Written by Logan F.
Quick answer: Model the hole at the insert's listed hole diameter (4.0 mm for OzFDM M3 Short and Long inserts) plus about 0.2 mm, because printed holes shrink, and make blind holes 1 mm deeper than the insert. Set your soldering iron about 10°C above your printing temperature, press the insert in straight, then push it flush with a flat tool.
Screwing straight into a 3D printed part works once or twice, then the plastic thread wears out. A heat-set insert fixes that: a soldering iron melts a knurled brass insert into a printed hole, the plastic sets hard around the knurling, and you get a metal thread you can assemble and take apart many times.
This guide uses the listed dimensions of our Brass Threaded Heat Inserts (Knurled), 13 sizes from M2 to M8 plus 1/4 inch, with design guidance from CNC Kitchen's published insert testing. Where a number is a starting point to test rather than a product spec, we say so.
Why heat-set inserts beat printed threads
- Reusable threads: brass survives repeated assembly, so lids, access panels, fan mounts and electronics enclosures can be opened and closed without stripping the plastic.
- Load spread over more plastic: the knurled body locks into a larger area of the part than a thread cut into the plastic, making the joint stronger and more tolerant of a screw tightened hard.
- Small threads are hard to print: a standard ISO metric coarse M3 thread has a 0.5 mm pitch, only a little wider than the 0.4 mm line a standard nozzle lays down, so printed M3 threads are hard to get clean and strong.
- Works with most filaments: our inserts suit PLA, PETG, ABS, nylon and almost all other FDM plastics.
For scale, CNC Kitchen's own M3 inserts pulled out of PLA test blocks at around 1,400 newtons on average in its 2026 datasheet re-test. Other inserts and prints will differ, but a small insert carries a lot when the hole is right.
If you would rather skip inserts, thread-forming screws cut their own thread in a printed hole with no insert needed.
Choosing the right insert size and length
Match the insert to the screw: an M3 insert takes a standard M3 machine screw. As a rough guide:
- M2 and M2.5: electronics, PCB mounts and compact parts.
- M3: the size most printer parts and maker projects call for.
- M4 and M5: brackets, mounts, structural joints and aluminium extrusion builds.
- M6, M8 and 1/4 inch: larger parts with room for a bigger, deeper hole.
Short inserts suit thin walls and shallow bosses. Long inserts give more thread engagement where there is room for a deeper hole. The M3 Thick is wider than the other M3 options and 4 mm long, a common size in printer builds. Choose a screw that engages the full insert without bottoming out in a blind hole.
Heat-set insert sizes and hole dimensions
Length, outer diameter and hole diameter are the listed specifications for our inserts. The last two columns are starting points to test, based on the CNC Kitchen guidance below.
| Size | Length | Outer Ø | Hole Ø (OzFDM spec) | First test Ø in CAD, printed hole | Minimum blind hole depth |
|---|---|---|---|---|---|
| M2 Long | 3 mm | 3.5 mm | 3.1 mm | 3.3 mm | 4 mm |
| M2.5 Long | 4 mm | 4.5 mm | 4.0 mm | 4.2 mm | 5 mm |
| M3 Short | 3 mm | 4.5 mm | 4.0 mm | 4.2 mm | 4 mm |
| M3 Thick | 4 mm | 5.0 mm | 4.4 mm | 4.6 mm | 5 mm |
| M3 Long | 5.7 mm | 4.6 mm | 4.0 mm | 4.2 mm | 6.7 mm |
| M4 Short | 4 mm | 6.0 mm | 5.5 mm | 5.7 mm | 5 mm |
| M4 Long | 8.1 mm | 6.3 mm | 5.5 mm | 5.7 mm | 9.1 mm |
| M5 Short | 6 mm | 7.0 mm | 6.2 mm | 6.4 mm | 7 mm |
| M5 Long | 9.5 mm | 7.0 mm | 6.2 mm | 6.4 mm | 10.5 mm |
| M6 Short | 8 mm | 8.0 mm | 7.1 mm | 7.3 mm | 9 mm |
| M6 Long | 12 mm | 8.0 mm | 7.1 mm | 7.3 mm | 13 mm |
| M8 Long | 12 mm | 10.0 mm | 9.1 mm | 9.3 mm | 13 mm |
| 1/4 Long | 12.7 mm | 8.0 mm | 7.3 mm | 7.5 mm | 13.7 mm |
Designing the hole
Diameter
Printed holes almost always come out smaller than the CAD model. In its datasheet re-test, CNC Kitchen measured printed holes about 0.25 mm under the designed size. Its M3 insert suits a hole that actually measures 4.0 mm, which meant 4.2 mm in CAD for holes used straight off the printer. At that size the insert sits in the hole before heating, no burr forms underneath, and strength was still about 90% of the maximum. For smaller inserts, CNC Kitchen adds about 0.2 to 0.3 mm in CAD.
