ISO Metric Thread Tolerances: 6H, 6g, Thread Fit, Coating Allowance and Inspection

What do 6H and 6g actually mean on an ISO metric fastener drawing?

Why might an M10 × 1.5 bolt pass dimensional inspection before coating but fail to assemble correctly after plating?

And does specifying a tighter thread tolerance automatically create a stronger or more reliable bolted joint?

These questions are important for design engineers, quality engineers, procurement managers and supplier-development teams sourcing:

• Bolts

• Screws

• Nuts

• Weld nuts

• Rivet nuts

• Self-clinching fasteners

• Threaded inserts

• Studs

• CNC machined threaded components

• Custom OEM fasteners

The most important starting point is:

Thread tolerance ≠ joint performance.

A thread tolerance class defines dimensional limits for the threaded feature. It does not, by itself, define:

• Joint preload

• Thread stripping strength

• Vibration resistance

• Sealing capability

• Galling resistance

• Corrosion resistance

• Functional performance

For ISO metric threads, engineering decisions should instead follow:

Basic Thread Geometry → Tolerance Class → Engagement Length → Material → Surface Treatment → Final Thread Condition → Mating Component → Installation → Functional Validation

This guide explains how to use 6H, 6g and other ISO metric thread tolerance classes correctly while avoiding several common specification mistakes.

Direct Answer: What Do 6H and 6g Mean?

For ISO metric screw threads, tolerance classes are defined through the ISO 965 system.

A designation such as:

M10 × 1.5–6H

identifies an internal thread tolerance class.

A designation such as:

M10 × 1.5–6g

identifies an external thread tolerance class.

The tolerance class combines:

Tolerance Grade + Tolerance Position

For example:

6H

• 6 = tolerance grade

• H = tolerance position for the internal thread

6g

• 6 = tolerance grade

• g = tolerance position for the external thread

The tolerance grade controls the width of the tolerance zone.

The tolerance position controls where that zone lies relative to the basic thread profile.

This distinction is more useful than describing 6H/6g simply as a "medium precision fit."

ISO Metric Thread Standards: What Does Each Standard Control?

Several ISO standards may appear in an engineered metric-thread specification, but they do not all control the same thing.

ISO 68-1

Defines the basic profile for ISO general-purpose metric screw threads.

ISO 261

Defines general-purpose metric screw thread combinations of nominal diameter and pitch.

ISO 724

Defines basic dimensions for ISO metric screw threads.

ISO 965 Series

Defines tolerances and tolerance systems for ISO general-purpose metric screw threads.

ISO 1502

Defines gauges and gauging principles for ISO general-purpose metric screw threads.

ISO 898-1

Defines mechanical properties for specified carbon and alloy steel bolts, screws and studs within its scope.

ISO 898-2

Defines mechanical properties for specified nuts within its scope.

These standards work together, but:

Thread dimensional compliance ≠ mechanical property compliance.

A bolt can have a correct 6g thread and still fail the required mechanical property specification.

Likewise, a nut can meet 6H dimensional requirements without necessarily satisfying the required proof-load or material requirements.

Why 6H and 6g Are Commonly Encountered

6H internal threads and 6g external threads are widely used general-purpose tolerance classes in ISO metric threaded fasteners.

Their widespread use supports:

• Manufacturing practicality

• Interchangeability

• Assembly clearance

• Global sourcing

• Standardized gauging

However, engineers should avoid turning this common practice into the rule:

"If no thread tolerance is specified, always assume 6H/6g."

That assumption may conflict with:

• Product standards

• Coating requirements

• Customer drawings

• OEM specifications

• Legacy designs

• Special thread requirements

The correct tolerance should come from the applicable product standard, drawing or engineering specification.

Common default ≠ universal default.

Tolerance Grade vs. Tolerance Position

These two concepts should not be confused.

Tolerance Grade

The number indicates the tolerance grade and therefore influences tolerance width.

A lower grade number generally represents a narrower tolerance zone than a higher grade number for comparable thread conditions.

