What a Helicoil Does and When You Need One

A helicoil is a small coiled wire insert that repairs a stripped bolt hole. When a threaded hole loses its grip — usually because a bolt has been removed and reinstalled too many times, or the hole was drilled in soft material — a helicoil restores the hole so a bolt can hold tight again. Instead of drilling a larger hole and using a larger bolt, you drill the hole bigger, install the helicoil, and use the original bolt size.

You'll need a helicoil when a bolt hole in metal, plastic, or composite material no longer holds a bolt securely. Common places this happens: engine blocks, cylinder heads, aluminum castings, and plastic equipment housings. The alternative is to drill oversize and use a larger bolt, which often isn't possible if space is tight or if you need to keep the original bolt size.

Key Takeaways

  • A helicoil kit includes the insert, a special drill bit, and a tool to wind the insert into the hole — buy the kit for your bolt size rather than buying pieces separately.
  • You drill out the stripped hole to a specific larger size, tap threads into that larger hole, then screw the helicoil in like a bolt.
  • The helicoil sits partially above the hole surface; you then break off the tang (the small wire loop sticking out) with pliers after installation.
  • The whole job takes 10 to 20 minutes per hole once you have the right kit and a working tap tool.

Gather the Right Helicoil Kit for Your Bolt Size

Helicoil kits are sold by the bolt size they repair — M6, M8, M10 for metric bolts, or 1/4", 5/16", 3/8" for SAE (inch) bolts. Do not guess the size. Measure the bolt that goes in the hole, or if the bolt is missing, measure the hole itself with calipers or a ruler. Write down the exact size.

Buy a helicoil kit that includes the insert, the drill bit, the tap (the tool that cuts threads), and the installation tool. Kits from Helicoil, Time-Sert, or equivalent brands come in small boxes with all four pieces. A single-insert kit costs $8 to $20. Do not buy just the inserts — you need the matching drill bit and tap, and they are specific to each bolt size.

If you're repairing multiple holes of the same size, a larger kit with 10 or 20 inserts is cheaper per insert than buying single kits repeatedly. Check the kit contents before checkout to confirm it includes the drill bit, tap, and installation tool.

Drill Out the Stripped Hole to the Larger Size

The helicoil kit includes a drill bit sized for that specific bolt. Use that bit, not a different one. Clamp or hold the part firmly so it doesn't spin when the drill catches. Mark the center of the old hole with a punch or marker so the drill bit doesn't wander.

Drill straight down through the center of the stripped hole. Go slowly and let the bit do the work — forcing it will break the bit or damage the hole. If the part is metal, use a cutting fluid or light machine oil to keep the bit cool. Drill all the way through the hole or to the depth marked on the kit instructions, whichever applies.

Stop drilling when the bit reaches full depth. Do not drill deeper. Wipe away chips and inspect the hole — it should be clean and straight.

Cut Threads into the Enlarged Hole with the Tap

The kit includes a tap — a tool that looks like a bolt with flutes (grooves) cut into it. This tap cuts new threads into the enlarged hole. Insert the tap into the hole by hand first, then use the tap handle (a T-shaped wrench that comes with the kit) to turn it clockwise.

Turn slowly and steadily. After every half turn, back the tap out a quarter turn to break the chips and clear the hole. Continue until the tap reaches the depth marked in the kit instructions — usually the tap will stop turning when it reaches the right depth because the kit is designed that way.

Remove the tap by turning it counterclockwise. Wipe the hole clean of metal chips. The hole now has fresh threads ready for the helicoil.

Install the Helicoil into the Threaded Hole

The helicoil is a coiled wire insert about the size of a small spring. It comes wound tightly on a mandrel (a thin rod). The kit includes an installation tool — usually a handle with a slot or socket that grips the mandrel.

Thread the helicoil into the newly tapped hole by hand as far as it will go, then use the installation tool to turn it clockwise the rest of the way. Turn until the top of the helicoil is slightly below the surface of the hole, or flush with it — the kit instructions show the correct depth. The helicoil should sit snug and not spin when you try to turn it by hand.

Do not over-tighten. The helicoil only needs to be tight enough to stay in place. Forcing it can damage the threads you just cut.

Break Off the Tang and Inspect the Finished Hole

After the helicoil is installed, a small wire loop — called the tang — sticks out from the top of the hole. This tang is meant to break off. Grab it with pliers and bend it back and forth until it snaps off. It usually breaks cleanly after two or three bends.

Wipe the hole clean. The helicoil is now ready to receive a bolt. The bolt will thread into the helicoil, not into the original hole, so it will grip as if the hole were new.

Install your bolt and tighten it to the torque specified for that bolt size. The repair is complete. The helicoil will hold the bolt securely for the life of the part.

Common Mistakes and How to Avoid Them

The most common error is using the wrong drill bit or tap — one that doesn't match the helicoil size. Always use the bit and tap that come in the kit. Mixing sizes will result in a helicoil that doesn't fit or threads that don't match.

The second mistake is drilling at an angle instead of straight down. A crooked hole will cause the tap to bind and may break it. Use a drill press if you have one, or clamp the part so it cannot move. Mark the center with a punch before you start.

The third mistake is not clearing chips from the hole between the drill and tap steps. Metal chips left in the hole will jam the tap or cause the helicoil to sit unevenly. Wipe the hole clean after each step.

Finally, do not over-tighten the helicoil during installation. It only needs to be snug. Forcing it can strip the threads you just cut, which defeats the purpose of the repair.

Frequently Asked Questions

Can I install a helicoil in plastic or aluminum?

Yes, helicoils work in both. Aluminum is softer than steel, so drill and tap more slowly to avoid tearing the threads. Plastic requires even more care — use a slow speed and light pressure. Some plastic materials are too soft for helicoils; if the tap spins without cutting, the material may not hold threads and a helicoil won't help.

What if the helicoil spins after I install it?

The helicoil is not tight enough. Remove it by turning counterclockwise, then reinstall it and turn it further clockwise. It should be snug enough that you cannot turn it by hand. If it keeps spinning, the threads you cut may be damaged — you may need to drill larger and use the next size helicoil up.

Can I reuse a hole if I remove the helicoil?

Not easily. Once a helicoil is installed, removing it usually damages the threads. If you need to remove it, drill it out and install a larger helicoil. Plan on the helicoil being permanent.

Do I need a drill press to install a helicoil?

A drill press makes the job easier and more accurate, but it's not required. A hand drill works if you clamp the part firmly and drill slowly and straight. The key is keeping the drill bit perpendicular to the hole surface — any angle will cause problems.

How long does a helicoil last?

A properly installed helicoil lasts as long as the bolt. It won't loosen or wear out under normal use. The repair is permanent unless the helicoil is physically damaged or the surrounding material cracks.