Our "first test" column adds 0.2 mm to the listed hole. Printers and materials differ, and that allowance was for smaller inserts, so print a test strip of holes 0.1 mm apart first, especially for M6, M8 and 1/4 inch. Our guide to tolerances and fit explains why holes print undersize.
Too small, and plastic gets pushed ahead of the insert and screws bind at the bottom of the thread. Too big, and less plastic fills the knurling, so pull-out strength drops. Keep holes straight, not tapered. Holes on a vertical face can print a different size from upright holes, so test in the orientation you will use.
Depth and chamfer
Make blind holes about 1 mm deeper than the insert, as CNC Kitchen recommends, so displaced plastic has somewhere to go and the insert can finish flush. Normally no chamfer is needed at the hole edge.
Wall thickness
CNC Kitchen stresses keeping to a minimum wall thickness around the insert. The knurling only grips solid plastic, so model a boss rather than setting an insert into a thin wall, and add wall loops (perimeters) in the slicer so the plastic around the hole is solid rather than infill. If a boss bulges or cracks as the insert goes in, thicken it. Our DFM basics guide covers wall thickness and orientation in more detail.
What you need to install heat-set inserts
- A temperature-controlled soldering iron, so you can set a specific temperature.
- An insert tip. A tip with a centring pin that sits in the insert's thread holds it square, so it goes in straight rather than at an angle. Our Soldering Iron Adapter Kit for Plastic Heat Inserts has brass adapters for M2 to M8 and suits most irons; check the sleeve sizes on the product page (3.85 mm inside, 6.6 mm outside diameter) against yours.
- A flat tool such as a screwdriver or tweezers.
Soldering iron temperature by material
Our insert guidance is the filament's printing temperature plus 10°C. CNC Kitchen's rule is 10 to 20°C above printing temperature, with these typical settings:
| Material | Soldering iron starting point | Source |
|---|---|---|
| PLA | About 225°C | CNC Kitchen |
| PETG | About 245°C | CNC Kitchen |
| ABS | About 265°C | CNC Kitchen |
| Other filaments (PLA+, ASA, nylon and so on) | Printing temperature from the product page or data sheet, plus 10°C | OzFDM insert guidance |
M5 and larger inserts hold more heat, so CNC Kitchen suggests a thicker tip and slightly higher temperature for them.
How to install a heat-set insert, step by step
- Check the fit. At the right diameter, the insert sits in the top of the hole on its own, smaller end down if it has one.
- Heat the iron. Fit the insert tip, set the temperature from the table and let it fully heat up.
- Locate the tip in the insert's thread, with the iron vertical and the part flat on a heat-safe surface.
- Let the heat do the work. Give the brass a few seconds, then apply light, steady pressure. If you have to push hard, the iron is too cool or the hole too small.
- Stop about 90% of the way in and lift the iron straight out.
- Press it flush with a flat tool and hold for a few seconds until the plastic sets.
- Let the part cool before tightening a screw into it.
- Test with a screw. It should run the full length freely. If it tightens near the end, a burr has formed under the insert.
The tip runs at over 200°C and a new insert stays hot for a while, so use the iron's stand and leave the brass to cool before touching it.
