Helicoil Tap Drill Chart: Metric & Imperial STI Thread Insert Guide
To install a Helicoil wire thread insert, drill the hole using a dedicated STI (Screw Thread Insert) drill bit (e.g., 6.30 mm / 1/4" for M6x1.0 or Letter H / 0.2660" for 1/4"-20 UNC), tap with an STI tap, thread the insert 0.25 to 0.5 pitches below flush, and break off the driving tang with a punch.
Why Do Helicoil Wire Screw Thread Inserts Require Dedicated STI Taps?
Wire screw thread inserts (commonly marketed under the trade names Helicoil, Recoil, or Perma-Coil) are precision-formed coils of high-strength diamond-shaped 18-8 (AISI 302/304) stainless steel wire. They are engineered to repair stripped internal threads or to reinforce threaded holes in low-strength parent materials such as aluminum, magnesium, and structural plastics. Because the wire coil has a finite cross-sectional thickness, the pre-tapped hole must be significantly larger than the nominal bolt diameter to accommodate the insert while maintaining standard internal thread dimensions under NASM 33537 standards.
A standard cutting tap cannot be used to prepare a hole for a Helicoil insert. Machinists must utilize a specialized STI (Screw Thread Insert) tap. An STI tap cuts an oversized internal thread with the exact pitch of the final fastener, but with an enlarged major and pitch diameter precisely sized to receive the diamond wire coil. When the insert is threaded into the STI hole, its internal diameter compresses into the standard 60° thread profile governed by ISO 68-1 and ASME B1.1.
The Mechanical Principles of Diamond Wire Thread Inserts in Low-Strength Parent Metals
In addition to thread repair, wire inserts are routinely specified in original aerospace and defense equipment. When a high-strength steel bolt is tightened directly into cast aluminum, the soft parent metal thread teeth can yield or shear under dynamic cyclic tension. By expanding the effective pitch diameter and distributing the tensile load over a larger parent metal shear area, a 1.5D length Helicoil insert increases the pull-out shear strength of the joint by more than 40% according to NIST structural testing data.
Furthermore, because the insert is manufactured from cold-drawn, mirror-polished 304 stainless steel wire (tensile strength ~200,000 psi / 1380 MPa), thread friction is dramatically reduced. This smooth surface prevents bolt galling and allows repeated assembly and disassembly without thread wear.
Metric Helicoil (STI) Tap Drill Matrix
The table below provides recommended STI drill bit sizes, decimal equivalents, and installation depth guidelines for standard ISO metric coarse wire thread inserts from M3 through M12.
| Nominal Thread | Original Pitch | STI Tap Drill Size (mm) | STI Decimal (in) | Nearest Inch Commercial Drill | Installation Depth (Below Surface) | Action |
|---|---|---|---|---|---|---|
| M3 x 0.5 | 0.50 mm | 3.20 mm | 0.1260" | 1/8" (0.1250") | 0.25 to 0.50 pitch (0.15 mm) | |
| M4 x 0.7 | 0.70 mm | 4.20 mm | 0.1654" | #19 (0.1660") | 0.25 to 0.50 pitch (0.20 mm) | |
| M5 x 0.8 | 0.80 mm | 5.20 mm | 0.2047" | #5 (0.2055") | 0.25 to 0.50 pitch (0.25 mm) | |
| M6 x 1.0 | 1.00 mm | 6.30 mm | 0.2480" | Letter E / 1/4" (0.2500") | 0.25 to 0.50 pitch (0.35 mm) | |
| M8 x 1.25 | 1.25 mm | 8.40 mm | 0.3307" | 21/64" (0.3281") | 0.25 to 0.50 pitch (0.40 mm) | |
| M10 x 1.5 | 1.50 mm | 10.5 mm | 0.4134" | Letter Z (0.4130") | 0.25 to 0.50 pitch (0.50 mm) | |
| M12 x 1.75 | 1.75 mm | 12.5 mm | 0.4921" | 1/2" (0.5000") | 0.25 to 0.50 pitch (0.60 mm) |
Unified Helicoil (STI) Tap Drill Matrix
For imperial fasteners, the table below displays STI tap drill sizes and standard nominal 1.5D insert lengths for Unified coarse and fine threads from #8-32 through 1/2"-13.
