Written by Logan F.
Quick answer: Start clearance holes at the ISO 273 medium sizes (M3 3.4 mm, M4 4.5 mm, M5 5.5 mm). Size hex nut traps from the nut's maximum across-flats plus 0.2–0.3 mm, and model countersinks at 90° to the screw's maximum head diameter. Printed holes shrink, so test these starting points first.
A screw hole that is 0.2 mm too small turns a five-minute assembly into a session with a drill. Most fit problems with printed parts come from modelling holes at the nominal screw size and forgetting that printed holes come out slightly undersized. This guide gives you the reference numbers for M2 to M8 screws and nuts from the ISO standards and our own product measurements, then explains how to turn them into clearance holes, nut traps and countersinks that work on a printer. Everything here matches the stainless screws, nuts and washers in our fasteners range.
Clearance holes: start with ISO 273
A clearance hole lets the screw pass through freely so it clamps the part rather than threading into it. ISO 273 defines three series of clearance holes for metric bolts and screws: fine, medium and coarse. Fine holes locate the screw most closely, while coarse holes leave the most room for misalignment.
| Thread | Fine | Medium | Coarse | Hex key (socket screws) |
|---|---|---|---|---|
| M2 | 2.2 mm | 2.4 mm | 2.6 mm | 1.3 mm |
| M2.5 | 2.7 mm | 2.9 mm | 3.1 mm | 1.5 mm |
| M3 | 3.2 mm | 3.4 mm | 3.6 mm | 2 mm |
| M4 | 4.3 mm | 4.5 mm | 4.8 mm | 2.5 mm |
| M5 | 5.3 mm | 5.5 mm | 5.8 mm | 3 mm |
| M6 | 6.4 mm | 6.6 mm | 7.0 mm | 4 mm |
| M8 | 8.4 mm | 9.0 mm | 10.0 mm | 5 mm |
Hex key sizes for M2, M3, M4, M5, M6 and M8 are from our countersunk hex head bolt listing. The M2.5 size is the ISO 10642 socket size.
Adjusting for a 3D printer
- Model at the medium size to start. Printed holes usually shrink. In CNC Kitchen's insert tests a hole had to be modelled at 4.1 mm to print at 4.0 mm in PLA. The medium series gives enough room to absorb that, while fine holes can print tight.
- Use coarse or a teardrop shape for holes printed on their side. A horizontal hole droops at the top. A pointed teardrop or flat-topped profile prints cleanly and still clears the screw.
- Watch the first layers. Holes that start on the build plate can be pinched by elephant's foot. A small chamfer on the bottom edge helps, and our elephant's foot guide covers the slicer fixes.
- Tune once in the slicer. OrcaSlicer's X-Y hole compensation grows or shrinks every hole by a set amount (positive values make holes bigger), as described in the OrcaSlicer precision settings. Dial it in with a test print and you do not need to edit every model.
Nut traps
A nut trap is a hexagonal or square pocket that holds a nut so you can tighten the screw from one side. Size it from the nut's across-flats (AF) dimension, the distance between two opposite flat faces, which is the number a spanner size refers to.
| Thread | ISO 4032 AF (max) | Our hex nut (AF × thickness) | Our thin nut (AF × thickness) | Starting pocket for our hex nut (AF × depth) |
|---|---|---|---|---|
| M2 | 4.0 mm | 3.9 × 1.6 mm | Not listed | 4.2–4.3 × 1.8 mm |
| M2.5 | 5.0 mm | Not listed (ISO max height 2.0 mm) | Not listed | 5.2–5.3 × 2.2 mm |
| M3 | 5.5 mm | 5.4 × 2.3 mm | 5.4 × 1.8 mm | 5.7–5.8 × 2.5 mm |
| M4 | 7.0 mm | 6.8 × 3.1 mm | 6.9 × 2.1 mm | 7.2–7.3 × 3.3 mm |
| M5 | 8.0 mm | 7.9 × 4.0 mm | 7.9 × 2.6 mm | 8.2–8.3 × 4.2 mm |
| M6 | 10.0 mm | 9.8 × 4.8 mm | 9.9 × 3.1 mm | 10.2–10.3 × 5.0 mm |
| M8 | 13.0 mm | 12.8 × 6.2 mm | 12.8 × 4.0 mm | 13.2–13.3 × 6.4 mm |
The ISO column is the maximum across-flats allowed by ISO 4032 for standard hex nuts. The "our nut" columns are the dimensions listed on our SS304 standard hex nuts and thin hex nuts. The starting pocket adds 0.2–0.3 mm to the ISO maximum and 0.2 mm to the nut thickness. Use the low end for a snug press and the high end for a nut that drops in. These allowances are starting points to test on your printer.
Modelling the hexagon in CAD
Most CAD polygon tools ask whether the diameter you type is inscribed (touching the flats) or circumscribed (touching the corners). Use inscribed with the AF value. If your tool only offers corners, multiply AF by 1.155: a 5.8 mm AF pocket for an M3 nut is about 6.7 mm across the corners.
Choosing the trap style
- Pocket on the far face. The nut sits in a hex recess in line with the screw and pulls against the whole face of the part. In CNC Kitchen's M3 tests in PETG, a nut in a bottom pocket averaged 166 kg of pull-out, the highest of every method tested.
