Metric · UNC · UNF Wire Insert Reference

Wire Thread Insert Size Chart: Metric, UNC & UNF Sizes and Lengths

Use this wire thread insert size chart to compare common Metric, UNC, and UNF thread sizes, pitch or TPI, and nominal 1D to 3D insert-length designations. The tables are intended for insert specification and RFQ planning; STI tap drill sizes are covered separately.

Quick Answer

A complete wire thread insert specification normally includes the thread standard, nominal size, pitch or TPI, D-length, free-running or screw-locking function, tanged or tangless design, material requirement, and quantity. D-length is a nominal insert-length designation—not the loose free-state coil length.

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Wire thread insert size chart and RFQ guide for metric UNC and UNF insert sizes, pitch and length

Metric Coarse Wire Thread Insert Size Chart

Common metric coarse wire insert sizes are shown with pitch and nominal 1D, 1.5D, 2D, 2.5D, and 3D length references. Values are in millimeters.

Swipe horizontally to view all length columns →

Metric coarse wire thread insert size and nominal length chart
Metric Thread	Pitch	1D	1.5D	2D	2.5D	3D
M2 × 0.4	0.4 mm	2.0 mm	3.0 mm	4.0 mm	5.0 mm	6.0 mm
M2.2 × 0.45	0.45 mm	2.2 mm	3.3 mm	4.4 mm	5.5 mm	6.6 mm
M2.5 × 0.45	0.45 mm	2.5 mm	3.8 mm	5.0 mm	6.3 mm	7.5 mm
M3 × 0.5	0.5 mm	3.0 mm	4.5 mm	6.0 mm	7.5 mm	9.0 mm
M3.5 × 0.6	0.6 mm	3.5 mm	5.3 mm	7.0 mm	8.8 mm	10.5 mm
M4 × 0.7	0.7 mm	4.0 mm	6.0 mm	8.0 mm	10.0 mm	12.0 mm
M5 × 0.8	0.8 mm	5.0 mm	7.5 mm	10.0 mm	12.5 mm	15.0 mm
M6 × 1.0	1.0 mm	6.0 mm	9.0 mm	12.0 mm	15.0 mm	18.0 mm
M7 × 1.0	1.0 mm	7.0 mm	10.5 mm	14.0 mm	17.5 mm	21.0 mm
M8 × 1.25	1.25 mm	8.0 mm	12.0 mm	16.0 mm	20.0 mm	24.0 mm
M10 × 1.5	1.5 mm	10.0 mm	15.0 mm	20.0 mm	25.0 mm	30.0 mm
M12 × 1.75	1.75 mm	12.0 mm	18.0 mm	24.0 mm	30.0 mm	36.0 mm
M14 × 2.0	2.0 mm	14.0 mm	21.0 mm	28.0 mm	35.0 mm	42.0 mm
M16 × 2.0	2.0 mm	16.0 mm	24.0 mm	32.0 mm	40.0 mm	48.0 mm
M18 × 2.5	2.5 mm	18.0 mm	27.0 mm	36.0 mm	45.0 mm	54.0 mm
M20 × 2.5	2.5 mm	20.0 mm	30.0 mm	40.0 mm	50.0 mm	60.0 mm
M22 × 2.5	2.5 mm	22.0 mm	33.0 mm	44.0 mm	55.0 mm	66.0 mm
M24 × 3.0	3.0 mm	24.0 mm	36.0 mm	48.0 mm	60.0 mm	72.0 mm
M27 × 3.0	3.0 mm	27.0 mm	40.5 mm	54.0 mm	67.5 mm	81.0 mm
M30 × 3.5	3.5 mm	30.0 mm	45.0 mm	60.0 mm	75.0 mm	90.0 mm
M33 × 3.5	3.5 mm	33.0 mm	49.5 mm	66.0 mm	82.5 mm	99.0 mm
M36 × 4.0	4.0 mm	36.0 mm	54.0 mm	72.0 mm	90.0 mm	108.0 mm
M39 × 4.0	4.0 mm	39.0 mm	58.5 mm	78.0 mm	97.5 mm	117.0 mm

Reference note: These are nominal thread-size and D-length references used for technical comparison. Confirm final availability, material, tolerances, and approved product data before production use.

