How to size a threaded insert: thread, length, material and style

Threaded inserts put a machine-screw thread into wood, particleboard, MDF or soft metal, so a bolt can go in and out without stripping the hole. Furniture, cabinet and fixture shops ask us about them often, and the question is nearly always the same: which size do I need? Four things decide it.

1. The thread inside the insert

The inside thread has to match the bolt or screw that goes into it.

• Inch threads are written size-threads per inch. 10-24 is a #10 screw with 24 threads per inch. 1/4-20 is a 1/4 in bolt with 20 threads per inch.
• Metric threads are written M and the diameter in millimetres, such as M6. The standard pitch for M6 is 1.0 mm, so M6 and M6 × 1.0 usually mean the same thing.

An inch bolt won't run into a metric insert, even when the sizes look close, so check the bolt first.

2. The length of the insert

An insert size is often written thread × length, so 1/4-20 × 12 mm is a 1/4-20 thread on an insert 12 mm long. The insert has to be shorter than the material is thick, and a longer insert holds better in softer material.

3. What it goes into

Hardwood, softwood, plywood, particleboard, MDF and metal each take a different style of insert, and each maker gives its own drill size for each material. Use the hole size the maker lists for your insert and your material. A hole that is too small splits the work, and a hole that is too big lets the insert spin.

4. The style

• Screw-in inserts have a coarse outside thread that cuts into wood and are driven in with a matching drive tool or hex key.
• Press-in inserts have barbs or ribs and are pressed or tapped into a drilled hole.
• Flanged inserts have a lip that stops at the surface. The lip also spreads the load.
• Tee nuts go in from the back face, and their prongs bite into the wood. See our 1/4 in SAE tee nuts.
• Helical (coil) inserts repair or reinforce threads in metal. They need the matching drill, tap and install tool, which is why they are usually bought as a kit, such as this 1/4-20 helical thread repair kit.

Ordering inserts by the box

Most screw-in and press-in inserts for wood are quoted to your spec. Use threaded inserts and insert nuts: request a quote and send:

• the thread size (for example 10-24, 1/4-20 or M6);
• the insert length;
• the material it goes into;
• the style you use now, and the part number if you have one;
• how many you use a month.

For thread sizes, diameters and inch-metric matches, see the threaded insert size chart. Bolts and screws to go with them are in screws and fasteners, and the fasteners buying guide covers the rest.

Matching the inserts and hardware you use now? Send us your list and we will match it, usually within 1 business day.