Hole sizes for heat-set inserts

Metric inserts, M2 to M10

All sizes in millimetres. L is the insert length, Ø its outside diameter over the knurl, and the hole column is the diameter each maker recommends for its own insert. Rows marked like this are the sizes where the two makers recommend different holes.

Blind holes: at least the insert length plus 1 mm deep, for both makers. Through holes need no extra length.

Inch inserts, #2-56 to 3/8"-16

Same columns, still in millimetres, because both makers publish their inch inserts in millimetres. The 1/4"-20 is the camera thread and the 3/8"-16 the heavy tripod thread.

Where the two makers differ

• From M4 to M8, CNC Kitchen’s hole is 0.1 mm larger. 5.6 against 5.7 mm at M4, 6.4 against 6.5 at M5, 8.0 against 8.1 at M6, and 9.6 against 9.7 at M8, where the inserts themselves also differ by 0.1 mm. At M10 they agree again, at 12.0. The inch sizes follow the same pattern from #8-32 up.
• The same name can mean a different insert. An M2 is 4.0 mm long from ruthex and 3.0 mm from CNC Kitchen, an M2.5 is 5.7 against 4.0 mm, and a short M3 is 4.0 against 3.0 mm. The hole diameter is the same; the hole depth is not.
• The Voron M3 insert (5 mm across, 4 mm long) takes a 4.4 mm hole from both, but ruthex asks for at least 1.3 mm of wall around it and CNC Kitchen for 1.6 mm.

A tenth of a millimetre is smaller than the error in most printed holes, which is the next section’s point. Use the numbers for the inserts you actually bought, then adjust for your printer.

These are finished sizes, not CAD sizes

Both tables give the hole the insert should go into, not the diameter to draw. CNC Kitchen says so directly:

Our numbers define the nominal hole size for best results and you as a user needs to make sure you get as close as you can either by measuring holes or using our test parts.

CNC Kitchen, “Are our heat-set insert datasheets wrong?”

In the same article, Stefan Hermann measured his own printed holes and found them about 0.25 mm smaller than drawn. For an M3 insert he settled on 4.2 mm in CAD against the table’s 4.0:

For the as-printed samples 4.2 mm in CAD is the sweet spot: the insert can be pre-seated, no burr forms underneath, and the strength is still about 90% of the maximum.

Typically, for smaller inserts, I simply add about 0.2 to 0.3 mm to my diameters.

ruthex takes the other route and sells a drill set to bring a printed hole back to size, because in its words “the holes designed in advance are for the threaded inserts contract again due to the heat. With the help of the drill these can simply be improved.”

So there are two honest ways to get there: draw the hole a few tenths larger than the table and test, or draw it at the table size and drill it out. Either way it takes one test print per printer and material, because every combination shrinks a hole differently.

Installing them

• Straight holes. CNC Kitchen: “Holes should be straight (not tapered).”
• Depth. Both makers: a blind hole at least 1 mm longer than the insert. A through hole needs no extra length.
• Wall. Keep at least the minimum wall in the tables between the hole and the outside of the part, or the boss can split as the insert goes in.
• Iron temperature. CNC Kitchen: “Set your soldering iron 10 to 20 °C above your printing temperature: around 225 °C for PLA, 245 °C for PETG and 265 °C for ABS.” ruthex: “The ideal temperature for insertion is 10-15 degrees above the hotend temperature of the 3D printing filament.”
• Technique. CNC Kitchen recommends melting the insert only about 90% of the way in, then pressing it flush with a flat tool and holding it while the plastic cools. Their full guide covers the rest.

Where these numbers came from

Every number in both tables was read on September 29, 2026 from the two makers’ own published drawings: ruthex’s size table from its product pages, and page 2 of CNC Kitchen’s dimensions and design guidelines poster. Nothing was copied from another chart.

Two gaps are left open rather than filled in. CNC Kitchen sells newer short M6 and M8 inserts that are not on its poster yet, so those cells say “not listed”. And the tables cover only these two makers. Other brands size their inserts and holes to their own knurl, so for anything else, use that maker’s drawing.

What Supra does with this

Supra is an AI CAD tool that runs engineering checks on the parts it builds and shows the result of each one. A heat-set insert is one of the places where a naive check gets it wrong: the insert is meant to overlap the plastic, so a rule that says parts must never overlap fails a correct design. When an assembly says a part is held by a heat-set insert, Supra treats that overlap as intended, and checks instead that the two actually engage and that the overlap stays under 2% of the smaller part’s volume.

Its built-in parts library carries a generic M2 to M5 insert rather than either maker’s exact one, and says so: confirm against the inserts in your hand. That is what the tables above are for.

See what Supra is. Or look up clearance holes for metric bolts.