For the common M3 brass insert (4.6 mm knurl, 5.7 mm long), model a 4.0 mm hole at least 6.7 mm deep, with at least 1.6 mm of plastic around it. Set the soldering iron 10 to 20 °C above your printing temperature to start.

This is a one-page, printable reference to keep next to the printer. It lists the hole, depth, minimum wall and full-strength boss for straight (Ruthex / CNC Kitchen style) metric inserts and tapered (PEM-style) inch inserts, and a starting iron temperature for each filament. To work out the hole for any other insert from your own caliper measurements, or to add your printer’s hole shrinkage, use the Heat-Set Insert Hole Size calculator.

Metric Straight Inserts (M2 to M8)

InsertKnurl OD (mm)Length (mm)Hole Ø (mm)Min. depth (mm)Min. wall (mm)Full-strength boss OD (mm)Screw clearance in mating part (mm)
M2 x 0.43.64.03.25.01.3≈ 3 perimeters7.22.2
M2.5 x 0.454.65.74.06.71.6≈ 4 perimeters9.22.7
M3 x 0.54.65.74.06.71.6≈ 4 perimeters9.23.2
M3 x 0.5 (short)common size; check maker4.63.04.04.01.6≈ 4 perimeters9.23.2
M4 x 0.76.38.15.69.12.1≈ 5 perimeters12.64.3
M5 x 0.87.19.56.410.52.6≈ 6 perimeters14.25.3
M6 x 18.712.78.013.73.3≈ 8 perimeters17.46.4
M6 x 1 (short)8.76.88.07.83.3≈ 8 perimeters17.46.4
M8 x 1.2510.112.79.613.74.5≈ 10 perimeters20.28.4

Straight knurled inserts in the widely published Ruthex / CNC Kitchen sizes. Depth = insert length + 1 mm for the displaced plastic. Perimeters are counted at a 0.45 mm line width. Boss = 2 × knurl OD for full pull-out strength. Clearance is the ISO 273 close fit. Other brands differ: check the datasheet or measure the insert.

Inch Tapered Inserts (#2-56 to 1/4-20)

ThreadInsert OD (in)Length short / long (in)Hole Ø at top (in)Hole Ø at bottom (in)Min. depth short / long (in)Boss OD (in)Screw clearance (in)
#2-560.1413.58 mm0.115 / 0.1882.9 / 4.8 mm0.1233.12 mm0.1072.72 mm (long)0.155 / 0.2283.9 / 5.8 mm0.2827.2 mm0.0890#43 drill, 2.26 mm
#4-400.1724.37 mm0.135 / 0.2193.4 / 5.6 mm0.1594.04 mm0.1413.58 mm (long)0.175 / 0.2594.4 / 6.6 mm0.3448.7 mm0.1160#32 drill, 2.95 mm
#6-320.2195.56 mm0.150 / 0.2503.8 / 6.3 mm0.2065.23 mm0.1854.70 mm (long)0.190 / 0.2904.8 / 7.4 mm0.43811.1 mm0.1440#27 drill, 3.66 mm
#8-320.2506.35 mm0.185 / 0.3124.7 / 7.9 mm0.2345.94 mm0.2085.28 mm (long)0.225 / 0.3525.7 / 8.9 mm0.50012.7 mm0.1695#18 drill, 4.31 mm
#10-240.2977.54 mm0.225 / 0.3755.7 / 9.5 mm0.2777.04 mm0.2466.25 mm (long)0.265 / 0.4156.7 / 10.5 mm0.59415.1 mm0.1960#9 drill, 4.98 mm
#10-320.2977.54 mm0.225 / 0.3755.7 / 9.5 mm0.2777.04 mm0.2466.25 mm (long)0.265 / 0.4156.7 / 10.5 mm0.59415.1 mm0.1960#9 drill, 4.98 mm
1/4-200.3759.52 mm0.300 / 0.5007.6 / 12.7 mm0.3639.22 mm0.3218.15 mm (long)0.340 / 0.5408.6 / 13.7 mm0.75019.0 mm0.2570F drill, 6.53 mm
1/4-280.3759.52 mm0.300 / 0.5007.6 / 12.7 mm0.3639.22 mm0.3218.15 mm (long)0.340 / 0.5408.6 / 13.7 mm0.75019.0 mm0.2570F drill, 6.53 mm

Tapered inserts (PEM SI type, the style many US suppliers sell). The hole has an 8° included taper; if your CAD or slicer makes a taper awkward, a straight hole at the bottom diameter works with slightly more force to install. Clearance is the close-fit drill.