But:

Tighter tolerance ≠ stronger thread.

A tighter manufacturing tolerance may improve dimensional control, but it does not automatically increase the load capacity of the threaded joint.

Tolerance Position

The letter defines the fundamental deviation of the tolerance zone relative to the basic thread profile.

For internal threads, H is a commonly encountered tolerance position.

For external threads, positions such as g and h may be encountered depending on the specification.

These positions influence available clearance and can become especially important when surface treatment is added.

Why 6H/6g Should Not Be Called a "Strength Fit"

A major mistake in thread design is to connect tolerance class directly with joint strength.

For example:

6H/6h is tighter, therefore it must be stronger than 6H/6g.

That conclusion is not generally valid.

Threaded-joint strength depends on variables such as:

• Bolt material

• Nut or parent material

• Mechanical properties

• Thread engagement length

• Thread geometry

• Diameter

• Pitch

• Manufacturing quality

• Load distribution

• Installation preload

A small reduction in thread clearance does not automatically produce a meaningful increase in joint load capacity.

Therefore:

Tighter Fit ≠ Higher Joint Strength.

Thread Engagement Length: Why It Matters

Thread engagement length can have a major effect on internal-thread load capacity, particularly when the mating material is weaker than the bolt.

Examples include threads in:

• Aluminum

• Plastics

• Cast materials

• Machined housings

• Thin components

Increasing engagement may increase the amount of thread material available to resist stripping.

However, the simple statement:

Longer engagement = proportionally higher strength

is also incorrect.

Thread load is not distributed equally across every engaged thread.

The required engagement depends on:

• Bolt strength

• Internal-thread material

• Thread size

• Pitch

• Geometry

• Required preload

• Failure mode

The design objective is to provide sufficient engagement for the required joint performance.

Stronger Bolt ≠ Stronger Internal Thread

Suppose an engineer replaces a bolt with a higher property class while keeping the same tapped aluminum component.

The bolt itself may become stronger.

The aluminum internal thread does not automatically become stronger.

In some cases, the limiting failure mode can shift toward thread stripping in the parent material.

Therefore:

Stronger Mating Screw ≠ Stronger Mating Thread.

This is particularly important for:

• Aluminum housings

• Castings

• Plastic assemblies

• Threaded inserts

• Lightweight structures

Thread tolerance should never be used as a substitute for proper thread-strength analysis.

6H/6g Does Not Guarantee Better Vibration Resistance

Another misconception is that a particular standard thread tolerance automatically provides vibration resistance.

Vibration loosening depends on the complete bolted joint.

Important variables include:

• Preload

• Joint stiffness

• Transverse movement

• Friction

• Bearing interfaces

• External loading

• Locking mechanism

A correctly manufactured 6H/6g thread can still loosen in an inadequately designed dynamic joint.

Therefore:

Thread Fit ≠ Anti-Loosening System.

Where rotational resistance is required, engineers may need to evaluate:

• Nylon insert lock nuts

• All-metal lock nuts

• Flange lock nuts

• Thread-locking systems

• Positive mechanical retention

• Other application-specific locking solutions

JUXIN FASTENERS supports several of these locking-fastener families for suitable OEM applications.

6H/6g Does Not Define Sealing Performance

The same principle applies to sealing.

ISO metric screw threads may be used in components that also carry:

• O-rings

• Bonded seals

• Gaskets

• Face seals

• Other sealing features

But the 6H/6g tolerance itself does not create a universal pressure-tight connection.

Therefore:

Metric Thread Fit ≠ Pressure Seal.

If sealing matters, the drawing must define the actual sealing interface and functional requirements.

The Most Important Thread-Tolerance Problem: Surface Treatment

For many OEM fasteners, the thread is not used in its bare manufactured condition.

The finished component may receive:

• Zinc plating

• Zinc-nickel coating

• PTFE-based coating

• Other customer-specified surface treatment

The coating occupies dimensional space.

That means the final thread condition may differ from the pre-coating condition.

