Key locking inserts are the ideal fastener for restoring broken threads, whether you're replacing an enlarged or misplaced hole or need to finish a repair on original equipment.

What exactly are key-locking inserts, and how are they different from regular threaded inserts for metal, and more importantly, how do they work? Read through our guide below to learn more:

Locking threaded inserts, otherwise known as key-locking inserts, are cylindrically tube-shaped pieces of metal with external and internal thread. Unlike self-locking threaded inserts, which feature adhesive or nylon on their external threads, these fasteners feature a positive mechanical lock.

This "locking mechanism" prevents rotation caused by vibration and torsion by driving the locking pins into the material. The general purpose of these key-locking inserts, often called keyserts, or keenserts, is thread repair and reinforcement of softer materials, as they provide a strong, permanent thread in the weaker parent material.

Key-locking thread inserts are easy to install and are frequently used to repair threads on ferrous, non-ferrous, and non-metallic metals, such as aluminum, iron, and magnesium. However, they're not really suitable for use in brittle materials, such as cast iron and plastics.

Depending on your application, there are four main types of threaded inserts:

·        Thin Wall — Thin wall inserts have a smaller external thread size, making them perfect for tight spaces.

·        Heavy Duty — Heavy duty inserts are general-purpose key locking inserts with thick and heavy-duty thread walls that are suited for most applications. These inserts are suited for applications that require the mating stud or bolt to be removed frequently.

·        Extra Heavy Duty — Extra heavy duty inserts offer a larger external thread area and a thicker wall thickness, providing greater pull-out strength, longer life, and stability. They work best with low ultimate shear strength materials, allowing for maximal pull-out strength and integrity.

·        Solid — These inserts are great for moving holes that have been misplaced, patching holes, preserving valuable castings, and filling in holes that are too wide.

Threaded inserts can be employed in a variety of applications due to their versatility and ease of use, which is why they've grown so popular in the manufacturing, assembly, building, and repair industries.

However, unlike regular threaded inserts, which can be used on a variety of different materials, key-locking threaded inserts are designed to work on metals more pliable and softer than carbon steel or passivated stainless steel. As such, they're best suited in the following applications:

·        Trucking — Trucking and RV drivers use key-locking threaded inserts during smaller repairs.

·        Molds — Installing thread inserts is much easier than drilling and tapping a hole in an object you're making a mold of.

·        Metalworking — These are used to repair damaged threads or to reduce the internal thread size.

·        Heavy machinery — locking threaded inserts are used to repair and maintain heavy machinery in sports where lead-bearing threads are required.

To install a key-locking insert to repair a damaged thread, first, you must assess the damage and remove the remaining bolt using a bolt extractor (if necessary). Then drill out the old thread using a standard drill; refer to the chart provided by the manufacturer/distributor of threaded inserts on what drill size to use.

Once you've drilled the hole, chamfer it with an 82°-100° countersink bit. For small inserts, use a smaller, 90° countersink bit. Keep in mind that all the rules associated with drilling and lubrication apply during insert installation. Next, tap the new thread with an appropriate tap.

Once the tapped hole is prepared, screw the key-locking threaded inserts in place, first by hand, with the pins facing outward. Use the supplied installation tool if necessary, and drive the rivet down until it's resting below the surface. Position the installation tool so that its grooves aren't aligned with the pins (keys) and tap the key down into the insert using a hammer.

It's important to note that keys bite into the threads of the parent material during installation. This creates perpendicular grooves inside the thread of the parent material, which ensures that the insert stays in place.

Drill out the material between the keys and the internal thread with a normal drill to remove a key locking insert. The keys should then be deflected inward and broken off. The insert can then be removed with a regular screw extraction tool, and a replacement insert of the same size can be placed into the original hole.

However, considering that the previous installation has caused perpendicular micro-grooves in the thread of the parent materials, some preparation might be required. You don't have to drill another hole — just use the same-size tapping tool to re-tap the thread. That will straighten out any micro-grooves in the material and allow for easy installation of a new threaded insert.

The disadvantage of utilizing a helical threaded insert is that these inserts are prone to breakage. If you're debating between a key locking insert and another insert, the former has a number of significant advantages.

The disadvantage of utilizing a helical threaded insert is that it usually has a "tang" or piece of material at the bottom of the insert that serves as a catch point for the installation tool. This can break off after installation and become loose in your application's chassis.

This shouldn't be an issue on its own, but if your application has exposed circuitry, it can result in an electrical short. A key locking insert eliminates the risk of a tang or other fastener component becoming free in the chassis, making it a safer option.

Key-locking threaded inserts are a standard part of thread repair kits for a good reason — they're reliable, easy to use, and not all that expensive. They're great for repairing damaged threads and require simple, standard tools, eliminating the need for a pre-winder. If you're interested in learning more about threaded inserts, check out our guides at Reid Supply.