Counterbore holes in 3D prints: sizes for socket head screws

Model an M3 counterbore 6.5 mm across and 3.4 mm deep, over a 3.4 mm clearance hole. An ISO 4762 socket head (5.5 mm across, 3 mm tall) then drops in with 0.4 to 0.5 mm to spare per side and sits 0.4 mm below the surface. If the counterbore faces the print bed, its ceiling is a bridge: turn on your slicer’s counterbore bridging or model two stepped bridge layers.

What size counterbore does a socket head screw need?

A socket head cap screw needs a counterbore about 1 mm wider than its head and 0.3 to 0.6 mm deeper than the head is tall. For an M3 screw (5.5 mm head, 3 mm tall), MISUMI’s machining table gives a counterbore 6.5 mm wide and 3.3 mm deep over a 3.4 mm clearance hole.

For a print, work it out from the head’s largest size:

counterbore diameter = largest head diameter + 2 × 0.4 mm
counterbore depth    = head height + 0.3 mm or more, rounded up to whole layers
clearance hole       = ISO 273 medium series (M3: 3.4 mm)

Then add your printer’s hole undersize (a test hole’s designed diameter minus its measured diameter) to both diameters.

• 0.4 mm per side, 0.8 mm total is the tolerance calculator’s loose, drop-in fit. QIDI Tech publishes 0.30 to 0.50 mm per side for drop-in parts. The clearance hole locates the screw, so the head only needs to drop in.
• Largest head diameter means the knurled (ridged) head. ISO 4762 allows a knurled M3 head up to 5.68 mm against 5.5 mm for a plain one. Head plus 0.8 mm lands within 0.02 mm of MISUMI’s machining sizes for M3 to M6.
• Whole layers: a slicer can only end the pocket on a layer boundary. At 0.2 mm layers, M3’s 3.3 mm becomes 3.4 mm.

Counterbore sizes for M2 to M8 screws

Screw	Head Ø, plain / knurled	Head height	Clearance hole	Machined counterbore Ø × depth	Model for a print, Ø × depth
M2	3.8 / 3.98 mm	2.0 mm	2.4 mm	4.4 × 2.0 mm	4.8 × 2.4 mm
M2.5	4.5 / 4.68 mm	2.5 mm	2.9 mm	5.4 × 2.5 mm	5.5 × 2.8 mm
M3	5.5 / 5.68 mm	3.0 mm	3.4 mm	6.5 × 3.3 mm	6.5 × 3.4 mm
M4	7.0 / 7.22 mm	4.0 mm	4.5 mm	8.0 × 4.4 mm	8.0 × 4.4 mm
M5	8.5 / 8.72 mm	5.0 mm	5.5 mm	9.5 × 5.4 mm	9.5 × 5.4 mm
M6	10.0 / 10.22 mm	6.0 mm	6.6 mm	11.0 × 6.5 mm	11.0 × 6.6 mm
M8	13.0 / 13.27 mm	8.0 mm	9.0 mm	14.0 × 8.6 mm	14.1 × 8.6 mm

• Head: maximum diameter and height from ISO 4762 (DIN 912), as Fasten.it and MISUMI’s JIS B 1176 excerpt reproduce them.
• Clearance hole: ISO 273 medium series, the same normal fit as the screw hole size calculator.
• Machined counterbore: MISUMI’s reference table for a head below the surface, M3 to M8. M2 and M2.5 come from Machinery’s Handbook as AmesWeb reproduces it, with the depth equal to the head height (flush). That chart runs wider from M4 up: 8.25, 9.75, 11.25 and 14.25 mm.
• Model for a print: the formula above at 0.2 mm layers. M3 to M8 depths are MISUMI’s below-surface depths rounded up to a whole layer; M2 and M2.5 add 0.3 mm, MISUMI’s margin for M3.

Counterbore sizes for #4 to 1/4 in screws

Screw	Head Ø × height, max	Clearance hole	Machined counterbore Ø	Model Ø	Model depth
#4	0.183 × 0.112 in	0.128 in	7/32 in (0.219)	0.215 in (5.45 mm)	0.126 in (3.2 mm)
#6	0.226 × 0.138 in	0.170 in	9/32 in (0.281)	0.258 in (6.54 mm)	0.157 in (4.0 mm)
#8	0.270 × 0.164 in	0.196 in	5/16 in (0.312)	0.302 in (7.66 mm)	0.181 in (4.6 mm)
#10	0.312 × 0.190 in	0.221 in	3/8 in (0.375)	0.344 in (8.72 mm)	0.205 in (5.2 mm)
1/4 in	0.375 × 0.250 in	0.281 in	7/16 in (0.438)	0.407 in (10.33 mm)	0.268 in (6.8 mm)

• Head: ASME B18.3 socket head cap screw maximums.
• Clearance hole: the ASME B18.2.8 normal fit, as in the screw hole size calculator.
• Machined counterbore: Machinery’s Handbook via AmesWeb, cut to the head height (flush). Its 0.45 to 0.80 mm per side also works in a print. The smaller model size keeps the bridge shorter if the counterbore faces the bed.
• Model depth: head height plus 0.3 mm (0.012 in), rounded up to a whole 0.2 mm layer.