This leads to one of the most important rules in fastener manufacturing:

Pre-Coating Thread Compliance ≠ Finished Thread Compliance.

Why Coating Changes External Thread Dimensions

Coating deposited on thread flanks changes the effective thread geometry.

The actual influence depends on:

• Coating thickness

• Thread flank angle

• Deposition uniformity

• Thread pitch

• Coating process

• Measurement convention

The old rule sometimes quoted as:

"Pitch diameter change ≈ four times coating thickness"

should not be used as a universal calculation for every thread and coating.

The geometric relationship depends on the thread profile and how coating thickness is defined and deposited.

For engineering release, use the applicable thread and coating standards rather than a generic multiplier.

Zinc-Plated Metric Threads

Electroplated zinc fasteners are commonly used in industrial applications.

Relevant engineering questions include:

• What is the required final thread tolerance?

• What is the coating thickness?

• Is the tolerance class defined before or after coating?

• Is the final component gauged after coating?

• Does the coating system affect friction?

• Is hydrogen embrittlement risk relevant to the fastener strength?

A zinc-plated bolt that fits a ring gauge before plating but fails after plating has not achieved the required finished thread condition.

Zinc-Nickel Coated Threads

Zinc-nickel systems can provide demanding corrosion performance, but they also require dimensional control.

Engineers should specify:

• Applicable coating specification

• Coating thickness

• Passivation

• Sealer/topcoat where required

• Final thread requirement

• Friction requirement where applicable

Remember:

Better Corrosion Resistance ≠ Automatic Thread Compatibility.

The coating system must satisfy both corrosion and dimensional requirements.

Hot-Dip Galvanized Threads Need Specific Engineering Treatment

Hot-dip galvanizing can create substantially greater coating build than many electroplated systems.

For threaded fasteners, ISO 10684 provides requirements for hot-dip galvanized fasteners and associated dimensional considerations.

Do not simply take a standard bare 6H/6g assembly, add a thick galvanized coating and assume it will remain interchangeable.

The thread system must be designed and manufactured according to the applicable hot-dip galvanized fastener requirements.

This is a good example of why:

Coating Name ≠ Complete Thread Specification.

Zinc Flake and Other Coating Systems

Non-electrolytically applied zinc flake coatings are covered by standards such as ISO 10683 for fasteners within its scope.

The dimensional impact still requires evaluation.

Even when the coating is relatively thin, engineers should consider:

• Final thread fit

• Coating accumulation

• Friction

• Installation

• Customer specification

JUXIN FASTENERS evaluates drawing-based surface-treatment requirements according to the requested fastener and application.

 Specific coating systems should be confirmed during technical review rather than assumed from a general product description.

Coating Changes More Than Thread Fit

A coating can also change friction.

This matters because:

Torque ≠ Preload.

And:

Same Torque + Different Friction ≠ Same Preload.

If a bolt changes from zinc plating to another coating, the final thread may still assemble perfectly while the tightening behavior changes.

Therefore, second-source qualification should evaluate both:

Dimensional Compatibility + Installation Behavior

not thread gauging alone.

Internal Threads: Nuts, Weld Nuts and Tapped Components

Internal thread control is especially important for JUXIN product families such as:

• Standard nuts

• Nylon insert lock nuts

• All-metal lock nuts

• Weld nuts

• Rivet nuts

• Self-clinching nuts

• Threaded inserts

• CNC machined components

The same 6H designation does not mean all of these products behave identically.

For example, a weld nut introduces another critical interface:

Nut Thread + Welded Joint

A weld nut can have a dimensionally correct internal thread while the installed assembly fails because of:

• Poor projection welding

• Misalignment

• Weld spatter

• Thread distortion

• Incorrect welding parameters

Therefore:

Strong Nut ≠ Strong Welded Joint.

And:

Correct 6H Thread ≠ Correct Weld Nut Installation.

Rivet Nut Threads: Thread Gauge Is Only One Qualification Step

Blind rivet nuts introduce a different system:

Internal Thread + Rivet Nut Body + Parent Sheet + Setting Process

A 6H internal thread does not determine:

• Pull-out resistance

• Spin-out resistance

• Sheet deformation

• Installation quality

For rivet nuts:

Pull-Out ≠ Spin-Out.

