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This is the contents of the Heli-Coil kit that I used for this review. They have these in a huge array of sizes and thread types>
click image to enlarge

Heli-Coil™ Insert Review

Ingenious little coils that have a huge capability to save the day

Text, photos and video by Tom Hintz

Posted - 6-23-2011

Being the ever-tough group that we are stripped out boltholes can be more of a certainty than a possibility. But with Heli-Coils™ the often-extreme expense and most of the lost time associated with a stripped out hole can be averted with a simple to use kit that often costs well under $30! And, you can do the repair yourself and get perfect results. In most cases the newly threaded bolthole is actually stronger when a Heli-Coil™ Insert is installed! I have lots of taps and dies but lacking a good way to add the necessary material into a stripped hole the Heli-Coil™ Inserts remain a much easier and far more economical way to make this kind of repair.

The Basics

Heli-Coil™ Inserts were developed over 75 years ago as a remedy for failing and weak threads in the then relatively new aircraft industry. Today the Heli-Coil™ Inserts are used in a staggering range of manufacturing and repair settings because they are simple, effective and usually stronger than threads cut into the original material. While most Heli-Coil™ Inserts that metalworkers will use are fabricated from super tough 304 stainless steel they are also available in Inconel X750, phosphor bronze, Nitronic 60 and titanium for special needs situations! The list of known applications is huge but it is the near instant way Heli-Coil™ Inserts can repair what was often a terminal problem not so long ago.

You will love a kit like this when you encounter the tell-tale loaded up bolt threads (left), the semicircular pieces of metal (used to be threads) and a now too smooth hole. When you use the specified drill very little additional material is removed from the hole (right) but it is the correct diameter for the special tap included in the kit.
Click images to enlarge

Heli-Coil™ Inserts create conventional 60° internal screw threads that work with the vast majority of common bolt and screw configurations in use today. In addition to repairing damaged threads Heli-Coil™ Inserts can be used to create high strength threads in soft materials such as aluminum. For our purposes here I will focus on the home metalworker as well as the DIY crowd who too frequently experience the panic of a stripped out a spark plug or other similarly important hole. A Heli-Coil™ Insert can easily and permanently repair those kinds of problems and virtually any others that involve damaged or weak threads!

One of the secrets behind how Heli-Coil™ Inserts work is that inserts are intentionally a bit larger in diameter before installation than when seated in the specially designed threads cut by the Heli-Coil™ tap. The insertion tool applies torque to the tang at the bottom of the insert which reduces the diameter of the leading coils. That allows the Heli-Coil™ Insert to screw into the threads easily. When the insertion tool is removed the coil expands outward like a spring and that outward tension locks the insert in place.

The Heli-Coil™ Kit

While Heli-Coil™ Inserts are available in many larger kits meant for the professional or manufacturing environments they are also sold in one size kits designed specifically for folks like the DIY and home metalworking crowd. These kits include several Heli-Coil™ Inserts in one size, a tap that prepares the hole for the insert and an insert installation tool. You have to supply the correct size drill needed and that is listed on each package. This keeps the price of the kit down plus you wind up with another drill bit and that is never a bad thing if your shop is anything like mine!

The included tap (left) cuts very clean, precise threads for the insert and is used just as any other threading tap. Here you can see the hole (right) with the Heli-Coil insert in place. Brand new, super tough threads in minutes!
Click images to enlarge

In addition to the simplicity of using the Heli-Coil™ Inserts, the remarkable range of sizes that these kits come in can be important as well. Very often we cannot simply expand a damaged hole and tap it for a larger fastener because one of the involved parts is not compatible with a larger diameter fastener. When we are working with virtually any kind of engine using a larger fastener can be especially risky because you never really know exactly how far the current hole diameter is from a water jacket or oil passage. Drilling or tapping into either can instantly takes us from a problem to a full-on disaster. The Heli-Coil™ Insert requires that the original hole be opened but only slightly so.

In the Shop

Using the Heli-Coil™ Inserts is at least as easy as threading a hole in the first place. Although it is a very good idea to use a drill press to produce a straight, clean and properly sized hole I have seen folks accomplish similar jobs with a handheld drill. I would stick with the drill press, even if I had to beg or borrow some time on one. There is just too much chance for messing things up with a handheld drill that could compromise the near magic of the Heli-Coil™ Inserts.

One note about cleaning out and drilling the hole. Look at the length of the Heli-Coil™ Insert. The hole has to be a little deeper than the insert is tall so that the top coil of the insert is just below the surface surrounding the hole when fully installed. After removing all debris from inside the hole use the included tap to cut the threads for the Heli-Coil™ Insert. (If you are new to using a tap see our story on Using Taps and Dies BEFORE YOU BUST THIS ONE!) Make sure that you thread as far down into the hole as you can. Then remove the tap and clean the hole out again.

Video Tour

Put the Heli-Coil™ Insert onto the installation tool and carefully turn it into the new threads. Note that the end of the insertion tool is square, like a tap so that you can use it in any tap handle to make installing the insert a little easier yet. Make sure that the insert is just below the top of the hole, remove the insertion tool and the hole is ready for the fastener. It is as simple as that.

The tang is designed to be snapped off with a punch or bar through the new threads. You can also "stack" inserts for deeper thread needs.


Certainly Heli-Coil™ Inserts are not an every-day tool for most of us but when the need arises they can save a ton of money, time and effort. This is an easy to use system that in many cases makes the repair stronger than the original threads. Heli-Coil™ Inserts also have utility when working with softer metals such as aluminum or even harder plastics and composites. Being able to install these tough threads where you need and in the exact thread type needed can give you lots of versatility when making or repairing things in your shop.

The Heli-Coil™ Insert kits sell for approximately $26 each (6-21-2011) depending on the size and how big of a kit you want. Remember that you get several inserts in the smaller kits so you just might be able to save more money and time down the road if you put the remainders of the kit someplace where you might actually find it later on.

Related Material

Repairing Threads
Using Taps and Dies

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