How do I fit a threaded insert into wood?
Giving timber a steel machine thread that can be undone hundreds of times — pilot sizes, driving it in square, and where an insert is the wrong answer.
- Difficulty
- intermediate
- Time
- 20 min
- Read
- 3 min
Short answer
Drill the pilot at exactly the size the insert specifies, start the insert square, and drive it in with a hex key or a bolt-and-nut driver until its top is just below the surface. Do not use a power tool, and do not force it in hardwood without piloting generously — the insert will split the timber or shear off.
A wood thread wears out. Every time a screw goes in and out of the same hole it takes a little more fibre with it, and after a handful of cycles it will not hold. A threaded insert replaces that wood thread with a steel one, and it is the correct answer for anything demountable: knock-down furniture, jig fixtures, machine feet, workbench accessories, and repairs to a hole that has already stripped.
What you'll need
- Threaded inserts of the right size
- A drill bit of exactly the stated pilot size
- A hex key, or a bolt with two nuts locked together
- A square
- Epoxy or thread-locking adhesive (optional)
Step by step
- Choose the insert type for the material.Hex-drive inserts with a coarse external thread are the strongest and most controllable. Slotted types driven with a screwdriver are cheaper and more prone to going in crooked. Knurled press-in inserts are for plastics rather than timber.
- Drill the pilot at exactly the stated size.This is not a place to judge by eye. Too small and the insert splits the timber; too large and the external thread never grips. The maker states a size, and in hardwood you generally take the top of the stated range.
- Drill deeper than the insert is long.The bolt has to be able to go past the insert without bottoming out, and the insert has to seat without packing dust ahead of it.
- Start it square, and check with a square.An insert that starts crooked stays crooked, and a bolt at an angle in a piece of furniture is very visible. Check from two directions in the first few turns.
- Drive it with a hex key or a nut-and-bolt driver.For a hex-drive insert, a hex key. For a slotted one, lock two nuts onto a bolt, wind the bolt into the insert, and drive with a spanner on the outer nut. Never with a power tool — the torque is uncontrolled and the insert shears or strips.
- Set the top just below the surface.A millimetre below flush means it cannot foul the part being bolted down. Standing proud is worse than being slightly deep.
- Use adhesive in weak material.In chipboard, MDF and end grain, a smear of epoxy in the hole before driving makes an enormous difference, because the insert is otherwise relying on material with very little strength across the thread.
- Avoid inserts in end grain if you have a choice.End grain holds threads poorly and splits readily. A cross dowel taking the bolt across the grain is a far stronger arrangement where the geometry allows it.
Tips
- A threaded insert is the standard repair for a hinge or a fitting whose screw hole has stripped for the third time, particularly on chipboard furniture and on machinery mounts.
- Buying a matched set — inserts and the bolts to suit — saves a second trip. M6 covers a great deal of furniture and jig work.
- In a through hole, a bolt with a nut and washer on the back is stronger and cheaper than an insert. Use inserts where you can only reach one side.
Common mistakes
- Driving an insert with a cordless drill — The torque required varies wildly as it cuts, and a drill supplies far more than the insert can take. They shear off level with the surface, which is very difficult to remove.
- Guessing the pilot size — The tolerance is small. A hole a millimetre out either splits the timber as the insert goes in or leaves an insert that spins in the hole the first time the bolt is tightened.