Heat-Set Insert & Screw Hole Size

A standard M3 brass heat-set insert (4.6 mm knurl, 5.7 mm long) needs a 4.0 mm hole about 6.7 mm deep. Look up typical sizes for M2 to M8 and #2-56 to 1/4-20 inserts below, or measure your own insert and get the hole size from the rule. Screw pilot, clearance and tap drill holes for printed parts are here too.

Published October 30, 2026

Hole for my insert

Units

Insert shape

OD over knurl (mm)

Length (mm)

Lead-in / body Ø (mm)

Line width (mm)

Measure the knurl at its widest with calipers. The lead-in is the plain, unknurled end that goes in first.

Hole Ø

Min hole depth

insert length + 1 mm

Boss Ø, full strength

2 × insert OD

Minimum wall

Insert and Screw Hole Tables

Search a size

Plastic (screw pilots)

Straight heat-set inserts (CNC Kitchen / Ruthex-style), mm

Thread	Knurl OD	Lead-in Ø	Length	Hole Ø	Min depth	Min wall	Boss Ø	Calc

Typical values from the widely published Ruthex table, the same size class as CNC Kitchen standard inserts. Min depth = length + 1 mm (rule). Boss Ø = 2 × knurl OD for full strength; the maker's minimum wall gives a smaller minimum boss. Rows marked typ are a common size not in that table.

Tapered heat-set inserts (PEM SI / McMaster-style)

Thread	Length	OD	Hole top Ø	Hole bottom Ø	Taper length	Min depth	Boss Ø

From the PEM SI bulletin for IUA/IUB/IUC tapered inserts (8° included taper): hole top Ø at the surface, bottom Ø at the end of the taper, then straight to the minimum depth. Two lengths per thread. Inch sizes in inches with mm below; metric in mm. Boss Ø = 2 × OD (PEM guideline).

Thread-forming screw pilot holes in printed plastic

Screw	Nominal Ø	Pilot hole Ø	Boss Ø	Min engagement

Clearance holes for printed parts

Screw	Close	Normal	Model at (typical)

Metric: ISO 273 close and medium series. Inch: close and free drill sizes, as in the Clearance Hole Chart. "Model at" adds a typical 0.2 mm for FDM holes printing small; find your own figure with the Hole & Fit Compensation tool.

Tap drills for tapping a printed hole

Thread	Tap drill	Ø

Standard tap drills (about 75% thread). See the Thread Size Chart for pitches and other sizes.

How to Size Holes for Heat-Set Inserts

Model the hole a little smaller than the insert's knurl, about the size of its plain lead-in, and at least 1 mm deeper than the insert. The hot insert melts its way in, the displaced plastic flows into the knurls and the extra depth gives it somewhere to go.

Insert sizes are not standard between makers. The tables give typical values for two common styles, but always check the datasheet for your inserts, or measure one and use the calculator.

The Rules Used

1. Straight insert hole

hole Ø = lead-in (body) Ø + 0.1 mm

no lead-in measured: hole Ø = knurl OD − 0.6 mm

The first rule reproduces every hole in the Ruthex table exactly. The second is within 0.2 mm of it for M2 to M8. The knurl should bite about 0.25 to 0.35 mm per side.

2. Tapered insert hole

top Ø = OD − 0.3 mm

bottom Ø = small-end Ø − 0.2 mm

taper length = (top − bottom) / (2 · tan 4°)

This matches the PEM table's top within 0.25 mm and bottom within 0.1 mm. An 8° included taper changes the diameter by 0.14 mm per mm of depth.

3. Depth and boss

min hole depth = insert length + 1 mm

boss Ø (full strength) = 2 × insert OD

min wall ≈ 0.35 × insert OD

PEM's own minimum depths are the insert length plus 1.0 mm (0.040 in), and PEM recommends a boss twice the insert diameter. The minimum wall follows the Ruthex table; thinner walls hold but crack or bulge more easily.

4. Thread-forming screw pilot

pilot Ø = factor × screw nominal Ø

PLA 0.85, PETG 0.80, ABS/ASA 0.80, nylon 0.75

boss Ø ≈ 2.5 d (nylon 2 d), engagement ≥ 2 d

Rules of thumb in line with thread-forming screw makers' guidance for moulded plastics (about 0.8 d for ABS, 0.75 d for nylon), with a larger pilot for brittle PLA.

