Inch STI Drill Reference · UNC & UNF Wire Thread Inserts

UNC & UNF STI Tap Drill Chart for Wire Thread Inserts

This inch STI tap drill chart provides common drill references for UNC and UNF wire thread insert installation threads, including 4-40, 6-32, 8-32, 10-24, 10-32, 1/4-20, 1/4-28 and larger sizes.

Use these values when planning helical wire insert repair, reinforcement, STI tapping, repair kits or tooling. The drill must match the selected STI tap and insert system rather than a normal standard tap.

Quick Answer

Common references include 4-40 UNC at 3.0 mm / No.31, 6-32 UNC at 3.7 mm / No.25, 8-32 UNC at 4.4 mm / 11/64, 10-32 UNF at 5.1 mm / 13/64, 1/4-20 UNC at 6.7 mm / 17/64, and 1/4-28 UNF at 6.6 mm / 17/64.

View UNC Chart View UNF Chart Metric STI Chart

STI Drill Sizes Are Not Standard Tap Drill Sizes

STI means Screw Thread Insert. An STI tap creates the oversized installation thread required to hold a wire thread insert. The final internal thread for the mating screw is created by the installed insert, so a normal UNC or UNF tap drill chart should not be substituted for an STI drill reference.

Original Screw Thread

Start with the required finished thread, such as 6-32 UNC, 10-32 UNF or 1/4-20 UNC.

STI Receiving Thread

A matching STI tap cuts the larger receiving thread into which the wire insert is installed.

Installed Insert

After installation, the insert restores the required internal UNC or UNF thread for the original mating screw.

Technical caution: Treat the values below as common STI drill references. Final production dimensions should be confirmed against the selected insert system, STI tap specification, parent material, hole condition and approved manufacturing or repair procedure.

Quick Answers for Common Inch Helicoil Drill Sizes

These are common inch wire insert sizes used in maintenance and industrial thread repair.

4-40 UNC 3.0 mm / No.31

Common STI drill reference before tapping the receiving thread for a 4-40 wire insert.

6-32 UNC 3.7 mm / No.25

Common STI drill reference for a 6-32 UNC wire thread insert.

8-32 UNC 4.4 mm / 11/64

Common drill reference before using the matching 8-32 STI tap.

10-24 UNC 5.0 mm / 13/64

Common STI drill reference for the coarse No.10 wire insert system.

10-32 UNF 5.1 mm / 13/64

Common STI drill reference for the fine No.10 wire insert system.

1/4-20 UNC 6.7 mm / 17/64

Common STI drill reference for a 1/4-20 UNC wire thread insert.

1/4-28 UNF 6.6 mm / 17/64

Common STI drill reference for the fine-thread 1/4-28 insert system.

3/8-16 UNC 9.9 mm / 25/64

Common STI drill reference for a 3/8-16 UNC wire insert.

3/8-24 UNF 9.8 mm / 25/64

Common STI drill reference for the fine-thread 3/8-24 system.

UNC STI Tap Drill Chart

UNC is the Unified National Coarse thread series. The table below lists common pre-drill references for STI tapping before installation of UNC wire thread inserts.

Finished Thread TPI Common Metric Drill Reference Common Inch Drill Reference Matching Tap
#2-56 UNC 56 2.3 mm 3/32 #2-56 STI tap
#3-48 UNC 48 2.7 mm No.36 #3-48 STI tap
#4-40 UNC 40 3.0 mm No.31 #4-40 STI tap
#5-40 UNC 40 3.4 mm No.29 #5-40 STI tap
#6-32 UNC 32 3.7 mm No.25 #6-32 STI tap
#8-32 UNC 32 4.4 mm 11/64 #8-32 STI tap
#10-24 UNC 24 5.0 mm 13/64 #10-24 STI tap
#12-24 UNC 24 5.8 mm 15/64 #12-24 STI tap
1/4-20 UNC 20 6.7 mm 17/64 1/4-20 STI tap
5/16-18 UNC 18 8.3 mm 21/64 5/16-18 STI tap
3/8-16 UNC 16 9.9 mm 25/64 3/8-16 STI tap
7/16-14 UNC 14 11.5 mm 29/64 7/16-14 STI tap
1/2-13 UNC 13 13.0 mm 17/32 1/2-13 STI tap
The drill listed for an STI tap is larger than the drill used for the corresponding normal internal UNC thread. Do not select the drill from a standard UNC tap chart when preparing a hole for a wire thread insert.

UNF STI Tap Drill Chart

UNF is the Unified National Fine thread series. Fine-thread insert systems require their own matching STI tap and drill preparation.

Finished Thread TPI Common Metric Drill Reference Common Inch Drill Reference Matching Tap
#4-48 UNF 48 3.0 mm No.31 #4-48 STI tap
#6-40 UNF 40 3.7 mm No.26 #6-40 STI tap
#8-36 UNF 36 4.4 mm 11/64 #8-36 STI tap
#10-32 UNF 32 5.1 mm 13/64 #10-32 STI tap
1/4-28 UNF 28 6.6 mm 17/64 1/4-28 STI tap
5/16-24 UNF 24 8.2 mm 21/64 5/16-24 STI tap
3/8-24 UNF 24 9.8 mm 25/64 3/8-24 STI tap
7/16-20 UNF 20 11.5 mm 29/64 7/16-20 STI tap
1/2-20 UNF 20 13.0 mm 33/64 1/2-20 STI tap
A fractional drill and a metric drill shown on the same row are manufacturer-style reference options, not necessarily exact mathematical conversions of each other. Follow the drill recommendation for the specific STI tap system being used.

