Stripped threads can be a nightmare, especially when dealing with soft materials such as aluminum, magnesium, or plastic. When you have a stripped fitting where a bolt just won’t hold or needs reinforcement, you need to find a way to make a fastener connection that is strong and worry-free. That’s when you need a helical insert.
Called by many names, such as screw thread inserts (STI) and helicoil inserts (from the Heli-Coil® brand from STANLEY Engineered Fastening), helical inserts are coiled wire inserts that are used to repair stripped threading and reinforce bolted connections. The coils are wider than the tapped hole. They are inserted into the hole, and as the bolt is screwed down, the coil expands, creating a tight, firm seal against the threads. When properly installed, helical inserts create a permanent bond that is stronger than the original bolt fastening.
Helical inserts provide a convenient way to repair stripped threads in soft or light materials, such as magnesium, wood, aluminum, plastic, and zinc die castings. Even after frequent use, thread reinforcement with helical inserts prevents wear of the threaded hole, boosting the integrity of the application and lengthening its lifespan to create a high-strength, heat-resistant solution.
Helical inserts come in a variety of materials and thread diameters to accept a range of thread size. These inserts have a wide variety of uses in countless industries, including:
- Plant engineering
- Electrical engineering
- Aerospace industry
- Transit products
- Medical industry
- Defense industry
- Automotive industry
One example of helical insert use in the automotive industry is engine cylinder head repair after the thread of a socket is stripped due to unintentional over-torquing or cross-threading of spark plugs. Helical inserts are also commonly used in exhaust systems, gear housings, hammer drills, printing presses, and more.
Installing Helical Coil Inserts 101
To begin installation, the helical coil is inserted into the pre-drilled hole. To screw the helical insert into the hole, you will need a means to secure the coil so it will rotate for proper installation. This is where you have to decide between tanged or tang-free coils.
A tanged helical insert extends the end of the coil wire across the radius of the coil. This provides a grip point for the helical insert tool so it can rotate the coil insert to snugly fit in the hole. After the coil is inserted, the tang must be removed before you can screw in the fastener or bolt.
A tang-free helical insert is installed in much the same way, although instead of using a metal coil tang that has to be removed after installation, tang-free inserts have a drive notch in the helical coil that is used as a grip point for the helical insert tools.
There are four basic helical coil designs:
- Tanged free-running coil: This is a stainless steel coil designed to resist corrosion. It can be used for quick repair of damaged threads. These types of coils meet military and aerospace standards.
- Tang-free free-running coils: These are similar to the tanged free-running coils, but they have no tang at the bottom. They instead use a special insert-driven notch for screw-in installation. This eliminates the need to remove the tang before inserting the bolt.
- Tanged screw lock coils: This helical coil design includes a straight segment in the coil that is flexed outward, which creates pressure on the bolt, retaining torque to secure the bolt in place. The coil is designed to maximize the contact area by applying pressure between the flanks of the bold thread, making it especially strong in applications where there is a lot of vibration.
- Tang-free screw lock coils: These have the same design as the tanged screw lock helical coils, but instead of a tang, they use a driving notch for installation.
Although both types of helical inserts are used across various industries, the tang-free insert eliminates a step—tang removal—which saves time and money. Kato estimates that eliminating tang removal can save from $0.15-$0.50 per hole in labor costs. It also eliminates the need for tang retrieval and the risk of loose metal getting into the machinery.
The size of the coil is determined using an STI gauge to match the insert to the hole size. The coil itself is then screwed in place so it presses against the existing screw threads. The coil design usually includes a “tang” at the bottom that acts as a catch point for the installation tool.
Once the coil has been screwed in place, the tang is snapped off so there is no obstruction to the bolt. When the bolt is screwed in place, the helical coil expands inside the drill hole, retaining a locking torque, which creates a firm, lasting connection.