M6 Size Guide M8 Size Guide Metric STI Tap Drill Chart

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Metric Fine Wire Thread Insert Size Chart

Fine-pitch inserts use the same nominal diameter as the corresponding metric size but a different pitch. The insert, STI tap, fastener, and prepared installation thread must all match the same fine-pitch specification.

Swipe horizontally to view all length columns →

Metric fine wire thread insert size and nominal length chart
Metric Fine Thread	Pitch	1D	1.5D	2D	2.5D	3D
M8 × 1.0	1.0 mm	8.0 mm	12.0 mm	16.0 mm	20.0 mm	24.0 mm
M10 × 1.0	1.0 mm	10.0 mm	15.0 mm	20.0 mm	25.0 mm	30.0 mm
M10 × 1.25	1.25 mm	10.0 mm	15.0 mm	20.0 mm	25.0 mm	30.0 mm
M12 × 1.25	1.25 mm	12.0 mm	18.0 mm	24.0 mm	30.0 mm	36.0 mm
M12 × 1.5	1.5 mm	12.0 mm	18.0 mm	24.0 mm	30.0 mm	36.0 mm
M14 × 1.5	1.5 mm	14.0 mm	21.0 mm	28.0 mm	35.0 mm	42.0 mm
M16 × 1.5	1.5 mm	16.0 mm	24.0 mm	32.0 mm	40.0 mm	48.0 mm
M18 × 1.5	1.5 mm	18.0 mm	27.0 mm	36.0 mm	45.0 mm	54.0 mm
M20 × 1.5	1.5 mm	20.0 mm	30.0 mm	40.0 mm	50.0 mm	60.0 mm
M22 × 1.5	1.5 mm	22.0 mm	33.0 mm	44.0 mm	55.0 mm	66.0 mm
M18 × 2.0	2.0 mm	18.0 mm	27.0 mm	36.0 mm	45.0 mm	54.0 mm
M20 × 2.0	2.0 mm	20.0 mm	30.0 mm	40.0 mm	50.0 mm	60.0 mm
M22 × 2.0	2.0 mm	22.0 mm	33.0 mm	44.0 mm	55.0 mm	66.0 mm
M24 × 2.0	2.0 mm	24.0 mm	36.0 mm	48.0 mm	60.0 mm	72.0 mm
M27 × 2.0	2.0 mm	27.0 mm	40.5 mm	54.0 mm	67.5 mm	81.0 mm
M30 × 2.0	2.0 mm	30.0 mm	45.0 mm	60.0 mm	75.0 mm	90.0 mm
M33 × 2.0	2.0 mm	33.0 mm	49.5 mm	66.0 mm	82.5 mm	99.0 mm
M36 × 2.0	2.0 mm	36.0 mm	54.0 mm	72.0 mm	90.0 mm	108.0 mm
M39 × 2.0	2.0 mm	39.0 mm	58.5 mm	78.0 mm	97.5 mm	117.0 mm
M36 × 3.0	3.0 mm	36.0 mm	54.0 mm	72.0 mm	90.0 mm	108.0 mm
M39 × 3.0	3.0 mm	39.0 mm	58.5 mm	78.0 mm	97.5 mm	117.0 mm

Selection warning: Do not substitute a coarse-pitch insert for a fine-pitch thread of the same nominal diameter. Pitch is part of the insert specification.

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UNC Wire Thread Insert Size Chart

Unified National Coarse (UNC) sizes are shown in inches. The thread designation already includes TPI; the separate TPI column is included for faster checking.

