How to Install a Threaded Insert: Wood, Metal & Plastic
A threaded insert presses or threads into a pre-drilled pilot hole to give a soft or thin material a reusable metal thread for machine screws — but the pilot-hole size and driving method both change with the material. Get either wrong and you split the wood, strip the plastic, or shear the insert under load. This guide covers pilot-hole sizes for wood, metal, and plastic, install steps for press-in, self-tapping, and heat-set inserts, and how to test the finished insert’s strength.
Quick Answer
Drill a pilot hole sized to roughly 90-95% of the insert’s outer diameter for wood or plastic, then drive it in by hand or with a hex key until it sits flush. For metal, tap the hole to the insert’s minor diameter first, or use a self-tapping insert that cuts its own threads as it’s driven in.
A threaded insert is a small hardware fastener — usually brass, steel, or zinc alloy — that presses or screws into a pilot hole to give a soft or thin base material a durable, reusable internal machine-screw thread. It lets you use a metal machine screw (removable, re-insertable) in wood, plastic, or thin sheet metal that couldn’t otherwise hold that thread on its own.
Threaded Insert Types by Material: Pilot Hole and Install Method
The install method and hole size aren’t universal — they’re set by what you’re driving the insert into. Use this table before drilling anything.
| Material | Insert Type | Pilot Hole | Install Method |
|---|---|---|---|
| Wood | Knife-thread / hex-drive brass | ~90-95% of insert OD | Hand-driven with hex key or bolt-and-nut jig; epoxy in soft/end grain |
| Metal | Helicoil / solid threaded bushing | Drilled + tapped to minor diameter | Tap hole first, thread insert in with an install tool, snap off the tang |
| Plastic | Heat-set / ultrasonic / molded-in | Slightly under insert OD | Press in with a heated soldering-iron tip so plastic re-flows around the knurling |
📊 A pilot hole sized correctly at 90-95% of the insert’s outer diameter is what determines maximum pull-out strength — oversized holes lose grip, undersized holes split wood or crack plastic during drive-in. Source: E-Z LOK Drill & Tap Chart Guide
Gather the Tools and Materials
Before you drill anything, get everything on hand — swapping tools mid-hole is how alignment gets lost.
- Power drill or tap handle: a drill for wood and plastic pilot holes, a tap and tap wrench for metal.
- Correct-size drill bit or tap: matched to the insert’s outer (wood/plastic) or minor (metal) diameter — see the table above.
- Threaded insert: sized and material-matched to the workpiece and the machine screw you’ll use.
- Hex key, insertion tool, or a long bolt with two locking nuts: to drive the insert in straight.
- Epoxy (wood only, optional): for soft or end-grain wood where the threads alone won’t hold under load.
- Safety glasses and a dust mask.
Step-by-Step: Installing the Insert
- Mark and drill the pilot hole: center-punch or mark the location, then drill straight and to the depth of the insert, sized per the material table above.
- Clear debris: brush or blow out drill dust and chips — leftover debris throws off flushness and thread engagement.
- Tap if it’s metal: run a tap matched to the insert’s thread through the hole before attempting to drive the insert.
- Lubricate (wood/metal only): a thin coat of wax or light oil on the insert’s threads reduces friction and driving force — skip this for heat-set plastic inserts.
- Drive the insert in straight: use a hex key, dedicated insertion tool, or a long bolt with two locking nuts threaded onto the tip to keep the insert plumb as you turn it clockwise.
- Stop at flush: the top of the insert should sit level with or very slightly below the surface — stop turning as soon as it’s flush; over-driving strips the hole.
- Test fit with the machine screw: thread the screw in by hand. It should engage smoothly with no binding or cross-threading.
According to a Fine Woodworking shop-tip column, using a bolt with two nuts locked near the tip — threaded into the insert — gives you a long, rigid handle that’s much easier to keep perpendicular than the short installation tool that ships with most kits, which is especially useful on a deep or narrow workpiece.
“Cross-threading happens when the insert is forced instead of installed straight — if something feels wrong partway in, back it out and fix the hole rather than pushing through.”
Checking Alignment, Flushness, and Strength
Run your finger over the surface — the insert should feel level with the material, not raised or sunk. If it’s crooked, don’t force it further; back it out, clean the hole, and re-square the tool before continuing.