The spring action of the insert is one advantage of using a helical coil. With conventional bolts, more than 75 percent of the load is carried by the first three threads of the assembly. The helical coil insert transfers shear stress into radial loading over the entire length of the insert. The result is a much stronger bond than you would get with drilling and tapping.
Helical coils are used in various applications, including manufacturing, electrical engineering, aerospace, medical devices, telecommunications, and more. A common example in the automotive industry is repairing an engine cylinder head; often, over-torquing or cross-threading of spark plugs will require a coil to repair the pre-drilled threads. Because they retain a tight seal, helical coils are particularly useful for gear housings, drills, printing presses, and machinery that generates continuous vibration.
Free-Running vs. Screw-Locking Inserts
In addition to deciding about tanged versus tang-free helical coils, there are other helical insert design choices to consider. Each type of helical insert is designed for different applications.
Free-running helical coils, for example, are extremely flexible inserts designed so that each coil in the helical insert can independently adjust to the fastener threads. This enables more surface contact between the coil and the parent materials, which creates a stronger and more reliable seal because the load is evenly distributed. One of the advantages of using a helical insert is that it increases fastener strength and longevity. Free-running coil inserts reduce stress and fatigue while maximizing thread strength and increasing reliability. Free-running helical inserts are available in both tanged and tang-free designs.
Screw-locking helical inserts provide added strength. In a screw-locking insert, the coil includes one or more straight segments. When the bolt is inserted, these segments flex outward, placing added pressure on the bolt thread and the walls of the drill hole. These locking coils make for even stronger, longer-lasting connections.
Typically, these types of coils are made from stainless steel, which makes the coil thread corrosion-resistant and extends the life of the fastener assembly. Because they are designed for critical applications such as transportation and aerospace manufacturing, these inserts also can withstand extreme temperatures, from -320° to +800° Fahrenheit.
Top Types of Helical Inserts
There are various helical coil manufacturers, and we carry inserts from two of the top manufacturers:
The Recoil helical coils from Arconic Fastening Systems are manufactured in standard sizes for thread forms measured in both centimeters and inches. Recoil helical coils are generally made from stainless steel and available in standard free-running or screw lock types. In addition to stainless steel, Recoil helical coils are available in nickel alloy for high temperature applications, phosphor bronze for electrical bonding joints, Nimonic 90 nickel-chromium-cobalt alloy, and Nitronic 60 chromium-nickel alloy.
There are a number of driver tools available for the Recoil helical coils, including the REC-20 Battery Driver, a lightweight, portable, battery-powered thread-insert driver for hard-to-reach tapped holes. This driver tool is ideal for small production and repair applications and features high and low speeds, forward and reverse directions, and a clutch torque controller. The REC-12US Electric Driver is a lightweight, handheld unit for tanged and tang-free coils, with two speeds and an auto-reverse function, ideal for low- or high-volume production. The REC-10K Air Driver can handle tanged and tang-free inserts and uses an air motor with small and large adapters. Recoil drivers are versatile and can handle virtually any job.
Kato Fastening Systems has been making helical coil inserts since the 1970s. Available in tanged and tang-free versions, the Kato CoilThread is made of cold-rolled stainless steel with a tensile strength of 200,000 psi. These helical coils are designed to resist corrosion and can be used at operating temperatures ranging from -320° to +800° Fahrenheit. Kato coil inserts also come in a wide range of materials and coatings for any job.
For installation, Kato has the Kato Hex Electric (KHE) tool, designed for medium- to high-volume applications. It can drive both tanged and tang-free CoilThread inserts and has a quick-connect keyless chuck system as well as auto reverse to cut installation time. Kato also offers the Prewinder Air (KPA) tool, which can be used with tanged and tang-free strip-feed belts for high-volume applications. The Kato Prewinder Electric (KPE) tool is an electric version, and there is also a wide variety of hand tools.
If you are looking for quality helical coils for manufacturing, installations, or repairs, we have a wide range of helical coils and installation tools for any application. Feel free to contact one of our experts to learn more about our comprehensive line of helical coils.
This article was updated in April 2021.