Soldering Iron Temperature by Filament

FilamentTypical print temp (°C)Iron: start at (°C)Do not exceed (°C)Notes
PLA / PLA+190–220215–230about 245Softens easily: too hot and the hole turns liquid and the insert sinks crooked.
PETG220–250240–260about 270Stringy when hot; wipe the tip between inserts.
ABS / ASA230–260250–270about 285Forms a good collar around the knurl; ventilate, it smells.
Nylon (PA)240–270260–280about 290Dry parts first. Nylon grips inserts very well.
Polycarbonate (PC)260–300280–300about 320Needs a hot iron and patience; let it sink slowly.
CF-filled gradesas baseas base polymeras baseFibres make the plastic less sticky: knurl grip is good but walls are more brittle. Add a perimeter.
TPU (flexible)210–235Not recommended–The soft plastic cannot hold the knurl under torque. Use a captive nut or a through-bolt instead.

Start values follow the common rule of print temperature + 10–20 °C (insert makers and CNC Kitchen give similar advice). Some people run hotter, up to about +40 °C, for speed; going hotter makes it easier to sink the insert too far or tilt it. Every iron reads differently, so test on a scrap print.

How to Use This Chart

  • Find your insert, then model the hole. Use the hole diameter and minimum depth from the table. If your printer makes holes undersize (most do, by 0.1–0.3 mm), add that on, or measure it with 3D Print Hole Compensation.
  • Check the wall. Set enough perimeters (walls) to cover the minimum wall thickness; infill around an insert adds very little. The Print Strength vs Infill calculator shows why walls matter more.
  • Use the boss figure for loaded joints. Twice the knurl diameter gives close to the full pull-out and torque strength. The minimum wall is fine for light-duty covers.
  • Insert from the top surface if you can, so the insert axis is vertical when printing and the hole is round.
Worked example: an M4 insert into a PETG bracket. The chart gives a 5.6 mm hole, 9.1 mm deep, with at least 2.1 mm of wall (5 perimeters at 0.45 mm). Your printer makes holes 0.15 mm small, so model the hole at 5.75 mm. Set the iron to about 245 °C, let the insert sink under light pressure, stop just short of flush, then press it flush with a flat cold block.

Installing tips

  • Use a dedicated insert tip that matches the thread size so the insert goes in straight.
  • Let the heat do the work: if you need to push hard, the iron is too cold. If the plastic bubbles or the insert drops in, it is too hot.
  • Leave the bottom of the hole open or deep enough for the plastic the knurl pushes down.
  • Screws into the insert should not bottom out: allow at least 1 mm below the screw tip.

Frequently Asked Questions

For the common M3 insert with a 4.6 mm knurl and 5.7 mm length, model a 4.0 mm hole at least 6.7 mm deep, with at least 1.6 mm of wall around it. Short M3 inserts (about 3 mm long) use the same hole at about 4 mm deep. Other makers’ inserts differ, so check the datasheet.

Start about 10–20 °C above the temperature you printed at: around 215–230 °C for PLA, 240–260 °C for PETG and 250–270 °C for ABS or ASA. Raise it a little if you have to push hard; lower it if the plastic goes runny.

As a minimum, about 0.35 times the insert’s knurl diameter (1.6 mm for an M3 insert). For full strength, make the boss around twice the knurl diameter, so about 9.2 mm across for an M3 insert.

Not reliably. TPU is too soft to hold the knurl when you tighten a screw. Use a captive hex nut pocket or a bolt right through the part instead.

No. Use the table for the insert you actually have. Inch tapered inserts and metric straight inserts have different shapes, so the hole sizes are not interchangeable even for similar thread sizes.

Metric straight-insert sizes are the widely published Ruthex / CNC Kitchen table; inch tapered sizes are from the PEM SI heat-set insert bulletin; clearance holes from ISO 273 and common close-fit drills. These are the same data used by our Heat-Set Insert Hole Size calculator. Insert makers’ dimensions vary, so check your insert’s datasheet. Iron temperatures are starting points only.