To confirm the insert will hold, thread a machine screw in by hand and check for at least one full thread of engagement with no wobble. For load-bearing hardware (drawer pulls, knock-down furniture fittings, jig fixtures), snug the screw down and apply the actual expected load by hand before trusting the joint — a insert that spins in place under light torque needs to be re-set with epoxy (wood) or a larger insert (metal/plastic), not just re-tightened.
Best Threaded Insert Kit Pick
Threaded Insert Kit for Wood (Brass, 1/4″-20, 10pc) – $19.49
A complete 1/4″-20 brass kit — insert, matching drill bit, and installation tool in one box, sized to the 90-95%-of-OD pilot hole this guide covers.
- Best for: a first wood-insert project where you don’t already own a matched drill bit
- Why we picked it: the bit is pre-sized to the insert, so you can’t pick the wrong pilot-hole size
- Main drawback: only covers one thread size (1/4″-20) — buy a different kit for #8-32 or metric hardware
Compare more threaded insert options
 Option 1 80Pc Hex Nut Threaded Inserts for Wood Furniture – $9.99
|
 Option 2 40Pc Threaded Inserts Assortment + Hex Wrench – $5.59
|
 Option 3 — Metal SEDY SAE Thread Repair Helicoil Kit (Metal) – $26.99
|
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Choosing the Right Insert for the Job
Match the insert to the material and the expected load before you drill anything — switching inserts after the hole is cut usually means starting over. For wood-specific sizing, grain considerations, and a deeper install walkthrough, see our dedicated wood-insert guide. For plywood specifically, this guide on threaded inserts in plywood covers how the layered grain changes pull-out strength versus solid wood.
- Material compatibility: match insert material (brass, stainless, zinc alloy) to the workpiece and to any moisture or corrosion exposure it will see.
- Load requirements: for heavier loads, check shear strength and pull-out strength, and consider a longer or larger-diameter insert.
- Thread size: the insert’s internal thread (1/4″-20, #8-32, M6, etc.) must match the machine screw you intend to use.
Common Mistakes to Avoid
- Drilling the pilot hole too small: forcing an oversized insert into an undersized hole splits wood and cracks plastic. Confirm your drill bit size against the insert’s actual outer diameter, not just its nominal thread size.
- Driving it in crooked: a tilted insert cross-threads the machine screw later. Keep the driving tool perpendicular to the surface the entire time.
- Skipping the tap on metal: pressing an insert into a bare, untapped metal hole strips the material instead of threading it — metal needs to be tapped first, or use a genuinely self-tapping insert.
- Overheating plastic: a soldering iron left too long on a heat-set insert melts more plastic than intended and weakens the surrounding material — press in short controlled bursts.
Frequently Asked Questions
How do you install a threaded metal insert?
Drill a hole to the insert’s minor diameter, tap it to cut matching threads, then thread the insert in with an installation tool until it’s flush. Self-tapping metal inserts skip the separate tapping step and cut their own threads as they’re driven in.
Do threaded inserts work in metal without tapping?
Only if the insert is specifically labeled self-tapping. A standard threaded insert pressed into an untapped metal hole will strip the hole rather than seat correctly, since bare metal doesn’t compress or deform around the insert the way wood or plastic does.
Should I glue threaded inserts in wood?
Yes, for soft wood or end grain, a thin coat of epoxy around the insert before driving it in adds meaningful pull-out resistance beyond what the threads alone provide. For hardwood side grain under light loads, threads alone are usually sufficient.
Can you use threaded inserts in plastic?
Yes — heat-set inserts are pressed in with a heated soldering-iron tip so the plastic re-flows around the insert’s knurled exterior as it cools. Ultrasonic and molded-in inserts are alternative methods used more often in manufacturing than home workshops.
How do you use a threaded insert kit?
Choose the correctly sized insert for your project, drill the pilot hole to the recommended depth and diameter, insert the threaded insert using the kit’s driving tool, then turn clockwise until it sits flush with the surface.
Conclusion
Installing a threaded insert comes down to two decisions: the right pilot-hole size for your material, and driving it in straight until it sits flush. Wood and plastic want a slightly undersized hole so the material grips the insert; metal needs to be tapped first. Test the fit with the actual machine screw before calling the job done, and reach for epoxy on soft or end-grain wood carrying real load. See our threaded insert pull-out strength data for how much load a properly installed insert can actually take in wood.