- Side slot. The nut slides in from the side, which is handy when the far face is not reachable. The same test measured 86 kg for a side pocket, so expect roughly half the pull-out strength. Make the slot width equal to the pocket AF, with two flats parallel to the slot walls.
- Square nuts in slots. Square nuts cannot rotate in a rectangular slot and are easy to line up. Our galvanised square nuts are 5.5 × 2 mm (M3), 7 × 2 mm (M4), 8 × 3 mm (M5) and 10 × 3 mm (M6).
- Fully enclosed nuts. Pause the print, drop the nut in, and print over it. The pause steps are the same as in our guide to embedding magnets.
If a hex pocket sits below a smaller screw hole, the roof of the pocket has to bridge. A common fix is a single sacrificial layer across the hole at that height, which you clear with a drill or knife after printing. Add a washer under nuts and screw heads that bear on plastic. Our fastener kit pages note that washers spread clamping force so nuts and heads do not pull into softer printed plastic.
Countersinks and head types
Countersunk screws
Countersunk screws sit flush in a 90° cone. Model the countersink to the theoretical maximum head diameter so every screw in the tolerance range sits at or slightly below the surface.
| Thread | ISO 10642 head diameter (theoretical max) | Head height (max) | Starting countersink in CAD |
|---|---|---|---|
| M2 | 4.70 mm | 1.35 mm | 90°, 4.7 mm at the surface |
| M2.5 | 5.88 mm | 1.69 mm | 90°, 5.9 mm at the surface |
| M3 | 6.72 mm | 1.86 mm | 90°, 6.7 mm at the surface |
| M4 | 8.96 mm | 2.48 mm | 90°, 9.0 mm at the surface |
| M5 | 11.20 mm | 3.10 mm | 90°, 11.2 mm at the surface |
| M6 | 13.44 mm | 3.72 mm | 90°, 13.4 mm at the surface |
| M8 | 17.92 mm | 4.96 mm | 90°, 17.9 mm at the surface |
Real heads come in smaller than the theoretical figure, so they sit a fraction below flush. Phillips countersunk screws follow a different standard, so measure one with calipers before modelling. The cone also wedges outwards as you tighten, so keep countersinks away from thin edges and stop once the head seats.
Button head socket screws
Button heads have a low, rounded head that sits on the surface and a wide bearing face that suits printed plastic. ISO 7380 lists maximum head diameters of 5.7 mm (M3), 7.6 mm (M4), 9.5 mm (M5), 10.5 mm (M6) and 14 mm (M8), with head heights of 1.65, 2.2, 2.75, 3.3 and 4.4 mm. To recess one, start the counterbore at least 0.5 mm wider than the head and test.
Socket cap screws
Socket cap screws have a taller cylindrical head and a deeper hex socket, so they suit counterbored holes where the head needs to disappear below the surface. Measure the head of the screws you use and size the counterbore the same way as for a button head.
Choosing screw length
For a through bolt, add up the clamped thickness, any washers and the nut thickness, then pick the next length up so a thread or two shows past the nut. Our button head and countersunk hex head screws are listed in 4–40 mm lengths for M2 to M3, 6–50 mm for M4, 8–50 mm for M5 and M6, and 10–40 mm for M8.
Shop the materials in this guide
- M3 Fastener Starter Kit (140 pcs): countersunk and button head screws in five lengths, with hex and nyloc nuts
- M4 Fastener Starter Kit (160 pcs): M4 screws, nuts and washers for brackets and heavier parts
- M5 Fastener Starter Kit (160 pcs): M5 hardware for extrusion builds and structural joints
- Small Fastener Kit (M2 and M2.5, 160 pcs): small screws, nuts and washers for electronics and PCBs
- Nut and Washer Assortment (290 pcs): standard, thin and nyloc nuts plus washers from M2 to M6
- Individual Hex Key Wrench Ball-End: the right key for each socket size
Browse the full nuts, bolts, screws and washers range or the starter kits.
Frequently asked questions
What size hole should I model for an M3 screw?
Start with the ISO 273 medium size of 3.4 mm. Use 3.2 mm where the screw must locate the part closely, and 3.6 mm for long holes, holes printed on their side or parts that need some adjustment. Test on your printer before a big print.
Can I screw a machine screw straight into a printed hole?
For light, one-off joints, yes. In CNC Kitchen's M3 tests, screws driven directly into PLA and PETG stripped at about 1 Nm, while heat-set inserts held 3 Nm or more. Use inserts or a nut for joints you will tighten hard or open often.
Should I use standard or thin nuts in a nut trap?
Standard nuts give more thread engagement and suit loaded joints. Thin nuts fit shallow traps and tight spaces. Use nyloc nuts where vibration could loosen the joint.
My nut spins in its pocket. What went wrong?
The pocket is too large, often because the hexagon was modelled across the corners instead of across the flats. Check the CAD dimension, reduce the across-flats by 0.1 mm at a time, or switch to a square nut in a slot.
Why do my countersunk screws sit proud?
Usually the countersink printed small or the hole below it is tight. Model the countersink to the theoretical maximum head diameter, check the clearance hole, and look for elephant's foot if the countersink is on the bottom face.
