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
Часто задаваемые вопросы
Helicoil (спиральная проволочная нить вставки) представляет собой катушечную вставку, изготовленную из алмазной проволоки, сформированной в спираль. Он устанавливается в специально пробитое отверстие для создания прочной внутренней резьбы. Геликойлы используются для ремонта резьбы в полосатых отверстиях, укрепления нитей в мягких материалах, таких как алюминий, и обеспечения износостойкости для часто собираемых соединений.
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.
Yes, Helicoils are commonly used in aluminum and other soft materials to strengthen threads. Aluminum threads strip easily, especially with repeated assembly. A Helicoil insert made of stainless steel provides a durable, wear-resistant thread that can withstand many assembly cycles. Use longer inserts (2D or 3D) for maximum pull-out strength in soft materials.
The countersink diameter (M diameter) is the diameter of the chamfer at the top of the Helicoil hole, typically cut at 120° or 45°. It allows the installation tool to engage the insert properly. For example, a 1/4-20 UNC Helicoil has a countersink diameter of 0.31-0.34 inches. The countersink must be within the specified range for proper installation.
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.
