STI Tap Size Chart for Wire Thread Inserts
An STI tap is the special tap used to create the receiving thread for a wire thread insert. The required tap normally carries the same nominal thread designation as the finished screw thread, but it is an STI tap—not a standard tap.
For example, an M8 × 1.25 wire thread insert requires an M8 × 1.25 STI tap. It should not be replaced with an ordinary M8 tap or a larger standard metric tap.
Match the STI tap to the final thread designation: M6 × 1.0 insert → M6 × 1.0 STI tap; M8 × 1.25 insert → M8 × 1.25 STI tap; M10 × 1.5 insert → M10 × 1.5 STI tap. For inch threads, a 1/4-20 UNC insert uses a 1/4-20 UNC STI tap, while a 10-32 UNF insert uses a 10-32 UNF STI tap.
Why an STI Tap Is Different from a Standard Tap
STI stands for Screw Thread Insert. The STI tap creates the larger receiving thread that holds the helical wire insert. After the insert is installed, the inside of the insert provides the required finished thread for the mating screw.
Same Nominal Thread
The STI tap is identified by the finished thread size, such as M8 × 1.25 STI or 1/4-20 UNC STI.
Different Thread Form
An STI tap is designed to produce the receiving thread required for the wire insert and is not interchangeable with a normal tap of the same designation.
Do Not Guess a Larger Tap
Do not substitute an M10 or M12 standard tap simply because the prepared hole for an M8 insert is larger than a normal M8 hole.
Common Metric Helicoil Tap Sizes
For common metric coarse wire insert systems, the STI tap designation follows the finished thread size and pitch.
Use a special M4 × 0.7 STI tap, not a standard M4 tap.
Match the STI tap to the M5 × 0.8 insert system.
Do not substitute an ordinary M6 × 1.0 machine tap.
The required tap is an M8 × 1.25 STI tap—not a standard M10 tap.
Use the dedicated M10 × 1.5 STI thread-insert tap.
Confirm pitch before ordering the insert and STI tap.
Metric STI Tap Size Chart
The table identifies the required STI tap designation for common metric coarse wire thread inserts.
| Finished Thread | Common Pitch | Required Tap | Standard Tap Substitute? | Drill Reference |
|---|---|---|---|---|
| M3 | 0.5 mm | M3 × 0.5 STI tap | No | Use Metric STI Drill Chart |
| M4 | 0.7 mm | M4 × 0.7 STI tap | No | Use Metric STI Drill Chart |
| M5 | 0.8 mm | M5 × 0.8 STI tap | No | Use Metric STI Drill Chart |
| M6 | 1.0 mm | M6 × 1.0 STI tap | No | Use Metric STI Drill Chart |
| M8 | 1.25 mm | M8 × 1.25 STI tap | No | Use Metric STI Drill Chart |
| M10 | 1.5 mm | M10 × 1.5 STI tap | No | Use Metric STI Drill Chart |
| M12 | 1.75 mm | M12 × 1.75 STI tap | No | Use Metric STI Drill Chart |
| M14 | 2.0 mm | M14 × 2.0 STI tap | No | Confirm system data |
| M16 | 2.0 mm | M16 × 2.0 STI tap | No | Confirm system data |
What Tap Size Is Used for M8 × 1.25 and M10 × 1.5 Inserts?
These two sizes cause frequent confusion because users often assume that the larger STI receiving thread can be made with an ordinary larger metric tap. That is not the correct way to specify a wire thread insert system.
M8 × 1.25 Wire Thread Insert
- Finished screw thread: M8 × 1.25.
- Required tap: M8 × 1.25 STI tap.
- A standard M8 × 1.25 tap is not suitable.
- Do not specify a normal M10 tap as the substitute.
- Use the matching STI drill reference before tapping.
M10 × 1.5 Wire Thread Insert
- Finished screw thread: M10 × 1.5.
- Required tap: M10 × 1.5 STI tap.
- The STI thread geometry is specific to insert installation.
- A normal larger metric tap should not be guessed.
- Confirm the selected insert system before production.
UNC & UNF STI Tap Size Chart
The same rule applies to Unified inch threads: the STI tap carries the finished UNC or UNF thread designation.