Swipe horizontally to view all length columns →

UNC wire thread insert size and nominal length chart
UNC Thread	TPI	1D	1.5D	2D	2.5D	3D
1-64	64	0.073 in	0.110 in	0.146 in	0.182 in	0.219 in
2-56	56	0.086 in	0.129 in	0.172 in	0.215 in	0.258 in
3-48	48	0.099 in	0.148 in	0.198 in	0.248 in	0.297 in
4-40	40	0.112 in	0.168 in	0.224 in	0.280 in	0.336 in
5-40	40	0.125 in	0.188 in	0.250 in	0.312 in	0.375 in
6-32	32	0.138 in	0.207 in	0.276 in	0.345 in	0.414 in
8-32	32	0.164 in	0.246 in	0.328 in	0.410 in	0.492 in
10-24	24	0.190 in	0.285 in	0.380 in	0.475 in	0.570 in
12-24	24	0.216 in	0.324 in	0.432 in	0.540 in	0.648 in
1/4-20	20	0.250 in	0.375 in	0.500 in	0.625 in	0.750 in
5/16-18	18	0.312 in	0.469 in	0.625 in	0.781 in	0.938 in
3/8-16	16	0.375 in	0.562 in	0.750 in	0.938 in	1.125 in
7/16-14	14	0.438 in	0.656 in	0.875 in	1.094 in	1.312 in
1/2-13	13	0.500 in	0.750 in	1.000 in	1.250 in	1.500 in
9/16-12	12	0.562 in	0.844 in	1.125 in	1.406 in	1.688 in
5/8-11	11	0.625 in	0.938 in	1.250 in	1.562 in	1.875 in
3/4-10	10	0.750 in	1.125 in	1.500 in	1.875 in	2.250 in
7/8-9	9	0.875 in	1.312 in	1.750 in	2.188 in	2.625 in
1-8	8	1.000 in	1.500 in	2.000 in	2.500 in	3.000 in

Units: All nominal length values in this UNC table are inches.

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UNF Wire Thread Insert Size Chart

Unified National Fine (UNF) inserts use a finer thread pitch than UNC. Confirm the exact thread designation and TPI before selecting the insert or STI tap.

Swipe horizontally to view all length columns →

UNF wire thread insert size and nominal length chart
UNF Thread	TPI	1D	1.5D	2D	2.5D	3D
0-80	80	0.060 in	0.090 in	0.120 in	—	—
3-56	56	0.099 in	0.148 in	0.198 in	0.248 in	0.297 in
4-48	48	0.112 in	0.168 in	0.224 in	0.280 in	0.336 in
6-40	40	0.138 in	0.207 in	0.276 in	0.345 in	0.414 in
8-36	36	0.164 in	0.246 in	0.328 in	0.410 in	0.492 in
10-32	32	0.190 in	0.285 in	0.380 in	0.475 in	0.570 in
1/4-28	28	0.250 in	0.375 in	0.500 in	0.625 in	0.750 in
5/16-24	24	0.312 in	0.469 in	0.625 in	0.781 in	0.938 in
3/8-24	24	0.375 in	0.562 in	0.750 in	0.938 in	1.125 in
7/16-20	20	0.438 in	0.656 in	0.875 in	1.094 in	1.312 in
1/2-20	20	0.500 in	0.750 in	1.000 in	1.250 in	1.500 in
9/16-18	18	0.562 in	0.844 in	1.125 in	1.406 in	1.688 in
5/8-18	18	0.625 in	0.938 in	1.250 in	1.562 in	1.875 in
3/4-16	16	0.750 in	1.125 in	1.500 in	1.875 in	2.250 in
7/8-14	14	0.875 in	1.312 in	1.750 in	2.188 in	2.625 in
1-12	12	1.000 in	1.500 in	2.000 in	2.500 in	3.000 in

Units: All nominal length values in this UNF table are inches. A dash means that the corresponding reference length is not included in this chart and should be confirmed separately.

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What Do 1D, 1.5D, 2D, 2.5D and 3D Mean?

D-length describes the insert’s nominal length class relative to the nominal thread diameter. It is useful for specification and ordering, but it should not be treated as the same measurement as free-state coil length or final installed depth.

1D

Nominal length class equal to one thread diameter. Example: M8 → 8 mm nominal reference.

1.5D

Nominal length class equal to one and a half thread diameters. Example: M8 → 12 mm nominal reference.