And:

Strong Rivet Nut ≠ Strong Installed Joint.

Thread inspection must therefore be combined with application-specific installation validation.

Self-Clinching Fastener Threads

For self-clinching nuts, studs and standoffs, thread compliance is again only one part of the product.

Installed performance also depends on:

• Sheet material

• Sheet hardness

• Sheet thickness

• Hole geometry

• Edge distance

• Installation tooling

• Press force

Therefore:

Correct Thread ≠ Correct Clinching Performance.

Push-out and torque-out requirements should be evaluated separately where applicable.

Threaded Inserts for Plastic

Threaded inserts used in thermoplastics introduce another important engineering distinction.

The metal internal thread may be dimensionally correct while the installed insert fails because of:

• Incorrect boss geometry

• Inadequate plastic around the insert

• Poor heat-staking process

• Incorrect ultrasonic installation

• Polymer creep

• Insufficient pull-out or torque resistance

Therefore:

Strong Insert ≠ Strong Plastic Joint.

The metal thread tolerance must be considered together with the polymer and installation system.

Stainless Steel Threads and Galling

Stainless steel threaded assemblies can present galling risk under certain combinations of:

• Material

• Surface condition

• Speed

• Pressure

• Lubrication

• Thread fit

It is tempting to solve galling by simply specifying a looser thread tolerance.

That may help in some designs, but it should not be treated as the only control.

Engineers should evaluate:

• Material pairing

• Lubrication

• Surface treatment

• Installation speed

• Thread condition

• Preload

Looser Thread ≠ Guaranteed Anti-Galling Solution.

And:

Stainless Steel ≠ Maintenance-Free Thread.

Aluminum Internal Threads

Aluminum housings and components require particular attention to internal-thread strength.

Important variables include:

• Aluminum alloy

• Temper

• Thread engagement

• Bolt strength

• Preload

• Repeated service

• Surface treatment

• Galvanic interaction

A very tight tolerance class does not compensate for insufficient parent-material strength.

Again:

Tighter Thread ≠ Stronger Aluminum Joint.

Plastic Internal Threads

Directly formed threads in plastics require a different design approach from metal threads.

Performance depends on:

• Polymer

• Reinforcement

• Temperature

• Moisture

• Creep

• Stress relaxation

• Thread geometry

• Installation torque

A standard metal 6H/6g mentality should not be transferred automatically to plastic joint design.

Depending on the application, engineers may instead consider:

• Thread-forming screws for plastics

• Heat-set inserts

• Ultrasonic inserts

• Molded-in inserts

• Other engineered threaded solutions

How Thread Engagement Influences Failure Mode

A threaded joint can fail through several mechanisms.

Bolt Tensile Failure

The bolt cross-section reaches its limiting load.

External Thread Failure

The bolt thread strips or shears.

Internal Thread Failure

The nut or parent-material thread strips.

Parent Material Failure

The surrounding component fails before the threaded fastener.

Insert Pull-Out

A threaded insert separates from the parent material.

The desired failure hierarchy depends on the application.

Thread tolerance is only one variable in this system.

Thread Inspection: Why GO/NO-GO Gauging Matters

For production inspection of ISO metric threads, limit gauges are commonly used according to the applicable gauging system, including ISO 1502.

For internal threads, inspection commonly uses thread plug gauges.

For external threads, thread ring gauges or other appropriate inspection methods may be used.

But an important quality principle is:

A GO gauge and NO-GO gauge do not individually measure every thread dimension.

They are functional limit-inspection tools used according to defined gauging principles.

For detailed process investigation, manufacturers may also use measurement methods appropriate to:

• Pitch diameter

• Major diameter

• Minor diameter

• Pitch

• Thread form

depending on the feature and manufacturing process.

GO Gauge Does Not Mean "Perfect Thread"

A thread that accepts the appropriate GO gauge satisfies a functional limit condition defined by the gauging system.

