A threaded insert that pulls out, spins in the hole or sits proud of the surface can turn a simple assembly job into a repeat repair. To install a threaded insert correctly, the hole size, insert type and fitting method must suit the parent material. The thread size alone is not enough.
Threaded inserts provide a durable internal thread where the base material is too soft, too thin or likely to be assembled repeatedly. They are commonly used in fabricated steelwork, aluminium parts, plastic panels, timber fixtures, machine guards and repair work. The best result comes from selecting the right insert before drilling the first hole.
Choose the insert for the material and application
Different inserts grip in different ways. A rivet nut, also called a blind threaded insert, is designed for sheet metal, box section and other applications where access is only available from one side. When installed with the correct setting tool, its body collapses behind the material to clamp in place.
Screw-in inserts have an external coarse thread and are generally used in timber, plastics and softer materials. They cut or form their own purchase in a correctly sized pilot hole. Press-in inserts rely on an interference fit and are often used in plastic components, particularly where the hole can be moulded or accurately machined. Heat-set inserts are another option for thermoplastics, using controlled heat to melt the surrounding material and form a secure bond.
Thread-repair inserts are a separate category. These restore a damaged thread in a drilled and tapped metal component, rather than creating a new fixing point in sheet, timber or plastic. Their installation sequence normally includes drilling, tapping with a dedicated tap and fitting the insert with the specified driver. Do not substitute a standard tap or assume a thread-repair insert uses the same hole dimensions as a rivet nut.
The application also matters. Consider the bolt diameter and pitch, required pull-out strength, available material thickness, vibration, corrosion exposure and the number of assembly cycles. An M6 insert may be suitable in more than one product, but the correct body length and grip range can be very different.
Measure before drilling the hole
Most failed inserts start with an incorrect hole. A hole that is too large reduces grip and can allow the insert to rotate under tightening. A hole that is too small can distort the insert, crack plastic, split timber or prevent a rivet nut from setting fully.
Use the manufacturer’s stated hole diameter for the exact insert, material and installation method. Do not select a drill by thread size. The hole for an M6 insert is not automatically 6 mm, and different styles of M6 insert can require different hole diameters.
Check the material thickness as well. Blind rivet nuts are rated for a grip range, meaning the thickness of material they can clamp effectively. If the sheet is thinner than the stated range, the insert may not clamp securely. If it is thicker, the body may not collapse as intended. In thin sheet, a larger flange can spread load over a wider area, while a reduced-head style may be preferable where a flush finish is needed.
Mark the centre accurately, then use a centre punch on metal to stop the drill wandering. Drill square to the surface using a pillar drill where practical. A hand drill can be suitable for maintenance work, but keep the drill steady and avoid opening the hole at an angle. On sheet metal, a step drill often produces a cleaner, rounder hole than forcing a large twist drill through thin material.
Deburr both sides after drilling. Sharp edges prevent an insert flange from sitting flat and can create stress points around the hole. Remove only the burr, not more parent material. On plastics and timber, clean away swarf so it does not interfere with seating.
Check fit without forcing it
Before setting several inserts, test one in a sample or an inconspicuous position. A rivet nut should enter the hole without excessive force and sit flat against the face. A screw-in insert should engage straight in its pilot hole without needing to be driven aggressively.
This simple test is especially worthwhile when working with coated steel, aluminium, hardwood, recycled plastic or material with variable thickness. Coatings, drilling burrs and real-world tolerances can affect fit even when the nominal specification is correct.
How to install threaded inserts correctly
For blind rivet nuts, use a dedicated rivet nut tool with the correct mandrel and nosepiece for the internal thread and insert body. Thread the insert fully onto the mandrel, place it squarely in the prepared hole and keep the tool perpendicular to the workpiece. Operate the tool until the insert has collapsed and clamped behind the material, then release it carefully.
The aim is a firm clamp, not maximum force. Over-setting can crush thin sheet, deform the flange or damage the internal thread. Under-setting leaves the insert loose, increasing the chance that it will spin when the bolt is tightened. If the tool is adjustable, set it using a trial piece and inspect the formed section on the reverse side.
For screw-in inserts in timber or plastic, use the recommended insertion tool rather than gripping the internal thread with pliers or a bolt where possible. Start the insert perpendicular to the surface and turn it steadily. It should sit flush or at the depth specified for the job. Driving it too deep can weaken a thin panel or cause a visible depression around the fixing.
Some plastic inserts are designed to be heat-set. Use a temperature-controlled soldering iron fitted with an appropriate tip, not an open flame. Apply light, even pressure and allow the insert to settle as the plastic softens. Stop at the required depth, then leave the part undisturbed until it has cooled. Twisting or loading the insert while the plastic remains warm reduces holding strength.
For threaded repair inserts, follow the kit sequence precisely: drill to the stated size, tap with the supplied or specified tap, clean the new thread and install the insert to the required depth. Break off any driving tang only where the insert design requires it, and make sure it cannot remain loose inside a component.
Avoid the fitting mistakes that cause failures
Using a bolt and nut to pull-set a rivet nut may work in an emergency, but it gives poor control compared with a proper tool. It can gall threads, apply uneven load and make repeatable installation difficult. For production, repair programmes or critical assemblies, use the correct setting tool and replace worn mandrels promptly.
Misalignment is another common cause of early failure. If the insert is installed at an angle, the fastener will enter under side load. This can cross-thread the bolt, damage the insert or prevent mating parts from locating properly. Check squareness before setting, not after.
Do not rely on adhesive to compensate for the wrong hole size or the wrong insert style. Some installations may specify threadlocking compound, sealant or adhesive for vibration resistance or fluid sealing, but these are additions to correct mechanical fit. They are not a substitute for it.
Material condition also deserves attention. Corroded sheet, delaminated coatings, cracked plastic and damp or split timber may not provide sufficient purchase. Repair or replace the parent material where needed. A high-quality insert cannot create strength in material that has already failed.
Inspect the finished fixing before assembly
Once installed, the insert flange or top face should sit evenly against the workpiece. Check that there is no gap, rocking or visible distortion. With rivet nuts, inspect the reverse side where access allows to confirm that the body has formed consistently.
Run the correct bolt into the insert by hand first. It should thread in smoothly for several turns. Resistance at the start can indicate swarf, damage to the thread or a misaligned insert. Do not force it with an impact driver.
Finally, tighten the assembled fastener to the appropriate torque for its size, grade and application. The insert’s stated torque capacity may be lower than the bolt’s capacity, particularly in thin sheet or soft materials. In vibration-prone equipment, use the specified locking method and recheck the joint after initial service where the maintenance schedule calls for it.
A few minutes spent confirming drill size, material thickness and tool setup prevents the familiar job of drilling out a spinning insert later. For workshop repairs and repeat production alike, fit one test insert first, inspect it closely and then proceed with confidence.