 6797 J-type legacy washers

• DIN 6798 J-type internal serrated washers

• ASME B18.21.1 inch-series washers

• Hardened spring-steel washers

• Stainless spring-material options

• Copper-alloy and special-material washers

• Customer-specific tooth geometry

• Controlled heat treatment and hardness

• Corrosion-resistant finishes

• Dimensional inspection

• Material and inspection documentation

• Lot traceability

• Production packaging

• Multi-SKU washer and fastener sourcing

View the internal tooth lock washer product range or submit a drawing for a custom requirement.

Request an Internal Tooth Lock Washer Quotation

A reliable quotation requires more than the nominal screw size.

Send your drawing, applicable standard, fastener-head dimensions, washer dimensions, tooth geometry, material, hardness, finish, mating-surface information,

 vibration or electrical requirements, annual quantity and quality-documentation requirements to:

info@juxinfasteners.com

JUXIN FASTENERS supports OEMs, electrical-equipment manufacturers, automotive suppliers, industrial machinery producers and global sourcing teams requiring internal tooth lock washers, 

serrated washers and application-specific locking components.