It does not mean every individual thread parameter is exactly nominal.

Likewise, thread quality should not be described as:

"GO passes, therefore the thread is perfect."

The correct statement is that the thread satisfies the applicable gauging acceptance when inspected correctly.

NO-GO Gauge Should Not Be Forced

A common shop-floor mistake is excessive force during NO-GO inspection.

Thread gauging requires controlled technique.

Forcing a gauge can:

• Damage the part

• Damage the gauge

• Produce inconsistent inspection decisions

Quality procedures should define:

• Correct gauge

• Gauge condition

• Inspection technique

• Calibration / verification

• Acceptance criteria

Gauge Wear Is a Supplier-Qualification Issue

Thread gauges are themselves precision inspection tools.

Over time they can wear.

A supplier's thread inspection system should therefore include appropriate control of:

• Gauge identification

• Calibration or verification status

• Wear monitoring

• Storage

• Handling

• Inspection records

A supplier claiming "100% thread gauging" still needs a controlled gauging system.

Inspection Quantity ≠ Measurement-System Quality.

Before Coating or After Coating: When Should Threads Be Gauged?

This depends on the drawing, coating system and applicable specification.

But for finished fasteners, the critical commercial question is usually:

Will the finished component assemble correctly with the specified mating thread?

Therefore, final thread condition after surface treatment must be addressed.

Depending on the product and process, inspection may involve:

• Pre-coating process control

• Coating thickness verification

• Post-coating functional gauging

• Assembly validation

The drawing and applicable standards should define the requirement.

Thread Inspection for Lock Nuts Requires More Than a Gauge

For prevailing-torque nuts such as:

• Nylon insert lock nuts

• All-metal lock nuts

• Flange lock nuts

thread dimensional inspection is necessary but not sufficient.

These products also have a functional locking characteristic.

Therefore:

Thread Gauge Compliance ≠ Prevailing-Torque Compliance.

The applicable product specification may require functional testing of the locking feature.

And:

Prevailing Torque ≠ Preload.

Installation requirements still need to be defined.

Thread Inspection for Coated Fasteners

A coated fastener can pass thread gauging and still produce unexpected assembly behavior because of friction.

This creates two separate quality questions:

Dimensional Question

Does the finished thread meet its specified dimensional limits?

Functional Question

Does the fastener produce the required installation and joint performance?

Both matter.

Thread Fit ≠ Torque-Preload Control.

Thread Tolerance Selection Framework

Instead of automatically choosing 6H/6g, use the following engineering sequence.

Step 1 — Identify the Thread System

Confirm:

• ISO metric thread

• Nominal diameter

• Pitch

• Coarse or fine pitch

Step 2 — Identify the Mating Component

Is the external thread mating with:

• Standard nut

• Lock nut

• Weld nut

• Tapped steel component

• Aluminum housing

• Rivet nut

• Self-clinching nut

• Threaded insert

Step 3 — Define Engagement Length

Determine whether sufficient parent material exists to resist the required load.

Step 4 — Select the Tolerance Class

Use the applicable product standard, drawing and functional requirement.

Step 5 — Evaluate Surface Treatment

Consider:

• Coating type

• Thickness

• Final thread condition

• Friction

Step 6 — Define Inspection

Specify:

• Applicable gauge

• Final gauging condition

• Additional dimensional measurements where required

Step 7 — Validate Assembly

Evaluate:

• Installation

• Torque/preload

• Locking behavior

• Galling

• Repeated service

• Functional requirements

This produces a more reliable result than selecting a tolerance class from a generic table.

When Should Engineers Consider a Different Tolerance Class?

A deviation from a common 6H/6g combination may be appropriate when required by:

• Product standard

• Surface-treatment system

• OEM specification

• Special assembly clearance

• Temperature

• Manufacturing process

• Special mating component

• Hot-dip galvanized fastener requirements

But a different class should solve a defined engineering problem.

Do not specify a tighter class merely because it appears more precise.