| Finished Thread | Series | Required Tap | Standard Tap Substitute? | Drill Reference |
|---|---|---|---|---|
| #4-40 | UNC | #4-40 UNC STI tap | No | Use Inch STI Drill Chart |
| #6-32 | UNC | #6-32 UNC STI tap | No | Use Inch STI Drill Chart |
| #8-32 | UNC | #8-32 UNC STI tap | No | Use Inch STI Drill Chart |
| #10-24 | UNC | #10-24 UNC STI tap | No | Use Inch STI Drill Chart |
| #10-32 | UNF | #10-32 UNF STI tap | No | Use Inch STI Drill Chart |
| 1/4-20 | UNC | 1/4-20 UNC STI tap | No | Use Inch STI Drill Chart |
| 1/4-28 | UNF | 1/4-28 UNF STI tap | No | Use Inch STI Drill Chart |
| 5/16-18 | UNC | 5/16-18 UNC STI tap | No | Use Inch STI Drill Chart |
| 5/16-24 | UNF | 5/16-24 UNF STI tap | No | Use Inch STI Drill Chart |
| 3/8-16 | UNC | 3/8-16 UNC STI tap | No | Use Inch STI Drill Chart |
| 3/8-24 | UNF | 3/8-24 UNF STI tap | No | Use Inch STI Drill Chart |
STI Tap Class of Fit Also Matters
Thread size alone may not completely define an STI tap for a controlled industrial application. The required thread class, tap geometry, hole type, parent material and tapping method should also be confirmed.
Metric STI Taps
Metric STI systems may use controlled metric fit classes such as 4H or 5H depending on the specified insert system.
Unified STI Taps
Unified STI systems may be specified in classes such as 2B or 3B depending on the application and product system.
Tap Geometry
Plug, bottoming, spiral-point, spiral-flute or other tap geometry should be selected according to hole type, material and production process.
How to Select the Correct STI Tap
1. Identify the Finished Thread
Confirm metric, UNC, UNF or another thread standard, including nominal size and pitch or TPI.
2. Add the STI Requirement
Specify an STI tap for the same nominal finished thread rather than selecting a normal standard tap.
3. Confirm Drill Preparation
Use the drill reference matched to the STI tap and insert system.
4. Confirm Hole Type
Through holes and blind holes can require different tap geometry and available thread depth.
5. Confirm Parent Material
Aluminum, steel and stainless steel can require different cutting geometry, coating and process controls.
6. Match the Insert
The STI tap, insert size, pitch, insert length and installation tooling must belong to the same system.
Common Helicoil Tap Size Mistakes
| Mistake | Why It Is Wrong | Correct Approach |
|---|---|---|
| Using a normal tap of the finished thread size | A normal tap does not create the special receiving thread required for a wire insert. | Use the matching STI tap. |
| Guessing the next larger standard tap | An STI thread is not defined simply by choosing a larger standard nominal thread. | Specify the original thread designation plus STI. |
| Ordering only “M8 helicoil tap” | Pitch, tap geometry and application remain undefined. | Specify M8 × 1.25 STI or the actual required pitch. |
| Confusing tap size with drill size | Drill preparation and STI tap designation are separate parts of the installation system. | Use the dedicated STI tap drill chart for hole preparation. |
| Ignoring thread class | Controlled applications may require a specified STI thread tolerance or class of fit. | Confirm drawing, standard or supplier specification. |
Related STI Tap and Wire Insert Resources
Metric STI Tap Drill Chart
Find common metric hole-preparation references before using the selected STI tap.
UNC / UNF STI Drill Chart
Check common inch drill references for 4-40, 6-32, 8-32, 10-32, 1/4-20 and other Unified sizes.
STI Taps
Review STI tap supply for wire thread insert repair, maintenance, tooling and OEM requirements.
FAQ: Helicoil and STI Tap Sizes
What tap size is required for an M8 × 1.25 helicoil type insert?
Use an M8 × 1.25 STI tap designed for screw thread inserts. Do not substitute a normal M8 × 1.25 tap or guess a larger standard metric tap.
What tap size is required for an M10 × 1.5 wire thread insert?
Use an M10 × 1.5 STI tap. The STI designation follows the required finished thread size and pitch.
What tap size is used for an M6 helicoil type insert?
For the common M6 × 1.0 coarse thread, use an M6 × 1.0 STI tap.
Is an STI tap the same size as a standard tap?
The nominal thread designation may be the same, but the tap is not the same tool. An STI tap creates the special receiving thread required for the wire insert.
Can I use an M10 tap for an M8 helicoil insert?
Do not select a normal M10 tap simply because the insert installation hole is larger than a standard M8 tapped hole. Specify the correct M8 × 1.25 STI tap for an M8 × 1.25 insert system.
What tap is required for a 10-32 wire thread insert?
A 10-32 UNF wire thread insert requires the corresponding 10-32 UNF STI tap.
What tap is required for a 1/4-20 wire insert?
Use the corresponding 1/4-20 UNC STI tap and prepare the hole according to the approved inch STI drill reference.
What does STI mean on a tap?
STI means Screw Thread Insert. It identifies a tap intended to create the receiving thread for a screw thread insert system.
Do I still need a tap drill chart?
Yes. The STI tap designation identifies the tap, while the tap drill chart identifies the hole preparation required before STI tapping.
Need the Correct STI Tap for Your Wire Thread Inserts?
Send the thread standard, nominal size, pitch or TPI, parent material, through or blind hole, quantity and insert requirement for tap and tooling review.