These charts serve as a practical reference for common Helicoil sizes. Whether the goal is thread repair, improving thread strength in softer materials, or preparing a new hole for repeated assembly, the correct helical insert dimensions help ensure the insert installs cleanly and the finished thread performs as expected. This page covers Unified Coarse (UNC), Unified Fine (UNF), Metric Coarse, and Metric Fine thread series.
Helicoil Size Lookup
Complete Helicoil Reference Tables
Understanding Helicoil Dimensions
Countersink "M" Diameter
The countersink diameter is the chamfer at the top of the Helicoil hole, cut at 120° or 45° included angle. It allows the installation tool to properly engage the insert. The M diameter must fall within the specified min-max range for correct installation.
Pitch Diameter "H"
The pitch diameter is the effective diameter of the STI tapped hole. It is measured between the thread flanks and determines how the Helicoil insert fits. The H diameter has tight tolerances (4H/5H for metric, 3B/2B for imperial) to ensure proper insert engagement.
Insert Length (D multiples)
Helicoil insert lengths are specified in multiples of the nominal diameter (D). Common lengths are 1D, 1.5D, 2D, 2.5D, and 3D. Longer inserts provide greater pull-out strength. For soft materials like aluminum, 2D or 3D lengths are recommended.
STI Tap vs. Standard Tap
Helicoil inserts require a special STI (Screw Thread Insert) tap, which cuts a larger internal thread than a standard tap. The STI tap size is always larger than the nominal thread size — for example, a 1/4-20 UNC Helicoil requires a 5/16 STI tap, not a 1/4 standard tap.
Installation Tips
Best Practices for Helicoil Installation
- Always use the correct STI tap — never substitute a standard tap
- Drill to the exact drill size specified; oversizing weakens the installation
- Use cutting oil when tapping, especially in steel and stainless steel
- Countersink the hole to the specified M diameter before installing the insert
- For blind holes, drill deeper than the minimum tapping depth to allow for chips
- Use longer inserts (2D or 3D) in soft materials like aluminum or plastic
- Remove the tang (driving tang) after installation using a punch — it is designed to break off
- For tangless inserts, no tang removal is needed, saving time in production
अक्सर पूछे जाने वाले प्रश्न
एक हेलिकॉइल (हेलिकल वायर थ्रेड डालने) एक कॉइल के आकार का आवेषण है जो हीरे के आकार के तार से बना होता है जो एक हेलिक्स में बनता है। यह एक टिकाऊ आंतरिक बनाने के लिए विशेष रूप से टैप किए गए छेद में स्थापित किया गया है। हेलिकॉइल का उपयोग स्ट्रिप्ड छेद में मरम्मत के लिए किया जाता है, जो एल्युमिनियम को नरम सामग्री]] में मजबूत करता है, और अक्सर इकट्ठे कनेक्शन के लिए पहनने के प्रतिरोध प्रदान करता है।.
The drill size depends on the thread size and pitch of the Helicoil insert. For example, a 1/4-20 UNC Helicoil requires a 17/64" drill bit, while an M6 x 1.0 metric Helicoil requires a 6.2 mm drill. Always use the drill size specified in the Helicoil chart — using the wrong size will result in improper insert installation and potential thread failure.
Helicoil inserts require a special STI (Screw Thread Insert) tap, which is different from a standard tap. The STI tap creates a larger internal thread to accommodate the insert. For example, a 1/4-20 UNC Helicoil uses a 5/16 STI tap. The tap size is always larger than the nominal thread size because the insert adds wall thickness between the bolt and the base material.
Tapping depth depends on the insert length, expressed in diameters (D). Common lengths are 1D, 1.5D, 2D, 2.5D, and 3D. For a 1/4-20 UNC with 1.5D length, the minimum tapping depth is 0.55 inches. Deeper inserts (2D, 3D) provide greater pull-out strength and are recommended for soft materials or high-load applications. For blind holes, drill 1-2 threads deeper than the minimum depth.
UNC (Unified National Coarse) has fewer threads per inch and is the standard for general-purpose fastening. UNF (Unified National Fine) has more threads per inch, providing finer adjustment and better vibration resistance. For the same nominal diameter, UNF Helicoils use different taps and have different pitch diameters. For example, 1/4" UNC is 20 TPI while 1/4" UNF is 28 TPI.
हाँ, हेलिकॉइल आमतौर पर एल्युमिनियम और अन्य सॉफ्ट सामग्री में धागे को मजबूत करने के लिए उपयोग किया जाता है। विशेष रूप से दोहराए गए असेंबली के साथ, एल्युमिनियम धागे आसानी से स्ट्रिप करते हैं। स्टेनलेस स्टील से बना एक हेलिकॉइल इंसर्ट एक टिकाऊ, पहनने वाले प्रतिरोधी धागा प्रदान करता है जो कई विधानसभा चक्रों का सामना कर सकता है। नरम सामग्री में अधिकतम पुल आउट ताकत के लिए लंबे आवेषण (2D या 3D) का उपयोग करें।.
काउंटरसिंक व्यास (M व्यास) हेलिकॉइल छेद के शीर्ष पर चैमर का व्यास है, आमतौर पर 120 डिग्री या 45 डिग्री पर काट दिया जाता है। यह स्थापना उपकरण को सम्मिलित करने की अनुमति देता है। उदाहरण के लिए, एक 1/4-20 UNC हेलिकॉइल में 0.31-0.34 इंच का एक काउंटरसिंक व्यास है। काउंटरसिंक उचित स्थापना के लिए निर्दिष्ट सीमा के भीतर होना चाहिए।.
Helicoil inserts can be removed using a specialized removal tool. The tool engages the top coil of the insert and unscrews it counterclockwise. For tangless inserts, removal may require carefully collapsing the insert with a small punch. After removal, the hole can be re-tapped and a new insert installed if needed. Always inspect the hole for damage before reinstalling.