Higher Manufacturing Precision ≠ Automatically Higher Joint Reliability.

Thread Tolerance Comparison Should Not Be Reduced to "Tight, Medium, Loose"

Terms such as:

• Precision fit

• Medium fit

• Loose fit

may help in informal discussion, but they are insufficient for an engineering drawing.

Procurement should request the exact thread designation.

For example:

M10 × 1.5–6H

is far more useful than:

M10 tight-fit internal thread.

Exact designations reduce interpretation differences across suppliers.

Second-Source Qualification: Thread Gauge Pass Is Not Enough

This is especially important for procurement.

Suppose Supplier A and Supplier B both deliver M8 × 1.25–6g screws.

Both pass the same thread ring gauge.

Are they functionally equivalent?

Not necessarily.

Compare:

Drawing + Revision + Material + Mechanical Properties + Heat Treatment + Thread Geometry + Tolerance Class + Surface Treatment 

+ Coating Thickness + Friction + Installation Behavior + Functional Testing + Documentation + Change Control

If a lock nut, weld nut, rivet nut or self-clinching fastener is involved, include its specific functional performance.

The principle remains:

Same Dimensions ≠ Functional Equivalence.

Common Thread Specification Mistakes

Mistake 1 — Assuming 6H/6g Automatically

Use the drawing or applicable product standard.

Mistake 2 — Specifying a Tighter Tolerance for More Strength

Thread tolerance and thread load capacity are different engineering questions.

Mistake 3 — Ignoring Coating Thickness

The final thread must still assemble correctly.

Mistake 4 — Gauging Only Before Coating

The finished component condition matters.

Mistake 5 — Using Thread Fit to Solve Vibration Loosening

Preload and locking strategy must be evaluated separately.

Mistake 6 — Using Tight Thread Fit to Create a Pressure Seal

Use the correct sealing architecture.

Mistake 7 — Approving a Second Source Because the Gauge Passes

Material, coating, friction and functional performance may still differ.

Mistake 8 — Treating Every Internal Thread as a Steel Nut

Aluminum, plastic, inserts, rivet nuts and weld nuts have different failure mechanisms.

Failure Mode Analysis for Thread Fit Problems

Supplier Qualification Questions for Threaded Fasteners

Procurement and supplier-development teams should ask:

• Which ISO thread standard applies?

• What exact tolerance class is required?

• Is the tolerance specified for the finished part?

• What material is used?

• What mechanical property class applies?

• Is heat treatment required?

• What surface treatment applies?

• What coating thickness is specified?

• Is friction controlled?

• How are threads inspected?

• Which GO/NO-GO gauges are used?

• Are gauges controlled for wear and verification?

• Is final gauging performed after coating where required?

• Are critical thread dimensions measured separately when necessary?

• Is functional assembly testing required?

• How are drawing revisions controlled?

• How are manufacturing or coating changes controlled?

This turns a thread RFQ into a supplier-qualification process rather than a simple diameter-and-pitch comparison.

Procurement Perspective: Tighter Tolerance Can Increase Cost Without Adding Value

A tighter thread tolerance can increase:

• Tooling control

• Inspection effort

• Scrap risk

• Process cost

• Manufacturing difficulty

If the application does not need it, the tighter tolerance may provide no functional benefit.

Conversely, an excessively loose or poorly controlled thread may create:

• Assembly inconsistency

• Alignment problems

• Reduced engagement

• Quality complaints

The correct objective is:

Required Functional Performance at Controlled Manufacturing Cost.

This can be evaluated through:

Total Installed Cost = Part Price + Tooling + Inspection + Assembly + Rework + Scrap + Downtime + Warranty Risk + Supply Risk

The lowest-priced threaded component is not necessarily the lowest-cost joint.