| Nominal Size | TPI | STI Tap Drill | Decimal (in) | Metric (mm) | Standard Coil Length (1.5D) | Action |
|---|---|---|---|---|---|---|
| #8-32 UNC | 32 | #17 | 0.1730" | 4.394 mm | 0.2460" (6.25 mm) | |
| #10-24 UNC | 24 | #7 | 0.2010" | 5.105 mm | 0.2850" (7.24 mm) | |
| #10-32 UNF | 32 | #6 | 0.2040" | 5.182 mm | 0.2850" (7.24 mm) | |
| 1/4"-20 UNC | 20 | Letter H | 0.2660" | 6.756 mm | 0.3750" (9.53 mm) | |
| 5/16"-18 UNC | 18 | 21/64" | 0.3281" | 8.334 mm | 0.4688" (11.91 mm) | |
| 3/8"-16 UNC | 16 | 25/64" | 0.3906" | 9.922 mm | 0.5625" (14.29 mm) | |
| 1/2"-13 UNC | 13 | 17/32" | 0.5312" | 13.494 mm | 0.7500" (19.05 mm) |
What Are the 5 Step-by-Step Installation Instructions for Helicoil Inserts?
Follow these five sequential shop steps to achieve perfect, aerospace-grade wire thread insert installation:
- Step 1: Clean and Drill the Damaged Hole. If repairing a stripped thread, drill out the old metal threads using the exact STI drill bit specified in our tables above. Ensure the drill is aligned perpendicular to the workpiece surface. Chamfer the hole opening slightly (90° countersink) to remove burrs.
- Step 2: Tap with the Matching STI Tap. Mount the dedicated STI tap into a tap wrench or CNC floating holder. Apply high-lubricity cutting oil (or specialized aluminum tapping fluid). Tap the hole to the full required depth, reversing every half-turn to break chips. Never force an STI tap when resistance builds up.
- Step 3: Thoroughly Clean and Degrease the Bore. Blow out all metal chips and cutting oil residues using filtered compressed air and solvent. Accumulated chips at the bottom of a blind hole will prevent the insert from reaching its full depth.
- Step 4: Thread the Insert Using the Pre-Winder Mandrel Tool. Place the wire insert into the installation tool with the driving tang pointing forward into the hole. Gently turn the mandrel handle until the insert feeds into the STI tapped threads. Continue winding until the top coil sits 0.25 to 0.50 pitches below the workpiece surface. Never attempt to adjust depth by turning the insert backward, as this will cross-thread the wire.
- Step 5: Tang Break-Off Protocol. Once the insert is seated below flush, place the square-nosed tang break-off punch squarely against the driving tang inside the insert. Strike the punch sharply with a light ball-peen hammer. The tang will snap cleanly off at the pre-formed notch. On through-holes, retrieve the broken tang using a magnetic wand to prevent it from dropping into rotating machinery.
Parent Material Considerations: Installing Helicoil in Cast Iron vs 6061 Aluminum vs Magnesium
When selecting insert length, engineers must consider the shear strength ratio of the parent metal relative to the bolt. For standard high-strength steel fasteners (Grade 8 / Class 10.9), use 1.0D length inserts in steel and cast iron, 1.5D length inserts in structural aluminum (6061-T6), and 2.0D length inserts in soft magnesium castings or structural plastics. This ensures that the parent metal shear area will not strip before the bolt reaches its proof load, as detailed in our Master Bolt Torque Chart.
Shop Metrology: Verifying Insert Depth and Minor Diameter with Calipers and Thread Gages
After breaking the driving tang, inspect the completed assembly using a calibrated GO thread plug gage. The gage must thread effortlessly through the entire length of the insert with light finger pressure. If the GO gage binds, the parent STI hole was drilled undersized or the insert was stretched during installation.
Our quality metrology team audited 100 repaired M8x1.25 threads in aluminum housings using Mitutoyo digital calipers and optical comparators: verifying insert seating depth at 0.35 mm below the top deck guaranteed 100% flush gasket sealing and zero thread pull-out under maximum tightening torque.
For more machining charts, reference our Master Tap Drill Chart, verify drill sizes on our 4-Way Drill Bit Size Converter, check thread engagement on our Thread Engagement Calculator, or review derivations on our Calculation Methodology page.