Printed (Modelled) Threads

M6 and larger, or 1/4-20 and coarser print usably on a 0.4 mm nozzle with 0.1 to 0.15 mm layers and the thread axis vertical. Finer threads (M3 to M5) come out rough: use an insert, a thread-forming screw or tap the hole.
Add clearance. Offset a modelled internal thread outwards (or an external thread inwards) by about 0.1 to 0.2 mm per side as a typical start, so a printed bolt and nut, or a metal screw, turn freely. Many CAD thread tools have a "modelled" option with a clearance setting.
To tap a printed hole, model it at the tap drill size plus your printer's hole undersize (or drill it afterwards), give it 3 or more walls, and tap slowly so the plastic does not melt.
Chamfer the first thread about 0.5 mm so it starts cleanly and the first layer's elephant's foot does not close it up.

Worked Example

Measuring an unknown M3 insert

Calipers read 4.62 mm over the knurl, 3.95 mm on the plain lead-in, and 5.7 mm long.
Hole: 3.95 + 0.1 = 4.05 mm. The knurl bites (4.62 − 4.05) ÷ 2 = 0.29 mm per side.
Depth: 5.7 + 1 = 6.7 mm. Boss for full strength: 2 × 4.62 = 9.24 mm. Minimum wall 0.35 × 4.62 = 1.62 mm, which is 4 perimeters at 0.45 mm line width.
If your printer makes holes 0.15 mm small, model the hole at 4.20 mm.

A #4-40 tapered insert, long version

From the PEM table: length 0.219 in, hole top 0.159 in (4.04 mm), bottom 0.141 in (3.58 mm), taper 0.129 in long, minimum depth 0.259 in (6.58 mm), boss 2 × 0.172 = 0.344 in (8.74 mm).
Check the taper: 2 × tan 4° × 0.129 = 0.018 in, and 0.159 − 0.141 = 0.018 in.

Tips for Installing Heat-Set Inserts

Use the right tip and temperature. About 20 to 40 °C above the printing temperature of the plastic is a typical start (around 220 to 245 °C for PLA). Too hot and the plastic turns to liquid and pulls out with the iron.
Let it go in under its own weight, keep it square, then press it flush with a flat, cold object while it cools for a few seconds.
Print walls, not infill, around the hole. Set enough perimeters to cover the minimum wall. The Strength vs Infill Estimator shows why walls matter.
Mind the hole shrink. Printed holes are usually 0.1 to 0.3 mm small; the Hole & Fit Compensation tool measures yours. Large prints may also need shrinkage compensation.
Through holes for screws into the insert from the mating part use the clearance sizes above or the Clearance and Counterbore Hole Chart.

Frequently Asked Questions

For the common M3 insert with a 4.6 mm knurl and 5.7 mm length, sold by CNC Kitchen and Ruthex, the recommended hole is 4.0 mm, at least 6.7 mm deep, with a wall of at least 1.6 mm. Other makers' M3 inserts differ, so check the datasheet or measure the insert.

Measure the plain lead-in at the bottom of the insert and add 0.1 mm. If there is no lead-in, use the knurl diameter minus 0.6 mm. Make the hole at least 1 mm deeper than the insert, and the boss around it about twice the insert diameter.

It works best tapered, because the insert lines itself up before it melts in. Model the 8° included taper from the top diameter down to the bottom diameter, then straight to the minimum depth. A straight hole at about the bottom diameter also works in a print, but the insert is easier to tilt.

A typical start is 85% of the screw's nominal diameter in PLA, so about 2.55 mm for an M3 screw, and 80% in PETG or ABS. Give the screw at least twice its diameter of engagement and a boss about 2.5 times its diameter. Print a test, because printed holes come out a little small.

For M6 or 1/4-20 and larger, yes, with fine layers, the thread axis vertical and about 0.1 to 0.2 mm clearance per side. Smaller threads print rough and wear quickly, so a brass insert, a thread-forming screw or a tapped hole is better for M2 to M5.

Insert dimensions vary by maker and the values here are typical. Check your insert maker's datasheet and test in a scrap print before production.