JUXIN FASTENERS Threaded Fastener Capabilities

JUXIN FASTENERS supports standard and drawing-based threaded components for OEM and industrial applications, including:

• Metric bolts

• Metric screws

• Standard nuts

• Nylon insert lock nuts

• All-metal lock nuts

• All-metal flange lock nuts

• DIN 929 hex weld nuts

• DIN 928 square weld nuts

• Projection weld nuts

• Floating cage weld nuts

• Blind rivet nuts

• Full-hex rivet nuts

• Half-hex rivet nuts

• Knurled round-body rivet nuts

• Self-clinching nuts

• Self-clinching studs

• Self-clinching standoffs

• Heat-set threaded inserts

• Ultrasonic threaded inserts

• Stainless steel threaded inserts

• Weld studs

• Custom cold-formed fasteners

• CNC machined threaded components

• Drawing-based OEM fasteners

• Second-source fastener development

Thread requirements can be evaluated together with:

• Material

• Mechanical properties

• Heat treatment

• Engagement

• Surface treatment

• Coating thickness

• Friction

• Mating component

• Installation

• Functional requirements

This is especially important when an OEM drawing combines a standard metric thread with a non-standard fastener body or application-specific surface treatment.

Engineer FAQ

What does 6H mean on a metric thread?

6H is an ISO metric internal-thread tolerance class. The number identifies the tolerance grade and H identifies the tolerance position.

What does 6g mean?

6g is a commonly used ISO metric external-thread tolerance class. The number identifies the tolerance grade and g identifies the tolerance position.

Is 6H/6g the standard fit for metric bolts and nuts?

It is a very common general-purpose combination, but engineers should use the tolerance required by the applicable product standard,

 drawing or OEM specification rather than assume it universally.

Is 6H/6h stronger than 6H/6g?

Not automatically. Threaded-joint strength depends on material, engagement, geometry and loading—not simply reduced clearance.

Does longer thread engagement always make a joint stronger?

Engagement can increase resistance to thread stripping, particularly in weaker parent materials, but the benefit is not unlimited or necessarily proportional.

Does 6H/6g prevent vibration loosening?

No. Thread tolerance does not replace correct preload, joint design or a suitable locking strategy.

Can 6H/6g threads seal pressure?

The tolerance class itself does not define a pressure seal. Sealing requires an appropriate sealing architecture.

Does zinc plating affect a 6g thread?

Yes. Coating occupies dimensional space and can affect the final thread condition. The finished thread requirement must be considered.

Is the coating effect always four times the coating thickness?

No. Do not use a universal multiplier for every thread and coating. Use the applicable thread/coating specification and dimensional requirements.

Should threads be gauged after coating?

The required final condition depends on the applicable specification, but finished-component thread compatibility must be controlled. 

Post-coating gauging may therefore be required.

Does a GO gauge measure pitch diameter?

A functional thread gauge evaluates specified limit conditions; it should not be treated as a direct numerical measurement of every individual thread parameter.

Procurement FAQ

What should I put on an RFQ for a metric threaded fastener?

Provide the exact thread designation, drawing revision, material, mechanical property class, surface treatment, coating thickness where applicable, 

mating component, functional requirements and annual volume.

Can I accept a second-source bolt if it passes the same ring gauge?

Gauge compliance is necessary but may not establish full functional equivalence. Compare material, heat treatment, coating, friction and installation performance as well.

Why does a plated bolt fit before coating but not afterward?

The deposited coating changes the effective thread geometry and can consume available clearance.

Can I specify a tighter tolerance to improve quality?

Only if the application needs it. Unnecessary tolerance tightening can increase cost and manufacturing risk without improving functional performance.

What should I check when sourcing weld nuts or rivet nuts?

Thread tolerance is only one requirement. Weld nuts require welding-process and alignment consideration; 

rivet nuts require grip range, hole geometry, setting and pull-out/spin-out validation.

Can JUXIN FASTENERS manufacture custom tolerance-controlled threaded parts?

JUXIN FASTENERS can evaluate drawing-based threaded fasteners and CNC machined components according to the specified thread, 

tolerance, material, surface treatment, mechanical and functional requirements.

