STI Thread Plug Gauges: What to Specify Before Ordering

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What Is an STI Thread Plug Gauge?

This guide focuses on holding threads for helical coil inserts. Other insert designs may have different hole preparation and inspection requirements.

1. Identify Which Thread You Are Inspecting

An insert application involves the prepared holding thread and the internal thread available after installation. State which one your inspection tool must check.

For the prepared holding thread, use the gauge specified for the insert system. Böllhoff identifies a dedicated plug gauge for checking prepared HELICOIL holding threads in its insert installation guidance.

Inspection after installation is a separate requirement. Follow the approved drawing and insert manufacturer’s instructions rather than assuming the pre-installation STI gauge applies.

2. Do Not Order from the Finished Screw Size Alone

A request such as “M6 plug gauge” does not establish whether you need an ordinary internal thread gauge or a gauge for a prepared insert holding thread.

Include the insert manufacturer’s part number or technical specification where available. Explain that the gauge is intended for inspection before insert installation, and provide the controlled drawing.

Illustrative example: a buyer needs to inspect holes prepared for an M6 insert but requests only an M6 thread plug gauge. The supplier could interpret this as an ordinary finished-thread requirement. Adding the insert system, STI requirement and governing specification resolves the ambiguity.

3. Confirm the Standard and Gauge Tolerance Basis

Vermont Gage documents differences among specification routes for inch-series STI gauges, including gauge tolerances. Its practical recommendation is to identify the applicable standard when ordering and arranging calibration. See its technical guide to STI gauges.

Do not select a standard or tolerance option simply because it appears in a supplier’s catalogue. Ask the responsible technical reviewer to confirm the requirement for your application and use that same basis in the quotation and documentation request.

4. Prepare These Details for the Manufacturer

5. Keep Thread Acceptance and Installation Checks Separate

Passing a holding-thread inspection does not, by itself, verify every aspect of the installed insert. Installation position and any other assembly requirements need the checks specified by the approved process.

Similarly, the ability to install an insert should not replace the required holding-thread inspection. Define the checks at each manufacturing stage and identify the tool used for each one.

6. Include Depth and Access Requirements

If the holding thread is recessed or in a blind hole, provide a section drawing. State any required thread depth and the reference surface used to define it.

A longer handle alone does not establish a depth-inspection method. Request technical review where special reach or depth indication is needed, and confirm whether those features are covered by the supplied verification records.

7. Review Identification and Documents on Receipt

Match the delivered gauge to the approved specification. Check the STI identification, thread designation, required members and agreed gauge ID. Verify that the supplied records refer to the same item and technical requirements.

Store the gauge so it can be distinguished from ordinary thread gauges for the same nominal screw size. Labels should support identification, but operators should also check the tool itself.

Buyer Questions

Can an ordinary thread plug gauge replace an STI gauge?

No. An ordinary gauge for the finished screw size does not establish conformity of the prepared STI holding thread. Use the specified gauge for that inspection stage.

Does STI mean the gauge fits every insert design?

No. Confirm the insert system and applicable specification. This guide concerns helical coil insert holding threads, not every type of threaded insert.

Can I order using the insert part number?

It is useful identification, but provide its technical specification or drawing and confirm the inspection requirements. A part number alone may not define the complete gauge order.

Are GO and NO-GO gauges the entire inspection requirement?

Not necessarily. Follow the applicable inspection plan, including any additional dimensional checks it requires. Confirm the complete tool scope before purchasing.