STI inserts (Helicoil): drill and tap chart
Full chart with the tap drill for every size. Press “Calculate” on any row to open the calculator with that thread.
Wire thread inserts (Helicoil, Recoil, V-Coil) repair a damaged thread or reinforce a thread in aluminium, magnesium or plastic. The hole is drilled and tapped with an STI tap, larger than the original thread, and the insert brings it back to the nominal size: an M8 x 1.25 screw goes into an M8 STI hole made with an 8.4 mm drill. The insert is chosen by length, in multiples of the diameter (1D to 3D).
| Thread | STI tap | Drilling | STI major Ø | Insert length (1D · 1.5D · 2D · 2.5D · 3D) | Calculate |
|---|---|---|---|---|---|
| M2 x 0.4 | M2 x 0.4 STI | ⌀ 2.1 | 2.52 | 2 · 3 · 4 · 5 · 6 mm | |
| M2.5 x 0.45 | M2.5 x 0.45 STI | ⌀ 2.6 | 3.085 | 2.5 · 3.75 · 5 · 6.25 · 7.5 mm | |
| M3 x 0.5 | M3 x 0.5 STI | ⌀ 3.2 | 3.65 | 3 · 4.5 · 6 · 7.5 · 9 mm | |
| M3.5 x 0.6 | M3.5 x 0.6 STI | ⌀ 3.7 | 4.279 | 3.5 · 5.25 · 7 · 8.75 · 10.5 mm | |
| M4 x 0.7 | M4 x 0.7 STI | ⌀ 4.2 | 4.909 | 4 · 6 · 8 · 10 · 12 mm | |
| M5 x 0.8 | M5 x 0.8 STI | ⌀ 5.2 | 6.039 | 5 · 7.5 · 10 · 12.5 · 15 mm | |
| M6 x 1 | M6 x 1 STI | ⌀ 6.3 | 7.299 | 6 · 9 · 12 · 15 · 18 mm | |
| M7 x 1 | M7 x 1 STI | ⌀ 7.3 | 8.299 | 7 · 10.5 · 14 · 17.5 · 21 mm | |
| M8 x 1.25 | M8 x 1.25 STI | ⌀ 8.4 | 9.624 | 8 · 12 · 16 · 20 · 24 mm | |
| M10 x 1.5 | M10 x 1.5 STI | ⌀ 10.5 | 11.949 | 10 · 15 · 20 · 25 · 30 mm | |
| M12 x 1.75 | M12 x 1.75 STI | ⌀ 12.5 | 14.273 | 12 · 18 · 24 · 30 · 36 mm | |
| M14 x 2 | M14 x 2 STI | ⌀ 14.5 | 16.598 | 14 · 21 · 28 · 35 · 42 mm | |
| M16 x 2 | M16 x 2 STI | ⌀ 16.5 | 18.598 | 16 · 24 · 32 · 40 · 48 mm | |
| M18 x 2.5 | M18 x 2.5 STI | ⌀ 18.75 | 21.248 | 18 · 27 · 36 · 45 · 54 mm | |
| M20 x 2.5 | M20 x 2.5 STI | ⌀ 20.75 | 23.248 | 20 · 30 · 40 · 50 · 60 mm | |
| M22 x 2.5 | M22 x 2.5 STI | ⌀ 22.75 | 25.248 | 22 · 33 · 44 · 55 · 66 mm | |
| M24 x 3 | M24 x 3 STI | ⌀ 25 | 27.897 | 24 · 36 · 48 · 60 · 72 mm | |
| M27 x 3 | M27 x 3 STI | ⌀ 28 | 30.897 | 27 · 40.5 · 54 · 67.5 · 81 mm | |
| M30 x 3.5 | M30 x 3.5 STI | ⌀ 31 | 34.547 | 30 · 45 · 60 · 75 · 90 mm | |
| M33 x 3.5 | M33 x 3.5 STI | ⌀ 34 | 37.547 | 33 · 49.5 · 66 · 82.5 · 99 mm | |
| M36 x 4 | M36 x 4 STI | ⌀ 37.25 | 41.196 | 36 · 54 · 72 · 90 · 108 mm |
| Thread | STI tap | Drilling | STI major Ø | Insert length (1D · 1.5D · 2D · 2.5D · 3D) | Calculate |
|---|---|---|---|---|---|
| M8 x 1 | M8 x 1 STI | ⌀ 8.3 | 9.299 | 8 · 12 · 16 · 20 · 24 mm | |
| M10 x 1 | M10 x 1 STI | ⌀ 10.3 | 11.299 | 10 · 15 · 20 · 25 · 30 mm | |
| M10 x 1.25 | M10 x 1.25 STI | ⌀ 10.4 | 11.624 | 10 · 15 · 20 · 25 · 30 mm | |