Common mistakes and how to fix them
| Problem | Likely cause | Fix |
|---|---|---|
| Screw binds near the end of the insert | Plastic burr pushed ahead of the insert, usually a hole too small | Enlarge the hole slightly and give blind holes 1 mm extra depth. Trim a burr off the back of a through hole. |
| Insert goes in crooked | Iron tilted, or tip not located in the insert | Use a centring tip and keep the iron vertical. Reheat and press square. |
| Insert sunk below the surface | Pushed all the way with the iron | Stop at about 90% and press flush with a flat tool. |
| Ring of plastic raised around the insert | Hole too small or too shallow | Open up or deepen the hole. Trim the ring with a hobby knife. |
| Insert spins or pulls out | Hole too big, thin walls, or loaded before cooling | Bring the hole back towards the listed size, add wall loops, and let it cool before tightening. |
Removing and re-seating an insert
To straighten or re-seat an insert, put the hot tip back in, let the plastic soften, then press it square or flush with a flat tool and hold until it sets.
To remove one, CNC Kitchen's removal method is to screw a screw into the insert for at least its full length, heat it with the iron about 20°C above the usual printing temperature (240°C for PLA), and pull gently once the plastic softens, about 10 seconds for PLA. Removed inserts can often be reused, but not from a PLA print into a PETG part, as leftover plastic can weaken the joint. If a replacement insert feels loose in the reshaped hole, reprint the part.
Bulk pricing on heat-set inserts
Our inserts are sold individually, so you only buy what you need. Buy 100 or more of one size (every size, including M6 Short) for 10% off, or 1,000 or more in any mix of sizes for 20% off.
| Size | Price each | 100 of one size at 10% off |
|---|---|---|
| M2 Long, M2.5 Long | $0.20 | $18.00 |
| M3 Short, M3 Thick, M3 Long | $0.25 | $22.50 |
| M4 Short, M4 Long | $0.30 | $27.00 |
| M5 Short, M5 Long | $0.35 | $31.50 |
| M6 Short, M6 Long | $0.55 | $49.50 |
| M8 Long | $0.60 | $54.00 |
| 1/4 Long | $0.70 | $63.00 |
Prices in AUD as listed in September 2026; check the product page for current pricing.
Heat-set insert kits and packs
Not sure which sizes you will use? The Heat Set Insert Starter Kit has 120 inserts in eight sizes from M2 to M5 for $28.95, against $32.50 for the same inserts bought individually. The M3 and M5 100-piece packs split 50 short and 50 long ($22.95 and $31.95). Browse all options in Knurled Brass Inserts and matching screw kits in Starter Kits.
Shop the materials in this guide
- Brass Threaded Heat Inserts (Knurled) - Metric: every size from M2 to M8 plus 1/4 inch, sold individually.
- Heat Set Insert Starter Kit (M2 to M5, 120 pcs): the most used sizes to design around.
- M3 Heat Set Insert Pack (100 pcs): short and long M3 inserts for printer mods and enclosures.
- Soldering Iron Adapter Kit for Plastic Heat Inserts: brass adapters that keep inserts square.
- M3 Fastener Starter Kit (140 pcs): stainless M3 screws and nuts to match.
Frequently asked questions
What size hole do I need for an M3 heat-set insert?
Our M3 Short and Long inserts list a 4.0 mm hole and the M3 Thick 4.4 mm. Printed holes come out small, so start at about 4.2 mm (4.6 mm for the Thick) in CAD and adjust after a test print.
What temperature should my soldering iron be for heat-set inserts?
Your filament's printing temperature plus 10°C. CNC Kitchen's typical settings are about 225°C for PLA, 245°C for PETG and 265°C for ABS.
Can I use heat-set inserts in resin prints?
Resin does not melt like FDM filament, so the insert cannot be heat-set. CNC Kitchen recommends gluing inserts into resin prints with thick CA glue or two-part epoxy.
Can I reuse a heat-set insert?
Often, yes. Inserts removed with heat usually come out clean. Avoid moving an insert from a PLA print into a PETG part.
