Clearance hole and heat-set insert hole chart

Metric clearance holes: the full ISO 273 table

ISO 273 gives three clearance holes for every metric screw. Fine (H12) is for holes that also locate the part; medium (H13) is the usual series and the one most charts mean when they give one value; coarse (H14) leaves room for misalignment. The standard dates from 1979 and was last confirmed in 2024, unchanged; Europe adopted it as EN 20273 (DIN EN 20273 in Germany).

The values below are read from IS 1821:1987, the Indian Standard that is identical to ISO 273, and agree on every row with the German Wikipedia table and Albany County Fasteners' chart. Some charts online give M12 as 13 / 14 / 15 and M16 as 17 / 18 / 19 — those are not ISO 273.

Printed holes: how much to add

A hole printed exactly at the ISO size usually comes out small. nophead's Polyholes post lists why: the STL's flat facets sit inside the circle, and the nozzle cuts the corner on every small curve. How much depends on the printer, the filament and whether the hole is vertical — so no standard gives a number.

What the sources do say: creative3dp's press-fit calculator puts printed holes about 0.2 mm small; Bambu Lab's own uncompensated M6 test hole printed 5.9; slicers ship their hole compensation at 0 and tell you to calibrate per filament. So the allowance here starts at 0.2 mm and is yours to change. Print one hole, try the screw, and set what you measured. If you would rather not calibrate, use the coarse series: it is already 0.2 (M3) to 0.4 mm (M6) larger than medium.

Heat-set insert holes from the makers' own tables

Heat-set inserts go into a straight hole a little smaller than their knurl; the iron melts the plastic around them. Each maker publishes its hole, and they do not all agree: CNC Kitchen's current table (April 2026) gives M4 5.7, M5 6.5 and M6 8.1, while ruthex — and older copies of CNC Kitchen's table still on some reseller pages — give 5.6, 6.4 and 8.0. PEM's industrial IUTB inserts have the same size and specify 5.67, 6.43 and 8.03 mm, +0.08. Use the table of the insert you bought.

Depth: at least the insert length plus 1 mm for a blind hole (CNC Kitchen, ruthex), so the insert never bottoms out; SPIROL's rule for industrial parts is the length plus two thread pitches. Tapered inserts are different parts: they need an 8° tapered hole (McMaster, PEM), never a straight one.

Worked example: a lid screwed onto an M3 insert

Base: a CNC Kitchen M3 × 5.7 insert needs a Ø4.0 hole at least 6.7 mm deep and 1.6 mm of wall — the Forgent3D insert boss makes it Ø7 outside (the insert's 4.6 knurl plus 1.2 mm a side). Lid: the M3 screw passes through an ISO 273 medium hole, Ø3.4, modelled at 3.6 with the 0.2 allowance.

One more check from SPIROL: the lid's hole must be bigger than the screw but smaller than the insert's pilot end (3.9 for this insert), or the screw can pull the insert out. 3.6 passes.

Installing them

• - Iron temperature: 10–20 °C above your printing temperature (CNC Kitchen: about 225 °C for PLA, 245 °C for PETG, 265 °C for ABS); ruthex says 10–15 °C above.
• - Press it about 90 % of the way with the iron, then push it flush with a flat tool and hold a few seconds while the plastic sets (CNC Kitchen).
• - Print the hole vertical where you can: it comes out round and the insert seats square.
• - In resin prints, don't melt: glue the insert with thick CA (CNC Kitchen).

FAQ

Sources

Every number on this page comes from these documents (the version or the date we read them is given).

1. ISO 273:1979 Fasteners — Clearance holes for bolts and screws (iso.org: confirmed 2024, current) — adopted as EN 20273 / DIN EN 20273; fine H12, medium H13, coarse H14
2. IS 1821:1987, identical with ISO 273:1979 (Bureau of Indian Standards, public copy) — the table the clearance values are read from
3. Wikipedia (de): Durchgangsbohrung — same values, M1–M24; medium is the preferred series
4. Albany County Fasteners, Clearance Hole Chart — Metric Series (ISO 273) — same values; close / normal / loose
5. CNC Kitchen, heat-set inserts — Dimensions & Design Guidelines, metric (store table of 2026-04-10) — L, insert Ø, hole Ø, minimum wall; blind hole ≥ L + 1 mm, straight
6. CNC Kitchen, Tips & Tricks for Heat-Set Inserts (2021-10-01) — straight holes, ~1 mm deeper, iron 10–20 °C above print temperature, melt ~90 % then press flush
7. ruthex threaded inserts — size table (2025-05-27) — d1 / d2 / d3 hole / L / min. wall; hole depth L + 1 mm min
8. PEM SI® Threaded Inserts for Plastics bulletin, © 2012 PennEngineering (copy hosted by LulzBot) — IUTB straight inserts: hole +0.08 / −0, minimum depth; wall ½–1 × insert Ø, boss 2 × insert Ø. M3 checked on the current pemnet.com catalogue
9. McMaster-Carr, Heat-Set Threaded Inserts for Plastic (metric, brass; read 2026-10-20) — “for max. hole dia.”; minimum material thickness on 94459A140 (M3) and 94459A220 (M6)
10. SPIROL white paper, How to Design the Proper Hole for Heat / Ultrasonic Inserts (© 2022) — depth ≥ insert length + 2 pitches; mating hole larger than the screw, smaller than the insert's pilot
11. nophead (Chris Palmer), Polyholes, HydraRaptor blog (Feb 2011) — why printed holes come out small; n = max(round(2d), 3) sides, r = (d/2) / cos(180°/n)
12. Bambu Lab Wiki, XY Hole / Contour compensation (edited 2026-05-08) — compensation ships at 0; their uncompensated M6 test hole printed 5.9; calibrate per filament
13. creative3dp press-fit calculator — printed holes come out about 0.2 mm small

Make the part