Helicoil drill sizes
Tap drill sizes for installing Helicoil and other wire-thread (screw-thread) inserts. Because the insert sits in an oversized, specially tapped hole, the drill is larger than a standard tap drill for the same thread.
Inch threads (STI)
| Thread | Drill size | Decimal (in) |
|---|---|---|
| 1/4-20 | 17/64 | 0.2656 |
| 5/16-18 | 21/64 | 0.3281 |
| 3/8-16 | 25/64 | 0.3906 |
| 7/16-14 | 29/64 | 0.4531 |
| 1/2-13 | 33/64 | 0.5156 |
| 5/8-11 | 41/64 | 0.6406 |
| 3/4-10 | 49/64 | 0.7656 |
Metric coarse threads
| Thread | Drill size (mm) |
|---|---|
| M3 x 0.5 | 3.2 |
| M4 x 0.7 | 4.2 |
| M5 x 0.8 | 5.2 |
| M6 x 1.0 | 6.3 |
| M8 x 1.25 | 8.4 |
| M10 x 1.5 | 10.5 |
| M12 x 1.75 | 12.5 |
Wire-thread inserts use an STI (screw-thread insert) tap, which cuts a larger thread than normal so the coiled insert can be wound in. The finished thread inside the insert matches the bolt. Drill sizes here are representative; always follow the chart supplied with your specific insert kit, since values differ slightly between brands.
Installing a standard tapped hole instead?
For ordinary threads without an insert, the Tap Drill Calculator gives the correct drill size.
Why insert holes are bigger
A Helicoil is a coil of diamond-section wire that forms a new thread inside a larger tapped hole. The STI tap cuts that larger thread, so the drill must be bigger than a standard tap drill for the same bolt size. Once the insert is wound in, its inside diameter matches the original bolt thread, restoring or strengthening the joint.
When to use thread inserts
Inserts repair stripped threads and reinforce threads in soft materials like aluminum and magnesium, where steel bolts would otherwise pull out. They spread the load over more material and resist wear from repeated assembly, which is why they are common in engine castings and aerospace parts.
FAQ
Why is the drill bigger for a Helicoil?
The insert needs a larger, specially tapped STI hole to wind into. After installation its inside threads match the original bolt size.
What drill do I use for a 1/4-20 Helicoil?
Commonly a 17/64 inch drill (0.2656), followed by the matching 1/4-20 STI tap. Confirm against your insert kit chart.
Can a Helicoil repair stripped threads?
Yes. Drilling out the damaged thread, tapping STI, and winding in an insert restores a full-strength thread, often stronger than the original in soft metal.