Heat-Set Insert Hole Size Chart: M2 to M8 Complete Reference

Quick reference. You’re designing a part, you need to drop in heat-set threaded inserts, and you want the exact hole diameter to model in CAD. This page is the chart — by insert size, by material, by nozzle. The full explanation of the math is below, but the table comes first because that’s what you’re here for.

For a calculator that handles your specific printer’s calibration (factoring in your flow rate, your linear advance tuning, your specific filament), the Hole Tolerance Calculator does it live. The values below are the typical defaults that apply to a tuned 0.4mm nozzle setup.

The chart — heat-set insert pocket sizes

Standard Ruthex / CNC Kitchen inserts (most common)

Insert size	Outer diameter (OD)	CAD pocket Ø in PLA	Pocket depth	Use case
M2	3.2mm	3.43mm	4.5mm	Electronics enclosures, small fixtures
M2.5	3.6mm	3.84mm	6.2mm	Camera mounts, small assemblies
M3 short	4.0mm	4.24mm	4.5mm	General-purpose, thin-walled parts
M3 long	4.0mm	4.24mm	6.2mm	Voron BOM standard, structural joins
M4	5.6mm	5.83mm	8.6mm	Furniture-grade assemblies
M5	6.4mm	6.63mm	10.0mm	Robotic assemblies, heavy brackets
M6	8.0mm	8.23mm	13.2mm	High-torque mounts
M8	10.0mm	10.23mm	15.0mm	Industrial fixtures

Pocket depth = insert length + 0.5mm (gives plastic somewhere to flow during install).

Material-adjusted CAD pocket diameter

For materials other than PLA, the compensation changes because of different shrinkage rates. Common adjustments to the PLA values above:

Material	M3 (4.0mm OD)	M4 (5.6mm OD)	M5 (6.4mm OD)	M6 (8.0mm OD)
PLA	4.24mm	5.83mm	6.63mm	8.23mm
PLA+	4.24mm	5.83mm	6.63mm	8.23mm
PETG	4.26mm	5.86mm	6.67mm	8.28mm
ABS / ASA	4.29mm	5.91mm	6.74mm	8.36mm
Nylon (PA)	4.30mm	5.93mm	6.77mm	8.40mm
Polycarbonate	4.29mm	5.91mm	6.74mm	8.36mm
PA-CF	4.26mm	5.86mm	6.67mm	8.28mm

The variation between materials at the same insert size is small (0.05–0.10mm) because most of the compensation is “slicer/extrusion overshoot” (a fixed effect) rather than “material shrinkage” (proportional to size, which barely matters at 4mm).

For nozzles larger than 0.4mm

Multiply the slicer compensation portion by nozzle / 0.4. The shrinkage portion stays the same. Practically:

• 0.6mm nozzle: add ~0.10mm to the values above
• 0.8mm nozzle: add ~0.20mm to the values above

A 0.4mm nozzle is what 95% of these inserts get installed with, so the base table is the right starting point.

Installation specs

Pocket geometry

• Chamfer at entry: 1mm × 45° — mandatory. Lets the insert self-align as you start it.
• Pocket depth: insert length + 0.5mm. The displaced plastic needs somewhere to flow.
• Wall thickness around pocket: minimum 1.5mm (M3), 2.0mm (M4), 2.5mm (M5+). Too thin and the wall splits during install.
• Spacing between pockets: minimum 2× pocket diameter center-to-center for parallel inserts.

Soldering iron temperature

Material	Iron set point
PLA	220–240°C
PETG	230–250°C
ABS / ASA	250–270°C
Nylon (PA)	280°C
Polycarbonate	290–310°C

Too cold: knurls don’t grip after the part cools. Too hot: plastic outgases and leaves voids. The numbers above target a slow, controlled melt.

Install procedure

1. Heat the iron to the target temp + give it 30 seconds to stabilize
2. Use a dedicated heat-set insert tip ($5–10) shaped to fit the insert ID
3. Rest the tipped iron on top of the insert, light downward pressure (~hand weight)
4. Let it sink slowly — 5–10 seconds per insert. Faster compromises knurl grip
5. Verify insert is flush or just slightly proud, perpendicular to the surface
6. Let cool 30+ seconds before torquing a screw in

Why the chart values are what they are

Three contributors to printed-hole shrinkage:

total_compensation = material_shrinkage + slicer_polygon_approximation + extrusion_overshoot

For an M3 insert (4.0mm OD), in PLA on a 0.4mm nozzle:

• Material shrinkage: 4.0 × 0.003 = 0.012mm (negligible)
• Slicer polygon approximation: ~0.04mm (slicer converts circle to 32-gon)
• Extrusion overshoot: ~0.18mm (inside-curve material pile-up)
• Total: ~0.24mm — model the pocket at 4.24mm

This is roughly fixed at small sizes (M2–M5) because the overshoot dominates. Larger sizes (M6+) start getting more material-shrinkage contribution but the formula compresses both into the chart values above.

When the chart doesn’t quite work

If you’ve installed inserts following the chart and they’re loose:

• Your printer over-extrudes — your printed pocket is larger than the formula expects. Calibrate flow rate down 2–3%. Or use the Hole Tolerance Calculator calibration override.
• Insert is the wrong spec — generic Amazon “M3” inserts vary wildly in OD. Stick with named-brand (Ruthex / CNC Kitchen) or measure yours.

If they’re so tight the insert won’t seat or the wall splits:

• Your printer under-extrudes — pocket is smaller than the formula expects. Calibrate flow rate up 2–3%.
• You’re at the wall thickness minimum — bump wall thickness 0.5mm and retry.
• Material shrinkage is higher than expected (especially Nylon) — bump pocket Ø by 0.1mm.

After one calibration print (~15 min), all future heat-set installs work first try. The Hole Tolerance Calculator saves the calibration to your browser so you don’t have to redo it.

How to use this chart in production

1. Pick the insert (M3 long is the most common — Voron BOM standard)
2. Look up the CAD pocket Ø for your material
3. Draw the pocket as a circle with the chart diameter, depth = insert length + 0.5mm, 1mm × 45° chamfer at entry
4. Print, install, torque screw in. Should hold 1000+ assembly cycles

For the deeper explanation of why hole compensation works the way it does — including the polygon-approximation effect, extrusion overshoot, and orientation considerations — see 3D Printed Holes Too Small? Exact mm Compensation by Material.

For the full install walkthrough with temperature, tooling, and common failures, Heat-Set Inserts in 3D Printed Parts: A Practical Guide covers it end to end.