← AskMachinist · Guides · Calculator · 🔎 Search

Helicoil Thread Insert Basics for Machinists

Guide · Updated October 28, 2026

Wire thread inserts (Helicoil is the best-known brand) repair stripped threads and give steel-strength threads in aluminum, magnesium and plastics. You drill an oversize hole, tap it with an STI (screw thread insert) tap, wind the coiled insert in, and break off the drive tang. The finished thread is the normal size, so a standard screw fits.

Check the finished thread

Check with a go/no-go thread plug gage after installing. The free Gage Block Calculator builds the stack.

Step by step

Pick the insert: Match the thread (for example 1/4-20 UNC) and choose length: 1D, 1.5D, 2D, 2.5D or 3D, where D is the nominal diameter.
Drill to the STI size: Use the drill listed for the STI tap, not the normal tap drill. For 1/4-20 it is 17/64 in.
Countersink lightly: A small 90–120° countersink keeps the first coil from being damaged.
Tap with the STI tap: Tap deep enough for the insert plus clearance. The STI tap is oversize so the insert ends up at nominal size.
Install: Wind the insert on the installation tool, tang first, and turn it in until the top coil sits 1/4 to 1/2 pitch below the surface.
Remove the tang: In through holes and blind holes with room, snap the tang off with the tang break-off tool, then pick it out.
Gage and clean: Blow out chips and check with a go/no-go plug gage.

Worked example

Repairing a 1/4-20 UNC hole in aluminum with a 1.5D insert. Insert length = 1.5 × 0.250 = 0.375 in. The ASME B18.29.1 STI chart lists a 17/64 in (0.2656 in) drill, with the STI minor diameter limits 0.2608–0.2704 in and the STI major diameter at least 0.315 in. That drill is well above the normal 1/4-20 tap drill (#7, 0.201 in), so drill out the old thread completely. Tap depth for a blind hole ≈ insert length + one pitch + the tap chamfer: 0.375 + 0.050 + about 0.100 = 0.525 in of drilled depth minimum. Install so the top coil is 0.0125–0.025 in below the surface (1/4 to 1/2 of the 0.050 in pitch).

Why inserts in aluminum

A steel or stainless wire insert spreads the screw load over a larger diameter of soft parent metal and resists wear from repeated assembly. That is why inserts are specified in aircraft and engine castings even when nothing is damaged.

Brand differences

Helicoil, Recoil and other kits follow the same STI thread standard, but always use the drill and tap that come with the kit. Some kits and charts call for a slightly different drill for hard or soft parent materials.

Common problems

An insert that will not start usually means a regular tap was used instead of the STI tap, or the countersink is missing. An insert that sits proud or crooked means the hole was not tapped deep enough or square. A tang that will not break in a blind hole means there is no room below it; use tangless inserts or drill deeper. A loose screw after install means the wrong STI tap class or a worn tap.

Tips

Tapping: tap on a mill to keep the STI tap square.
Drill chart: tap drill chart explained.
Gages: thread gages.

FAQ

What drill for a 1/4-20 Helicoil?

17/64 in (0.2656 in), per the STI chart, not the normal #7 tap drill.

What is an STI tap?

An oversize tap sized so the coiled insert ends up at the nominal thread size inside it.

What insert length should I use?

1.5D or 2D is common; longer for soft materials or high loads.

Do I have to remove the tang?

Yes in through holes so the screw can pass; tangless inserts avoid it in blind holes.

Are all thread insert kits the same?

They use the same STI standard, but follow the drill and tap supplied with your kit.

Related guides and calculators

Build your stack in seconds

The free Gage Block Calculator finds the fewest-block combination for inch and metric sets, with wear blocks. No login.

Questions? Ask a machinist.

General shop information only — follow your machine and tooling manufacturer's instructions and your shop's safety rules.