| M12 x 1.25 | M12 x 1.25 STI | ⌀ 12.4 | 13.624 | 12 · 18 · 24 · 30 · 36 mm | |
| M12 x 1.5 | M12 x 1.5 STI | ⌀ 12.5 | 13.949 | 12 · 18 · 24 · 30 · 36 mm | |
| M14 x 1.25 | M14 x 1.25 STI | ⌀ 14.25 | 15.624 | 14 · 21 · 28 · 35 · 42 mm | |
| M14 x 1.5 | M14 x 1.5 STI | ⌀ 14.5 | 15.949 | 14 · 21 · 28 · 35 · 42 mm | |
| M16 x 1.5 | M16 x 1.5 STI | ⌀ 16.5 | 17.949 | 16 · 24 · 32 · 40 · 48 mm | |
| M18 x 1.5 | M18 x 1.5 STI | ⌀ 18.5 | 19.949 | 18 · 27 · 36 · 45 · 54 mm | |
| M20 x 1.5 | M20 x 1.5 STI | ⌀ 20.5 | 21.949 | 20 · 30 · 40 · 50 · 60 mm | |
| M22 x 1.5 | M22 x 1.5 STI | ⌀ 22.5 | 23.949 | 22 · 33 · 44 · 55 · 66 mm | |
| M24 x 2 | M24 x 2 STI | ⌀ 24.5 | 26.598 | 24 · 36 · 48 · 60 · 72 mm |
| Thread | STI tap | Drilling | STI major Ø | Insert length (1D · 1.5D · 2D · 2.5D · 3D) | Calculate |
|---|---|---|---|---|---|
| #4-40 UNC | #4-40 UNC STI | 1/8"0.125" · 3.18 mm | 0.1445" · 3.67 mm | 0.112" · 0.168" · 0.224" · 0.28" · 0.336" | |
| #5-40 UNC | #5-40 UNC STI | 9/64"0.1406" · 3.57 mm | 0.1575" · 4 mm | 0.125" · 0.1875" · 0.25" · 0.3125" · 0.375" | |
| #6-32 UNC | #6-32 UNC STI | 9/64"0.1406" · 3.57 mm | 0.1786" · 4.536 mm | 0.138" · 0.207" · 0.276" · 0.345" · 0.414" | |
| #8-32 UNC | #8-32 UNC STI | 11/64"0.1719" · 4.37 mm | 0.2046" · 5.197 mm | 0.164" · 0.246" · 0.328" · 0.41" · 0.492" | |
| #10-24 UNC | #10-24 UNC STI | 13/64"0.2031" · 5.16 mm | 0.2441" · 6.201 mm | 0.19" · 0.285" · 0.38" · 0.475" · 0.57" | |
| #12-24 UNC | #12-24 UNC STI | 15/64"0.2344" · 5.95 mm | 0.2701" · 6.861 mm | 0.216" · 0.324" · 0.432" · 0.54" · 0.648" | |
| 1/4"-20 UNC | 1/4"-20 UNC STI | 17/64"0.2656" · 6.75 mm | 0.315" · 8 mm | 0.25" · 0.375" · 0.5" · 0.625" · 0.75" | |
| 5/16"-18 UNC | 5/16"-18 UNC STI | 21/64"0.3281" · 8.33 mm | 0.3847" · 9.771 mm | 0.3125" · 0.4688" · 0.625" · 0.7813" · 0.9376" | |
| 3/8"-16 UNC | 3/8"-16 UNC STI | 25/64"0.3906" · 9.92 mm | 0.4562" · 11.587 mm | 0.375" · 0.5625" · 0.75" · 0.9375" · 1.125" | |
| 7/16"-14 UNC | 7/16"-14 UNC STI | 29/64"0.4531" · 11.51 mm | 0.5303" · 13.47 mm | 0.4375" · 0.6563" · 0.875" · 1.0938" · 1.3126" | |
| 1/2"-13 UNC | 1/2"-13 UNC STI | 17/32"0.5313" · 13.49 mm | 0.5999" · 15.238 mm | 0.5" · 0.75" · 1" · 1.25" · 1.5" | |
| 9/16"-12 UNC | 9/16"-12 UNC STI | 19/32"0.5938" · 15.08 mm | 0.6708" · 17.038 mm | 0.5625" · 0.8438" · 1.125" · 1.4063" · 1.6876" | |
| 5/8"-11 UNC | 5/8"-11 UNC STI | 21/32"0.6563" · 16.67 mm | 0.7431" · 18.875 mm | 0.625" · 0.9375" · 1.25" · 1.5625" · 1.875" | |
| 3/4"-10 UNC | 3/4"-10 UNC STI | 25/32"0.7813" · 19.84 mm | 0.8799" · 22.35 mm | 0.75" · 1.125" · 1.5" · 1.875" · 2.25" | |
| #4-48 UNF | #4-48 UNF STI | 1/8"0.125" · 3.18 mm | 0.1391" · 3.532 mm | 0.112" · 0.168" · 0.224" · 0.28" · 0.336" | |