How do you print a counterbore upside down without supports?

If the part allows, print the counterbore facing up. The step is then a floor laid on the layers below, and nothing needs support.

Facing the bed, the step becomes a ceiling: a ring of plastic between the counterbore wall and the screw hole with nothing under it. OrcaSlicer’s documentation warns that this ring sags and that the hole’s perimeters “may not be printed correctly.” The result is gaps or weak spots around the screw. There are three fixes.

Turn on counterbore bridging in the slicer

• OrcaSlicer: Bridge counterbore hole, under Quality > Bridging, in Advanced mode.
• Bambu Studio: Counterbore hole bridging under Quality > Advanced, added in version 2.8.0 (June 2026 public beta). The default is None.

Both offer the same three modes. Partially bridged covers only the overhanging ring and leaves the hole open. Sacrificial layer prints one full bridged layer across the hole, which you break through after printing. PrusaSlicer 2.9.6 and UltiMaker Cura have no such option, so model one of the fixes below.

Model two stepped bridge layers

Pantheon Design, a printer maker, documents this method at 0.2 mm layers:

1. On the counterbore floor, sketch a slot as wide as the clearance hole, running across the full counterbore.
2. Cut the slot one layer (0.2 mm) deeper than the floor.
3. Using the same sketch, cut a square with sides equal to the clearance hole diameter, two layers (0.4 mm) deeper than the floor.

The first layer bridges from wall to wall on both sides of the slot. The second bridges across the slot and leaves a square opening, and the round hole starts on the square’s edges. The head still bears on the floor around the slot. At other layer heights, use one and two of your layers.

Model a sacrificial layer

Close the clearance hole with one layer of plastic at the counterbore floor (0.2 mm at 0.2 mm layers). It bridges as a flat disc. After printing, push it out with the screw or drill it to the clearance size. This is the modeled version of the slicers’ sacrificial layer mode.

Counterbores for button head and low head screws

The same formula works for other socket heads. ISO 7380-1 button heads start at M3:

Screw	Button head (ISO 7380-1), Ø × height	Model Ø × depth	Low head (DIN 7984), Ø × height	Model Ø × depth
M3	5.7 × 1.65 mm	6.5 × 2.0 mm	5.5 × 2.0 mm	6.3 × 2.4 mm
M4	7.6 × 2.2 mm	8.4 × 2.6 mm	7.0 × 2.8 mm	7.8 × 3.2 mm
M5	9.5 × 2.75 mm	10.3 × 3.2 mm	8.5 × 3.5 mm	9.3 × 3.8 mm
M6	10.5 × 3.3 mm	11.3 × 3.6 mm	10.0 × 4.0 mm	10.8 × 4.4 mm
M8	14.0 × 4.4 mm	14.8 × 4.8 mm	13.0 × 5.0 mm	13.8 × 5.4 mm

Head sizes are the standards’ maximums. The model columns add 0.8 mm to the diameter and 0.3 mm to the height, rounded up to a 0.2 mm layer. A low head has the same diameter as a plain ISO 4762 head, so an existing ISO 4762 counterbore fits it, just deeper than needed.

For a flat head (countersunk) screw, the hole needs a countersink, a cone rather than a stepped counterbore.

Should you use a washer under a screw head in plastic?

Use one when the joint must stay tight. Plastic under a screw head creeps, and the clamp load drops. SPIROL, a maker of hardware for bolted plastic joints, lists a washer or a flanged bolt as ways to give the head enough bearing surface on plastic. A plain washer is wider than the head, so the counterbore grows. For an ISO 7089 washer (normal series):

Screw	Washer outer Ø × thickness	Model Ø × depth
M3	7 × 0.5 mm	7.8 × 3.8 mm
M4	9 × 0.8 mm	9.8 × 5.2 mm
M5	10 × 1.0 mm	10.8 × 6.4 mm
M6	12 × 1.6 mm	12.8 × 8.0 mm
M8	16 × 1.6 mm	16.8 × 10.0 mm

The depth is head height plus washer thickness plus 0.3 mm, rounded up to a 0.2 mm layer. Where the clamp load must hold, SPIROL’s fix is a metal compression limiter: a sleeve as long as the plastic is thick or slightly shorter. The screw then clamps the metal sleeve instead of the plastic.

Why won’t the screw head fit the counterbore?

• The head won’t go in. Measure the printed counterbore with calipers. If it came out smaller than modeled, add the shortfall to the model, or correct every hole in the slicer with the hole compensation calculator.
• Only the mouth is tight. On a counterbore facing the bed, the first layer is squished wider than drawn. Prusa calls this elephant foot and offers Elephant foot compensation under Print settings > Advanced. Otherwise, shave the lip off with a deburring tool or a hobby knife.
• The head sits proud. Check for sagged bridge strands or an unopened sacrificial layer at the bottom of the pocket, then for a depth the slicer rounded down. Model the depth as a whole number of layers.
• The head rocks or won’t sit flat. The fillet (the curve where the head meets the shank) can be wider than the hole: up to 3.6 mm on an M3, against a 3.4 mm hole. ISO 273 recommends a chamfer on the hole’s edge where the fillet would otherwise interfere.