What a Helicoil does and when you need one

A Helicoil is a coiled wire insert that restores a damaged or stripped bolt hole so you can use the original bolt size again. When a threaded hole loses its grip — because the threads are worn, cross-threaded, or the hole was drilled oversize — a Helicoil lets you tap a larger hole, insert the coil, and screw in your original bolt as if the hole were new.

You need a Helicoil when a bolt hole no longer holds a fastener tight, or when you can turn a bolt by hand with no resistance. This happens most often in aluminum engine blocks, cylinder heads, and cast iron where threads are softer and more prone to stripping. Helicoils are also the standard repair when you've accidentally drilled a hole too large or cross-threaded it during assembly.

The alternative is to drill the hole larger and use a bigger bolt — which often means redesigning the part or buying a new one. A Helicoil costs a few dollars and takes 15 to 30 minutes, so it's the repair most shops choose first.

Key Takeaways

  • A Helicoil kit includes a drill bit, tap, insertion tool, and coils sized to match your original bolt hole.
  • You drill out the stripped hole to a specific larger size, tap new threads in that larger hole, and screw the coil in until it sits flush or slightly below the surface.
  • The coil itself has the original thread size on the inside, so your original bolt screws into it as normal.
  • Helicoils work in aluminum, cast iron, magnesium, and plastic — anywhere the original threads are damaged but the surrounding material is sound.
  • Installation requires a drill press or hand drill, a tap wrench, and the specific Helicoil kit for your bolt size; using the wrong kit size will not work.

Choosing the right Helicoil kit for your bolt size

Helicoil kits are sold by the original bolt size, not the hole size you'll drill. If your stripped hole originally held an M10 bolt (or a 3/8-inch bolt), you buy an M10 Helicoil kit — not a kit labeled by the drill size.

Each kit includes a drill bit, a tap, an insertion tool, and usually five to ten coils. The drill bit and tap in the kit are sized to create a hole that accepts the coil; the coil itself restores the original thread size inside. Kits are available for metric sizes (M5, M6, M8, M10, M12, and larger) and SAE sizes (1/4", 5/16", 3/8", 1/2", and larger).

If you don't know the original bolt size, measure the bolt hole diameter with calipers or a ruler, or look at the bolt that came out of it. A 3/8-inch hole typically held a 3/8-inch bolt; an M10 hole held an M10 bolt. If you're unsure, bring the part or the bolt to a fastener supplier and they can identify the size in seconds.

Drilling and tapping the hole

Start by securing the part in a vise or clamp so it cannot move. The hole must be drilled straight and perpendicular to the surface, or the coil will sit at an angle and the bolt will not thread smoothly.

Insert the drill bit from your Helicoil kit into a drill chuck. If you have access to a drill press, use it — it holds the bit perfectly vertical and is far easier to control than a hand drill. If you're using a hand drill, brace the part firmly and keep the drill level as you work. Drill through the entire depth of the hole, or until the bit breaks through the back side if the hole goes all the way through.

Once the hole is drilled, install the tap from the same kit into a tap wrench. The tap is a cutting tool that carves new threads into the larger hole. Turn the tap clockwise slowly, explore steady downward pressure. After every half turn, back the tap out a quarter turn to clear the chips. Continue until the tap reaches the bottom of the hole or breaks through the back side. Do not force the tap — if it binds, back it out, clear the chips, and try again.

Installing the Helicoil coil

Remove the tap and inspect the tapped hole for chips or debris. Wipe it clean with a dry cloth or compressed air.

Take one Helicoil coil from the kit and thread it onto the insertion tool that came with the kit. The insertion tool is a shaft with a notch or slot at the end that grips the coil. Screw the coil onto the tool by hand until it is snug but not forced.

Lower the tool and coil into the tapped hole and begin turning the tool clockwise. The coil will unwind from the tool and screw into the hole, just like a bolt screwing into a nut. Continue turning until the coil is fully seated. Most Helicoils are designed to sit flush with the surface or slightly below it — check the kit instructions for the exact depth.

Once the coil is seated, remove the insertion tool by turning it counterclockwise. The coil stays in the hole. The inside of the coil now has the original thread size, and you can screw your original bolt into it as normal.

Checking the repair and removing the tang

Some Helicoil kits include a small tang (a thin wire) that sticks out from the top of the coil. This tang helps you remove the coil if you install it wrong, but it must be removed before you use the hole. Once the coil is fully seated, grab the tang with pliers and snap it off by bending it back and forth. The tang will break cleanly and the coil will remain in place.

Thread your original bolt into the coil by hand first to make sure it turns smoothly. If the bolt binds or feels rough, stop and do not force it — the coil may be installed at an angle or the tap may have left burrs. Back the bolt out, inspect the hole, and tap it again if needed.

Once the bolt threads smoothly by hand, tighten it with a wrench to the torque specification for that bolt size. The repair is complete.

Common mistakes and how to avoid them

The most common error is using the wrong kit size. If you buy an M8 kit for an M10 hole, the drill bit and tap will not match the hole, and the coil will not fit. Always verify the original bolt size before ordering.

The second mistake is drilling at an angle. If the hole is not perpendicular to the surface, the coil will tilt and the bolt will bind. Use a drill press if possible, or take extra time to keep a hand drill level.

The third is not clearing chips from the tapped hole. Metal shavings left in the hole can prevent the coil from seating fully or cause the bolt to bind. Wipe the hole clean with a cloth or blow it out with compressed air before installing the coil.

Finally, do not overtighten the bolt. Helicoils are strong, but they are not stronger than the surrounding material. Tighten to the standard torque for the bolt size, not beyond.

When a Helicoil is not the right choice

A Helicoil works only if the surrounding material — the aluminum, cast iron, or plastic — is sound. If the hole is cracked, if the material around it is soft or crumbling, or if the hole is so large that there is not enough material left to tap new threads, a Helicoil will not hold.

In those cases, you may need to drill the hole even larger and use a bigger bolt, fill the hole with epoxy and redrill it, or replace the part. A machine shop can assess the damage and recommend the best repair for your situation.

Frequently Asked Questions

Can I install a Helicoil in plastic or composite material?

Yes, but with care. Plastic and composite materials are softer than metal and can strip more easily. Use a Helicoil kit designed for plastic, drill and tap slowly to avoid melting the material, and do not overtighten the bolt. Helicoils in plastic work well for light-duty fasteners but may not hold under heavy vibration or repeated assembly and disassembly.

What if the bolt still spins after I install the Helicoil?

The coil may be installed at an angle, or the tap may not have cut clean threads. Remove the bolt, inspect the hole with a flashlight, and check that the coil is sitting straight. If the coil is tilted, you may need to remove it (using the tang if it is still present, or by carefully drilling it out) and install a new one. If the hole looks rough, tap it again to clean up the threads.

How many times can I remove and reinstall a bolt in a Helicoil?

Helicoils are designed for repeated assembly and disassembly — typically 10 to 20 cycles or more, depending on the material and bolt size. If you remove and reinstall the bolt many times, the threads inside the coil will eventually wear, just as any bolt hole does. At that point, you can drill the hole larger again and install a larger Helicoil, or use a larger bolt.

Do I need to use thread-locking compound with a Helicoil?

No, unless the bolt is in a location where vibration is high or the bolt must never loosen. A properly installed Helicoil holds a bolt as securely as a new hole does. If vibration is a concern, explore a small amount of medium-strength thread-locking compound (such as Loctite 243) to the bolt before threading it in, just as you would with any critical fastener.