| #5-44 UNF | #5-44 UNF STI | 1/8"0.125" · 3.18 mm | 0.1545" · 3.925 mm | 0.125" · 0.1875" · 0.25" · 0.3125" · 0.375" | |
| #6-40 UNF | #6-40 UNF STI | 9/64"0.1406" · 3.57 mm | 0.1705" · 4.33 mm | 0.138" · 0.207" · 0.276" · 0.345" · 0.414" | |
| #8-36 UNF | #8-36 UNF STI | 11/64"0.1719" · 4.37 mm | 0.2001" · 5.083 mm | 0.164" · 0.246" · 0.328" · 0.41" · 0.492" | |
| #10-32 UNF | #10-32 UNF STI | 13/64"0.2031" · 5.16 mm | 0.2306" · 5.857 mm | 0.19" · 0.285" · 0.38" · 0.475" · 0.57" | |
| #12-28 UNF | #12-28 UNF STI | 15/64"0.2344" · 5.95 mm | 0.2624" · 6.664 mm | 0.216" · 0.324" · 0.432" · 0.54" · 0.648" | |
| 1/4"-28 UNF | 1/4"-28 UNF STI | 17/64"0.2656" · 6.75 mm | 0.2964" · 7.528 mm | 0.25" · 0.375" · 0.5" · 0.625" · 0.75" | |
| 5/16"-24 UNF | 5/16"-24 UNF STI | 21/64"0.3281" · 8.33 mm | 0.3667" · 9.313 mm | 0.3125" · 0.4688" · 0.625" · 0.7813" · 0.9376" | |
| 3/8"-24 UNF | 3/8"-24 UNF STI | 25/64"0.3906" · 9.92 mm | 0.4291" · 10.9 mm | 0.375" · 0.5625" · 0.75" · 0.9375" · 1.125" | |
| 7/16"-20 UNF | 7/16"-20 UNF STI | 29/64"0.4531" · 11.51 mm | 0.5025" · 12.763 mm | 0.4375" · 0.6563" · 0.875" · 1.0938" · 1.3126" | |
| 1/2"-20 UNF | 1/2"-20 UNF STI | 33/64"0.5156" · 13.1 mm | 0.565" · 14.35 mm | 0.5" · 0.75" · 1" · 1.25" · 1.5" | |
| 9/16"-18 UNF | 9/16"-18 UNF STI | 37/64"0.5781" · 14.68 mm | 0.6347" · 16.121 mm | 0.5625" · 0.8438" · 1.125" · 1.4063" · 1.6876" | |
| 5/8"-18 UNF | 5/8"-18 UNF STI | 41/64"0.6406" · 16.27 mm | 0.6972" · 17.708 mm | 0.625" · 0.9375" · 1.25" · 1.5625" · 1.875" | |
| 3/4"-16 UNF | 3/4"-16 UNF STI | 49/64"0.7656" · 19.45 mm | 0.8312" · 21.112 mm | 0.75" · 1.125" · 1.5" · 1.875" · 2.25" |
How to read the chart
STI tap: the one that taps the hole before fitting the insert. It is designated as the nominal thread plus STI ("M8 x 1.25 STI").
Drill: the tap drill for the STI tap. It is larger than for the normal thread because the hole also has to hold the insert coil.
Insert length: a multiple of the nominal diameter. 1D for light loads, 1.5D for general use and 2D or more in aluminium and soft materials.
Guide values calculated from geometry; the exact drill and tap are given by the insert maker's kit. "Calculate" opens the repair tool with that size.
Related tools
Cutting parameters calculator
Cutting speed (Vc), rpm, feed and theoretical surface finish for turning, milling, drilling and threading by material and tool. A starting point.
GSM Code List
List of G and M Codes for CNC Programming. Search and filter ISO commands by control, machine or function.
CNC G-code Simulator
CNC IDE: editing with G/M tooltips, line inspector, conversion between controls and 2D/3D toolpath.