UNC vs UNF Wire Thread Insert Sizes

The nominal diameter alone is not enough to specify an inch wire insert. The thread series and TPI must also be identified.

UNC — Coarse Thread

  • Examples include 6-32, 8-32, 10-24 and 1/4-20.
  • Identify the nominal diameter and TPI.
  • Use the corresponding UNC STI tap.
  • Match insert length and installation tooling separately.

UNF — Fine Thread

  • Examples include 10-32, 1/4-28 and 3/8-24.
  • Do not substitute the UNC STI tap of the same diameter.
  • Confirm TPI before ordering taps or repair kits.
  • Check the mating screw before final specification approval.

How to Use an Inch STI Tap Drill Chart

Treat the drill, STI tap and wire insert as one matched system. Selecting only the original bolt size is not enough.

1. Confirm Thread Size

Identify the original finished screw thread, including UNC or UNF and the correct TPI.

2. Select STI Drill

Use the reference intended for the selected wire insert STI tap, not the normal tap-drill value.

3. Match the STI Tap

Use an STI tap with the same finished thread designation as the insert system.

4. Select Insert Length

Specify insert length such as 1D, 1.5D, 2D or another required engagement length.

5. Confirm Installation Tool

Tanged and tangless inserts can require different compatible installation tooling.

6. Inspect the Finished Thread

Confirm installation depth, cleanliness and mating-screw engagement according to the approved process.

STI Taps Installation Guide Installation Tools

Common Inch STI Drill Selection Mistakes

Mistake Why It Matters Better Practice
Using a standard UNC/UNF tap drill chart A standard tap does not create the receiving thread required for a wire insert. Use the STI drill and matching STI tap for the insert system.
Confusing 10-24 and 10-32 Both use No.10 nominal size but have different thread pitches. Confirm whether the finished thread is UNC 10-24 or UNF 10-32.
Confusing 1/4-20 and 1/4-28 Same nominal diameter does not mean the STI taps are interchangeable. Specify the full thread designation including TPI.
Assuming metric and fractional drills are exact conversions Manufacturer charts can recommend nearby standard drill sizes. Follow the selected STI tap manufacturer’s approved value.
Ignoring parent material Hole quality, tapping behavior and tool life vary with material. Confirm aluminum, steel, stainless steel or other parent material.

UNC / UNF Wire Insert RFQ Checklist

For quotations, send enough information to match the insert, STI tap, drill and installation tooling correctly.

Thread

  • UNC or UNF.
  • Nominal size.
  • Threads per inch (TPI).
  • Through hole or blind hole.

Insert

  • Free-running or screw-locking.
  • Tanged or tangless.
  • Insert length.
  • Material requirement.

Tools & Quantity

  • STI tap requirement.
  • Drill requirement.
  • Installation tool requirement.
  • Quantity and packaging.
RFQ example: 1/4-20 UNC tanged free-running wire inserts, 1.5D, matching STI taps and installation tools, sample required before bulk order.

Related STI Tap and Wire Insert Resources

Metric STI Tap Drill Chart

Use the existing metric chart for M3, M4, M5, M6, M8, M10 and M12 wire insert systems.

View metric chart →

STI Taps

Review taps designed to produce the receiving thread required for wire thread insert installation.

View STI taps →

Wire Thread Inserts

Compare tanged, tangless, free-running and screw-locking wire thread insert options.

View wire thread inserts →

FAQ: UNC & UNF STI Tap Drill Sizes

What drill size is commonly used for a 4-40 UNC wire thread insert?

A common STI drill reference is 3.0 mm or No.31. Confirm the final value against the selected 4-40 STI tap system.

What is the common 6-32 helicoil drill size?

A common STI drill reference for 6-32 UNC is 3.7 mm or No.25.

What is the common 8-32 STI drill size?

A common reference for 8-32 UNC is 4.4 mm or 11/64.

What drill size is commonly used for a 10-32 wire insert?

For 10-32 UNF, a common STI drill reference is 5.1 mm or 13/64.

What is the common 1/4-20 helicoil tap drill size?

A common STI drill reference for 1/4-20 UNC is 6.7 mm or 17/64.

What is the common 1/4-28 helicoil tap drill size?

A common STI drill reference for 1/4-28 UNF is 6.6 mm or 17/64.

Can I use a normal UNC or UNF tap instead of an STI tap?

No. A wire thread insert requires the oversized receiving thread produced by the corresponding STI tap.

Why can UNC and UNF sizes use the same fractional drill?

Standard fractional drills are discrete sizes rather than exact metric conversions. A manufacturer’s chart may recommend the same fractional drill for two nearby STI preparation diameters while listing slightly different metric references.

Should these drill sizes be treated as final production dimensions?

They should be treated as common references. Confirm the selected STI tap, insert system, tolerance, parent material and approved process before production or critical repair work.

HELICOIL and Heli-Coil are trademarks of their respective owner. Taiyi Thread Inserts is an independent manufacturer of generic wire thread inserts and a supplier of matched STI taps and thread repair products. References to HELICOIL-related terminology are descriptive only and do not indicate affiliation, authorization or endorsement by the trademark owner.

Need UNC or UNF Wire Thread Inserts and STI Taps?

Send the thread size, UNC or UNF designation, TPI, insert length, quantity, parent material and tooling requirement for specification review.