2D

Nominal length class equal to two thread diameters. Example: M8 → 16 mm nominal reference.

2.5D / 3D

Longer nominal length classes for applications where part geometry, parent material, load, and available depth support them.

Important measurement note: A loose wire insert is spring-like and its free-state length is not the same dimension as the nominal D-length shown in a specification table. Do not reject or approve an insert by comparing a loose coil directly with the nominal D-length alone.

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How to Read a Wire Thread Insert Specification

“M8 × 1.25, 1.5D, free-running, tanged” contains four separate decisions. Each one affects the supplied insert, compatible tooling, and application fit.

Thread Size

M8 identifies the nominal metric internal thread size that the installed insert is intended to receive.

Pitch / TPI

1.25 mm defines metric pitch. For UNC or UNF, use the exact thread designation and TPI.

D-Length

1.5D identifies the nominal insert-length class used for specification and ordering.

Insert Type

Free-running, screw-locking, tanged, and tangless designs require different selection and tooling decisions.

Example RFQ line: M8 × 1.25, 1.5D, free-running, tanged wire thread insert, stainless steel requirement, 5,000 pcs bulk packed.

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Select the Correct Wire Thread Insert Type

Size and D-length are only part of the specification. Insert function and installation design must also be defined.

Free-Running

General-purpose helical wire insert for repair or reinforcement where normal screw movement is required.

View free-running inserts →

Screw-Locking

Selected where additional screw retention is required. Locking-torque requirements should be defined.

View screw-locking inserts →

Tanged

Uses a driving tang during installation. Tang removal requirements depend on the application.

View tanged inserts →

Tangless

Uses a tangless installation system where loose-tang control matters. Matching tangless tools are required.

View tangless inserts →

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Thread Insert Size Chart vs STI Tap Drill Chart

These references answer different questions. This page focuses on the insert itself; the STI tap drill chart focuses on preparing the receiving hole and installation thread.

Comparison of thread insert size chart, STI tap drill chart, and repair kit specification
Reference	Main Purpose	Information Covered	Use It When
Thread Insert Size Chart	Select and specify the insert.	Metric / UNC / UNF thread size, pitch or TPI, D-length, insert type, material, and quantity.	Preparing a product list, refill pack, distributor range, or RFQ.
STI Tap Drill Chart	Prepare the hole and STI thread.	STI drill reference, matching STI tap, and installation preparation.	Planning drilling, STI tapping, and thread repair installation.
Repair Kit Specification	Define a complete kit.	Inserts, drill bit, STI tap, installation tool, tang-break tool, case, label, and refill plan.	Buying or private-labeling single-size or assortment repair kits.

Open STI Tap Drill Chart View STI Taps View Thread Repair Kits Repair Kit Buying Guide

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Do Not Mix Different Thread Insert Families

Wire thread inserts, key-locking inserts, self-tapping inserts, and thin-wall solid inserts use different product dimensions and installation systems. A size from one family cannot be assumed to match another.

Comparison of different threaded insert families
Insert Family	Typical Identification	Installation Requirement	Selection Warning
Wire Thread Inserts	Thread size, pitch/TPI, D-length, free-running/locking, tanged/tangless.	Matching STI hole preparation, STI tap, and installation tool.	Use this page for wire insert size and D-length planning.
Key-Locking Inserts	Internal thread, external thread, body length, key configuration.	Family-specific drill, tap, installation, and key-locking steps.	Do not apply wire-insert D-length tables.
Self-Tapping Inserts	Internal thread, external body geometry, length, cutting-slot design.	Prepared receiving hole and family-specific installation method.	External body size is a separate selection variable.
Thin-Wall Solid Inserts	Internal thread, external thread, wall thickness, length, locking method.	Family-specific tapped hole and installation procedure.	Use the actual product drawing or approved data.

Key-Locking Inserts Self-Tapping Inserts Thin-Wall Threaded Inserts

Thread Insert RFQ Checklist

Send a complete specification instead of only a nominal thread size. This reduces quotation revisions and helps prevent mismatched inserts, tools, and packaging.

