What a Helicoil Does and When You Need One

A helicoil is a small coiled wire insert that restores a bolt hole when the threads inside have been stripped or damaged. Instead of replacing the entire part or drilling to a larger bolt size, you install the helicoil into the damaged hole, and it creates new threads that hold a standard bolt securely. This repair works on metal, plastic, and composite materials.

You need a helicoil when a bolt spins freely in its hole without tightening, or when you remove a bolt and the threads come out with it. Helicoils are common repairs in automotive work, machinery maintenance, and anywhere bolts take repeated stress. The repair is permanent and costs far less than replacing the part.

Key Takeaways

  • A helicoil kit includes the insert, a drill bit, a tap tool, and an installation tool — all sized for your bolt diameter.
  • You drill out the stripped hole to a larger diameter, cut new threads with a tap, then screw the helicoil insert into those new threads.
  • The helicoil sits flush or slightly proud of the surface, and you then install your bolt into the helicoil as if it were a normal threaded hole.
  • The entire process takes 10 to 20 minutes per hole once you have the correct kit size.

Choosing the Right Helicoil Kit Size

Helicoil kits come in standard sizes matched to bolt diameters. The kit size tells you what size bolt will fit after installation. Common sizes are M5, M6, M8, M10, M12 (metric) or #6, #8, #10, #1/4", #5/16" (SAE). If you do not know what bolt originally went in the hole, measure the bolt you plan to use, or look at the part's documentation.

Buy a kit that includes the drill bit, tap (the cutting tool), the helicoil insert itself, and the installation tool. Most kits come with one or two inserts. If you are repairing multiple holes, buy a kit with extras or buy several kits. Hardware stores and online retailers stock helicoil kits by bolt size — ask for "helicoil repair kit" and specify your bolt size.

Drilling Out the Stripped Hole

find the part in a vise or clamp so it cannot move. Mark the center of the stripped hole with a marker or punch so the drill bit stays centered. Use the drill bit that came in your helicoil kit — do not guess at the size. Insert the bit into a power drill or hand drill and drill straight through the hole, perpendicular to the surface. Drill slowly and steadily; if the bit binds, back it out, clear the chips, and continue.

The hole should now be clean and enlarged. Wipe away metal shavings with a cloth or brush. Do not skip this step — chips left in the hole will interfere with the tap and the helicoil.

Cutting New Threads with the Tap

The tap is a tool that cuts threads into the enlarged hole. It looks like a bolt with grooves and a square drive on top. Insert the tap into a tap wrench (a handle with a square socket that holds the tap). The tap wrench comes in most helicoil kits or can be bought separately.

Position the tap perpendicular to the hole — use a small level or square to check. Turn the tap clockwise slowly and steadily, explore light downward pressure. After every half turn, reverse direction a quarter turn to break the chips. Continue until the tap reaches the bottom of the hole and stops. Do not force it. Back the tap out by turning counterclockwise, then remove it.

Wipe the hole clean again. The hole now has fresh threads cut into it, but they are the wrong size for your original bolt — they are sized for the helicoil insert.

Installing the Helicoil Insert

The helicoil insert is a coiled wire that looks like a tiny spring. It comes pre-wound and ready to install. Thread it onto the installation tool — the tool usually has a tang or pin that fits into the helicoil's inner hole. The installation tool is often included in the kit or sold separately.

Screw the helicoil into the tapped hole by turning the installation tool clockwise. Turn slowly and steadily until the helicoil is fully seated. Most helicoils should sit flush with the surface or protrude slightly — check your kit's instructions. Once the helicoil is in place, remove the installation tool by turning it counterclockwise. The tang or pin will break free, and the tool will come out, leaving the helicoil behind.

The helicoil is now permanent. The hole is ready for your bolt.

Installing Your Bolt and Checking the Repair

Thread your bolt into the helicoil as you would into any normal threaded hole. The bolt should turn smoothly and tighten firmly without spinning. If the bolt spins or feels loose, the helicoil may not be seated correctly — back the bolt out and inspect the helicoil. If it is crooked or not fully seated, you may need to remove it and start over, though this is rare.

Tighten the bolt to the torque specification for your process. If you do not have a specification, tighten it firmly by hand or with a wrench until it is snug, but do not over-tighten. The repair is complete.

Common Mistakes to Avoid

Using the wrong drill bit size is the most common error. The bit must match your kit — do not substitute a bit that is close in size. A bit that is too small will not enlarge the hole enough, and a bit that is too large will make the hole too big for the tap to cut proper threads.

Failing to clean chips from the hole before tapping will cause the tap to bind or break. Stop frequently and clear the hole. Forcing the tap or helicoil will damage the threads or break the tool — if something feels stuck, back off and check what is wrong.

Installing the helicoil crooked or not fully seated will cause the bolt to sit at an angle or feel loose. Take your time and may support the installation tool is perpendicular to the surface as you turn it.

Frequently Asked Questions

Can I reuse a hole if the helicoil fails?

Yes. If a helicoil strips out (rare), you can drill the hole larger and install a bigger helicoil. Most parts can accept one or two size increases before you run out of room. After that, the part may need replacement.

Do I need a power drill or can I use a hand drill?

A hand drill works, but a power drill is faster and more consistent. For the tap, a hand wrench is standard — most taps are turned by hand, not by power tool. Go slowly with either method.

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

The helicoil may not be fully seated, or the tap may not have cut clean threads. Remove the bolt and inspect the helicoil from above. If it is crooked or loose, you may need to remove it and reinstall it, or drill and tap the hole again with a larger kit size.

Can helicoils be used in plastic or aluminum?

Yes. Helicoils work in aluminum, plastic, and composite materials. Plastic and composite require the same process but demand slower drilling and tapping to avoid cracking or melting. Let the tools do the work — do not force them.

How long does a helicoil last?

A properly installed helicoil is permanent. It will not loosen or wear out under normal use. The repair is as strong as the original threads would have been.