Screw & Bolt Size Chart
The hole to print depends on whether the screw should pass through freely or cut its own threads into the plastic.
| Size | Clearance hole (free pass-through) | Self-tap pilot hole |
|---|---|---|
| M2 | 2.4mm | 1.6–1.7mm |
| M2.5 | 2.9mm | 2.0–2.1mm |
| M3 | 3.4mm | 2.5mm |
| M4 | 4.5mm | 3.3mm |
| M5 | 5.5mm | 4.2mm |
| M6 | 6.6mm | 5.0mm |
| M8 | 9.0mm | 6.8mm |
Clearance hole vs. self-tapping pilot hole
A clearance hole is sized so the screw's threads never touch the plastic at all — use this when the screw is meant to pass through into a nut, a heat-set insert, or threads on the other side. A self-tapping pilot hole is deliberately undersized so the screw cuts its own threads into the plastic as it's driven in; this works because printed plastic is soft enough to tap into directly, but it's weaker and more prone to stripping than a proper insert or nut, especially with repeated assembly and disassembly.
When to use a heat-set insert instead
Self-tapping directly into plastic is fine for a connection made once and rarely revisited. For anything assembled and disassembled repeatedly, or under real mechanical load, a heat-set insert gives real metal threads that don't degrade the way a self-tapped plastic thread does after a few cycles.