# 3D-printed hole too tight for a bearing or screw: add clearance

> Fix printed holes that are too small: add clearance in the model, test with a calibration print, ream with a drill bit and print holes vertically where you can.

Canonical: https://roboticsengineering.in/fix/3d-print-bearing-hole-too-tight-tolerance/ | Published: 2026-11-03 | Updated: 2026-11-04

**Printed holes come out smaller than designed.** Add clearance in the model and print a test piece first.

## What the message means

Plastic spreads slightly at the edges and the printer rounds corners. A hole drawn at exactly the bearing size becomes a tight press fit.

## Fix it in this order

1. Print a small test with holes in steps of 0.1 mm to learn what your printer needs.
2. Add that clearance to every hole in the model.
3. Ream a tight hole with a drill bit of the right size, turned by hand.
4. Orient holes so they print round, for example axis vertical for small holes.
5. Use a slightly smaller clearance for press fits and a larger one for sliding fits.
6. Do not force a bearing in a hole that is too tight, since it can crack the part.

## Why it happens

Printed holes usually come out slightly undersized, often by a fraction of a millimetre, for two reasons: the extruded line swells at the edge, and the slicer approximates a circle with straight segments that cut inside the true curve. The exact amount depends on your printer, filament and settings, which is why a test piece with holes stepped by 0.1 mm gives a better answer than a guess.

Holes printed with the axis horizontal sag at the top because the roof has no support below it, so a teardrop shape or a vertical orientation prints rounder. Print the final part in the same orientation as the test.

## If you see this, do this

| You see | Do this |
| --- | --- |
| Bearing will not go in | Add clearance or ream. |
| Bearing is loose | Reduce the clearance. |

## Related

[part slips on the shaft](https://roboticsengineering.in/fix/3d-printed-wheel-or-gear-slips-on-motor-shaft/), [warping](https://roboticsengineering.in/fix/3d-print-warping-corners-lift-bed-adhesion/).

Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.

## Why this happens

A slicer approximates a circle with short straight segments, and each segment lies inside the true curve, so a hole is slightly smaller than drawn. On top of that the extruded line is slightly wider than its nominal width at the edge and the plastic pushes into the opening as it cools. The result is a hole that is commonly smaller than the model, by an amount that depends on your printer, nozzle, material and speed.

Orientation changes it too. A hole whose axis is horizontal has no support above the top of the bore, so that part sags, and the hole becomes slightly egg-shaped. Printing the axis vertical, or using a teardrop shape in the model, gives a rounder and more predictable bore.

## How to confirm the cause

- Measure the printed hole with a caliper or with drill bits of known size, and compare it with the value in the model. The difference is your clearance to add.
- Check the hole at both ends and in the middle. A different size at the first layer points to elephant foot or first-layer squash.
- Print the same hole in two orientations. A big difference means orientation, not slicer scaling, is the problem.
- Try the bearing or screw in each step of a test piece and note which hole gives a firm press fit and which gives a free slide.

## Worked example

A test piece with six holes in 0.1 mm steps for an M3 screw (nominal 3.0 mm), written in OpenSCAD:

```
for (i = [0:5])
  translate([i * 12, 0, 0])
    difference() {
      cube([10, 10, 3]);
      translate([5, 5, -1])
        cylinder(d = 3.0 + i * 0.1, h = 5, $fn = 64);
    }

// A 608 bearing is 22 mm outside diameter.
// Start a press-fit pocket near 22.0 + the value
// you found on your own test piece.
```

## Common mistakes

- Applying one fixed clearance to every hole size; small holes lose more in proportion than large ones.
- Testing in one orientation and printing the final part in another.
- Forcing a bearing into a tight hole, which can crack thin walls.
- Setting the clearance so large that a bearing that needs a press fit now spins in its seat.
- Using too few segments for a circle; use a high segment count so the bore is closer to round.

## If this did not fix it

- Over-extrusion or flow set too high, which makes every feature fatter.
- A thin wall around the hole that flexes, so the hole springs back out of size.
- Slicer settings such as outer wall first or small-feature speed that change how holes print.