Insert Specification

Metric, UNC, UNF, or other thread standard.
Nominal thread size and pitch or TPI.
Length class: 1D, 1.5D, 2D, 2.5D, 3D, or specified custom requirement.
Free-running or screw-locking.
Tanged or tangless.

Application Details

Parent material and part type.
Repair, reinforcement, or production installation.
Available hole depth and wall thickness.
Load, vibration, temperature, and corrosion requirements.
Drawing, sample, or approved specification when available.

Commercial Details

Quantity by size and length.
Bulk inserts, refill packs, or complete repair kits.
STI tap and installation tool requirement.
Bulk, retail, barcode, or private-label packaging.
Sample quantity and target bulk order.

RFQ example: M8 × 1.25, 1.5D, free-running, tanged wire thread inserts, stainless steel requirement, 5,000 pcs bulk packed, with optional M8 × 1.25 STI tap and installation tool.

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FAQ: Wire Thread Insert Sizes and Lengths

What does 1D, 1.5D, or 2D mean for a wire thread insert?

It identifies a nominal insert-length class relative to the nominal thread diameter. For an M8 insert, the nominal references are 8 mm for 1D, 12 mm for 1.5D, and 16 mm for 2D. Do not treat these values as the same measurement as free-state coil length.

Is a thread insert size chart the same as an STI tap drill chart?

No. This page identifies insert thread size, pitch or TPI, and D-length. The dedicated STI tap drill chart covers the drill reference and matching STI tap used to prepare the installation thread.

Can metric coarse and metric fine wire thread inserts be interchanged?

No. The nominal diameter may be the same, but the pitch is different. The insert, STI tap, fastener, and prepared installation thread must use the same thread specification.

Can UNC and UNF inserts of the same diameter be interchanged?

No. UNC and UNF use different threads per inch. Always specify the complete thread designation, such as 1/4-20 UNC or 1/4-28 UNF.

How do I choose between free-running and screw-locking inserts?

Free-running inserts support normal screw movement. Screw-locking inserts are selected where additional screw retention is required. Define vibration, removal-cycle, and torque requirements before approval.

What information should I send for a wire thread insert quotation?

Send the thread standard, nominal size, pitch or TPI, D-length, insert type, material requirement, quantity, tool requirement, packaging format, and any drawing or approved specification.

Technical reference note: Chart values are for size comparison and specification planning. Final product availability, materials, tolerances, locking performance, coatings, and installation dimensions should be confirmed against the approved product drawing or applicable specification.

HELICOIL is a trademark of its respective owner. Taiyi Thread Inserts supplies generic helical wire thread inserts, STI taps, installation tools, and repair kits. This page uses generic product terminology for engineering reference and quotation guidance.

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Need Help Reviewing a Wire Thread Insert Size List?

Send the thread standard, size, pitch or TPI, D-length, insert type, material requirement, quantity, tooling requirement, and packaging format. Taiyi can review bulk inserts, refill packs, repair kits, OEM supply, and private-label requirements.

Request a Quote

Wire Thread Insert Size Chart: Metric, UNC & UNF Sizes and Lengths

Metric Coarse Wire Thread Insert Size Chart

Metric Fine Wire Thread Insert Size Chart

UNC Wire Thread Insert Size Chart

UNF Wire Thread Insert Size Chart

What Do 1D, 1.5D, 2D, 2.5D and 3D Mean?

1D

1.5D

2D

2.5D / 3D

How to Read a Wire Thread Insert Specification

Thread Size

Pitch / TPI

D-Length

Insert Type

Select the Correct Wire Thread Insert Type

Free-Running

Screw-Locking

Tanged

Tangless

Thread Insert Size Chart vs STI Tap Drill Chart

Do Not Mix Different Thread Insert Families

Thread Insert RFQ Checklist

Insert Specification

Application Details

Commercial Details

FAQ: Wire Thread Insert Sizes and Lengths

Need Help Reviewing a Wire Thread Insert Size List?

Taiyi Thread Inserts

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