Heat-set insert hole size chart
The pilot hole has to sit in a narrow window: wide enough that the insert smooth leading tip starts straight, narrow enough that the knurls still have plastic to bite into. Too wide and the insert spins under load. Too narrow and the boss splits.
These figures are for the common tapered brass inserts sold for 3D printing, in PLA and PETG. Insert brands differ in knurl geometry and outer diameter, and two M3 inserts from different suppliers can want different holes. Where your supplier publishes a drawing, the drawing wins.
Starting hole diameters
| Thread | Hole diameter (mm) | Note |
|---|---|---|
| M2 | 3.0 – 3.2 | 3.2 mm is the usual starting point |
| M2.5 | 3.4 – 3.6 | Often shares tooling with M3 |
| M3 | 4.0 | The most consistent size across brands |
| M4 | 5.6 | |
| M5 | 6.4 | |
| M6 | 8.0 | One common figure; verify on the drawing |
| M8 | — | No universal figure. Use the rule below. |
When the chart does not cover your insert
Measure the knurled outer diameter and subtract roughly 0.1 to 0.2 mm. That gives a hole the knurls can displace material into without wedging the boss apart. It is also how the M8 row should be handled: there is no figure the whole industry agrees on.
Boss and depth
- Leave at least 1.6 mm of plastic around the hole. For load-bearing joints, a boss around twice the insert diameter is a safer target.
- Model the hole deeper than the insert is long. Published guidance ranges from 0.5 mm to 2 mm; the point is to give displaced plastic somewhere to go rather than letting it push the insert back out.
- Expect the printed hole to come out under the modelled size. FDM over-extrudes on tight curves, so a 4.0 mm hole often measures nearer 3.8 mm.
- Print a test coupon with two or three hole sizes before committing a part you cannot reprint.
Common questions
- What size hole does a heat-set insert need?
- For the common tapered brass inserts in PLA and PETG, start at 3.0–3.2 mm for M2, 3.4–3.6 mm for M2.5, 4.0 mm for M3, 5.6 mm for M4 and 6.4 mm for M5. M6 is commonly quoted at 8.0 mm, but confirm it against your supplier's drawing.
- How much deeper than the insert should the hole be?
- Deeper than the insert is long, so displaced plastic has somewhere to go. Published guidance ranges from about 0.5 mm to 2 mm. If the hole is too shallow, molten plastic pushes the insert back out as it seats.
- How thick should the wall around the hole be?
- Leave at least 1.6 mm of plastic all the way around. For anything load-bearing, aim for a boss roughly twice the insert outer diameter: a 4.6 mm M3 insert wants a boss around 8 to 9 mm across.
- Why does my printed hole come out smaller than the model?
- FDM over-extrudes slightly on tight curves, so a hole modelled at 4.0 mm often measures nearer 3.8 mm. Print a test coupon and measure before committing to the final part.