Heat Set Insert Size Guide: How to Choose the Right Thread Size for Your Project

by Initeq April 10, 2026

Brass heat set inserts let you add strong, reusable machine-screw threads to 3D-printed parts, injection-molded plastics, and other thermoplastic materials. But with over a dozen thread sizes on the market — from tiny M1.6 inserts up to beefy 1/4"-20s — picking the right one for your project isn't always obvious.

This guide covers everything you need to know: how heat set inserts work, which thread size to choose, the difference between Short and Long styles, recommended hole sizes, and tips for getting clean, strong installations every time.

What Are Heat Set Inserts?

A heat set insert is a small, knurled brass cylinder with internal machine-screw threads. You press it into a pre-drilled or printed hole using a soldering iron (or a dedicated heat-press tool), and the heat softens the surrounding plastic just enough for the insert's knurled exterior to melt into the material and lock in place.

Once cooled, you have a permanent metal thread embedded in your part. It's far stronger and more reliable than threading a screw directly into plastic, and unlike self-tapping screws, inserts allow repeated assembly and disassembly without wearing out the hole.

Common applications include:

3D-printed enclosures, jigs, and fixtures
Prototyping and small-batch production
Attaching panels, lids, brackets, and PCBs to printed housings
Robotics, drones, and CNC machine assemblies
Any project where a printed part needs to bolt together with standard hardware

Metric vs. Imperial: Which System Should You Use?

This depends almost entirely on what hardware you already have — or what hardware the rest of your project uses.

Choose Metric (M2, M2.5, M3, M4, M5, M6, M10) if:

You're working with a metric 3D printer (most consumer machines use metric fasteners)
Your design files specify metric screws
You're outside the US, or your project follows ISO standards

Choose Imperial/SAE (#2-56, #4-40, #6-32, #8-32, #10-24, #10-32, 1/4"-20, 1/2"-28) if:

You're building something that interfaces with US-standard hardware
Your screws are already SAE/UNC thread
You're working on automotive, woodworking, or US industrial applications

The most popular sizes for 3D printing are M3 (metric) and #4-40 (imperial). These are the sizes you'll see recommended in most Voron, Prusa, and functional-print build guides. If you're starting a new project from scratch and don't have a specific constraint, M3 is a safe default — M3 screws and hex drivers are widely available and hit a sweet spot between strength and compactness.

Thread Size Quick-Reference Table

Here's a summary of every Durathread insert size we carry, organized from smallest to largest. The "Best For" column gives you a starting point — but any size will work in any thermoplastic material that softens with heat.

Thread Size	System	Best For
M1.6	Metric	Tiny enclosures, watch-scale projects, delicate electronics housings
#2-56	Imperial	Miniature assemblies, PCB standoffs, small sensor mounts
M2.5	Metric	Raspberry Pi mounting holes, small electronics enclosures
M3×0.5	Metric	Most popular metric size. Voron builds, functional prints, general purpose
#4-40	Imperial	Most popular imperial size. General purpose, enclosures, brackets
M4×0.7	Metric	Medium-duty brackets, larger enclosures, panel mounts
#6-32	Imperial	Mid-size assemblies, rack panels, general mechanical
M5×0.8	Metric	Structural joints, heavy brackets, CNC fixtures
#8-32	Imperial	Heavier mechanical assemblies, larger fixtures
M6×1.0	Metric	Structural applications, furniture-scale, heavy loads
#10-24	Imperial	Heavy-duty brackets, jigs, large enclosures
#10-32	Imperial	Same as #10-24 but fine thread for precision adjustment
1/4"-20	Imperial	Maximum strength, large structural parts, heavy assemblies
M10×1.5	Metric	Industrial scale, heavy structural, large functional prints
1/2"-28	Imperial	Specialty applications, large assemblies, firearms barrel devices

Short vs. Long: What's the Difference?

Every Durathread insert comes in two styles:

Short — a lower-profile insert designed for thinner walls and shallow bosses. Use these when your wall thickness is limited, or when you don't need maximum pull-out strength.

Long — a taller insert that engages more material, providing higher pull-out resistance. Use these for structural joints, parts under repeated load, or anywhere you have room for a deeper boss.

Rule of thumb: If your wall thickness allows it, go Long. The extra thread engagement gives you a stronger joint with very little size penalty. If your wall is thin (under ~4mm for M3 or under ~6mm for 1/4"-20), go Short.

How to Install Heat Set Inserts

Installation is straightforward, but a few details make the difference between a clean flush insert and a crooked mess.

What you need:

A soldering iron with a temperature-controlled tip (recommended: 220–260°C / 430–500°F for PLA and PETG; lower end for PLA, higher end for ABS/Nylon)
A printed or drilled hole sized for your insert

Steps:

Print or drill the receiving hole to the correct diameter (you may need to test-fit and adjust by 0.1mm for your specific printer and filament)
Place the insert on the hole, threaded end up, knurled end facing the plastic
Heat your soldering iron to the target temperature for your material
Press the iron tip gently into the insert — let the heat do the work. Don't force it.
Push the insert down until it sits flush with (or just below) the surface
Remove the iron straight up and let the part cool for 30 seconds before handling

Common mistakes to avoid:

Too much heat melts excessive material and creates a sloppy, weak joint. Start low and increase if needed.
Rushing it pushes the insert crooked. Slow and steady gets a perpendicular result.
Undersized holes cause the plastic to bulge around the insert. If you see material pushing up around the rim, enlarge the hole by 0.1–0.2mm.
Inserting from the wrong end — the knurled barbs face outward (toward the plastic). The smooth internal threads face inward.

How Many Do I Need?

Durathread inserts are available in several pack sizes to match your project scale:

20-pack — great for a single project or prototype
50-pack — covers a multi-part build or a few iterations
100-pack — workshop stock for ongoing projects
1,000-pack — production runs, makerspaces, and volume users

Buying in bulk significantly reduces the per-unit cost. Our 1,000-packs are the best value for anyone who uses inserts regularly — a single 1,000-pack of 1/4"-20 inserts, for example, will stock a workshop for dozens of builds.

Tips for Getting the Best Results

Print orientation matters. If possible, orient your part so that the insert hole axis is vertical during printing. This gives you solid layer lines around the insert boss rather than bridging or overhangs, which improves pull-out strength.

Design your bosses with extra wall thickness. A good rule of thumb is to make the boss outer diameter at least 2× the insert's outer diameter. This gives the knurling plenty of material to grip.

Test-fit before committing. Print a small test coupon with 3–4 holes at slightly different diameters (e.g., 4.0mm, 4.1mm, 4.2mm for M3 inserts) and install one insert in each. This tells you the optimal hole size for your specific printer, filament, and slicer settings.

Still Not Sure Which Size?

If you're building a 3D-printed project and don't have a specific screw size in mind, start with M3×0.5 Short inserts. They're the most widely used size in the maker and 3D printing community, M3 screws are cheap and everywhere, and the Short style works in most standard wall thicknesses.

For imperial projects — especially anything that needs to interface with US-standard hardware — #4-40 Short is the equivalent go-to.

Browse the full Durathread line in our insert collection or check out our assortment packs if you want to try several sizes at once.

All Durathread inserts are machined from CDA 360 free-machining brass — the same alloy used in precision fittings and electrical connectors. Every order ships free from Georgia (USA).