What's the Difference between STI and Heli-Coil/EG gauges?

At TruThread Europe, we often receive requests for EG or Heli-Coil gauges, but when we send a quotation for STI gauges, customers sometimes ask why. The answer is simple: there is practically no difference! The only distinction lies in the name. However, understanding the terminology and technical aspects behind these gauges can help ensure you select the right tools for your threading needs.

What is Heli-Coil?

Heli-Coil is a well-known brand of wire thread inserts designed to reinforce and repair threads in softer materials like aluminum, magnesium, and plastics. These inserts provide a durable internal thread that resists wear, corrosion, and stripping. Since "Heli-Coil" is a brand name, it has become synonymous with this type of insert, much like how some people refer to all adhesive bandages as "Band-Aids." The main function of Heli-Coil inserts is to create stronger and more reliable threads in materials that might otherwise be too weak to sustain repeated use.

These inserts are widely used in various industries, including aerospace, automotive, and manufacturing, where precision and durability are essential. By using Heli-Coil inserts, companies can extend the lifespan of their components and avoid costly repairs due to damaged or stripped threads.

What is STI gauge?

STI, or Screw Thread Insert, is a more general term that describes the same type of threading system. When using a helical wire insert like Heli-Coil, an STI tap is required to create a properly sized hole for the insert. The thread dimensions are slightly larger than a standard internal thread to accommodate the insert, which then restores the original thread size once installed.

STI gauges are specifically designed to inspect these threads, ensuring that the inserts fit correctly and maintain the intended tolerances. Using an STI gauge helps verify that the hole has been prepared correctly and that the insert will provide the desired performance once installed. Without proper gauging, there is a risk of misalignment, which can lead to thread failure or component malfunction.

What is EG gauge?

EG (Effective Diameter) gauges are specialized tools used to measure the effective diameter of internal threads. This measurement is crucial to ensure that tapped holes are manufactured to the correct size, allowing screws, bolts, and other threaded components to fit properly during assembly. By focusing on the effective diameter rather than just the nominal size, these gauges provide a more accurate representation of how a threaded part will engage with its mating component.

Why the Confusion?

The confusion arises because people often use "Heli-Coil" as a generic term for all wire thread inserts, while "STI" or “EG”refers to the technical specifications used for these inserts. In reality, if you are looking for gauges to inspect Heli-Coil threads, what you actually need are STI thread gauges.

Another reason for this confusion is that many suppliers and engineers are accustomed to referring to inserts by their most recognized brand name rather than by their technical classification. While this is common practice, it is important to remember that STI/EG gauges can be used for a variety of thread inserts, not just Heli-Coil.

Choosing the Right Gauges for Your Application

When selecting thread gauges, it is important to consider the specific requirements of your project. STI gauges come in various types, including STI for Metric or Unified threads. Each type serves a different purpose in ensuring that the threading process is precise and meets industry standards.

At TruThread Europe, we provide a range of high-quality STI thread gauges designed for use with Heli-Coil inserts, EG and other screw thread inserts. Our gauges ensure accuracy, reliability, and compliance with industry specifications, helping you achieve optimal results in your threading applications.

Same Function, Different Name

Whether you call them Heli-Coil gauges, EG gauges or STI gauges, they serve the same purpose—ensuring precision in thread inserts. The key takeaway is that Heli-Coil is a brand of inserts, while STI refers to the tools and standards used for those inserts. Understanding this distinction can help you avoid confusion and select the right gauges for your needs.

At TruThread Europe, we are committed to providing the best solutions for your thread gauging requirements. If you need assistance selecting the right gauges, feel free to contact us—we’re always happy to help!