RFQ Checklist for Threaded Fasteners and Custom Threaded Components

For accurate engineering and commercial evaluation, provide:

• 2D drawing

• 3D model if available

• Part number

• Drawing revision

• Application

• Thread system

• Nominal thread diameter

• Thread pitch

• Coarse or fine thread

• Internal or external thread

• Required tolerance class

• Engagement length

• Mating component

• Mating thread tolerance

• Base material

• Material grade

• Mechanical property class

• Heat treatment

• Hardness where applicable

• Surface treatment

• Coating specification

• Coating thickness

• Passivation or topcoat where applicable

• Final thread-gauge requirement

• Friction requirement where applicable

• Lubrication condition

• Installation torque

• Target preload where applicable

• Locking requirement

• Prevailing-torque requirement where applicable

• Reuse requirement

• Corrosion requirement

• Operating temperature

• Vibration environment

• Sealing requirement if applicable

• Pull-out or spin-out requirement for rivet nuts

• Push-out or torque-out requirement for self-clinching fasteners

• Welding requirement for weld nuts

• Parent material for inserts or tapped threads

• Functional testing

• Inspection documentation

• PPAP or customer-specific documentation where applicable

• Annual volume

• Existing approved sample for second-source development where available

For coated threaded parts, clearly identify whether the specified thread tolerance applies to the final finished component.

Related JUXIN FASTENERS Engineering Resources

This page should connect internally with existing JUXIN FASTENERS resources covering:

• Bolt and nut strength class matching

• BSP vs. NPT pipe threads

• Bolt tightening and over-torque

• Torque, friction and preload

• Fastener surface finish selection

• Zinc plating vs. zinc-nickel coating

• Zinc plating color variation

• Hydrogen embrittlement

• Rivet nut selection and installation

• Self-clinching fasteners

• Weld nuts and projection welding

• Threaded inserts for plastics

• Nylon insert lock nuts

• All-metal lock nuts

• Custom CNC threaded components

• Second-source fastener qualification

Use the existing indexed JUXIN FASTENERS URL when implementing each internal link.

The intended content path is:

Thread Identification → Tolerance Selection → Material & Engagement → Surface Treatment 

→ Final Thread Inspection → Assembly Validation → Supplier Qualification → RFQ

Final Engineering Rule: Thread Gauge Compliance Is Necessary, but It Is Not the Joint

The most important lesson is not that every industrial metric fastener should use 6H/6g.

The more useful principle is:

Specify the thread tolerance required by the actual product and application, then control the finished component accordingly.

The complete engineering chain is:

Thread Profile → Diameter & Pitch → Tolerance Class → Engagement → Material → Mechanical Properties → Surface Treatment

 → Final Thread Condition → Friction → Mating Component → Installation → Functional Validation

6H internal threads and 6g external threads are widely used because they provide practical general-purpose thread tolerances for many ISO metric fastener applications.

But:

6H/6g ≠ Universal Requirement.

Tighter Tolerance ≠ Higher Strength.

Thread Fit ≠ Anti-Loosening.

Thread Fit ≠ Pressure Seal.

GO Gauge Pass ≠ Complete Product Qualification.

Same Dimensions ≠ Functional Equivalence.

For design engineers, this approach prevents thread tolerances from being used to solve problems that actually belong to material strength, preload, coating, sealing or locking design.

For quality teams, it separates dimensional gauging from functional product qualification.

For procurement and supplier-development teams, it provides a more reliable framework for comparing suppliers and qualifying second sources.

JUXIN FASTENERS supports OEM, Tier-1, Tier-2 and industrial manufacturers with metric bolts, nuts, lock nuts, weld nuts, blind rivet nuts,

 self-clinching fasteners, threaded inserts, weld studs, custom cold-formed fasteners, CNC machined threaded components and drawing-based second-source development.

For a metric thread project, coating-related thread-fit problem, custom tolerance requirement, assembly complaint or alternative-source qualification, 

send your drawing, revision, exact thread designation, material, mechanical requirements, surface treatment, mating component, installation conditions and annual volume for technical review.

JUXIN FASTENERS

Email: info@juxinfasteners.com

Website: www.juxinfasteners.com