M8 tap drill size

Drill a 6.8 mm hole (0.2677") for an M8 × 1.25 tap. That leaves about 74% thread, the standard for general-purpose tapping.

Tap drill
6.8 mm
0.2677" · 6.80 mm · 74% thread
Inch drill
17/64" 77% thread
Clearance (close)
8.4 mm
Heat-set insert hole
9.70 mm
Helicoil (STI) drill
8.3 mm
M8 tap drill: 6.8 mm (0.2677 in / 6.80 mm), 74% thread

M8 thread specs

SeriesISO metric coarse
Major diameter8.00 mm (0.3150")
Pitch1.25 mm (20.3 TPI)
Basic minor diameter6.65 mm (0.2617")
Common internal class6H

Every drill that works for M8

Drills from all four sets that give 50–85% thread, largest first. Lower percentages tap easier; see how much thread you need.

DrillThreadinmmSetGood for
9/32"53%0.28137.14Fractionaldeep holes only
K53%0.28107.14Letterdeep holes only
7.1 mm55%0.27957.10Metricdeep holes only
J59%0.27707.04Letterdeep holes only
7.0 mm62%0.27567.00Metrichard or tough metals
I67%0.27206.91Letterhard or tough metals
6.9 mm68%0.27176.90Metrichard or tough metals
6.8 mmstandard74%0.26776.80Metricgeneral use
H77%0.26606.76Lettergeneral use
17/64"77%0.26566.75Fractionalgeneral use
6.7 mm80%0.26386.70Metricmore torque, more broken taps
G84%0.26106.63Lettermore torque, more broken taps

M8 clearance hole

The hole a M8 screw passes through. ISO 273 fine, medium and coarse series.

Fitmmin
Close (fine)8.4 mm0.3307
Normal (medium)9.0 mm0.3543
Loose (coarse)10.0 mm0.3937

M8 counterbore and countersink

For a socket head cap screw to sit flush, and for a flat head socket screw.

Socket head counterboreDiameterDepth
NBK14.00 mm8.60 mm
AMES14.25 mm8.00 mm

Flat head socket screw: countersink at 90° (metric screws, ISO 10642) to 17.92 mm (0.7055") across for a flush head. Inch flat heads use 82°, so don't mix countersinks.

M8 heat-set insert hole

Hole to model for a brass heat-set insert in a 3D print. Brands size their knurl differently, so use your brand’s number.

BrandHole
CNC Kitchen9.70 mm
Generic (Albany County)10.20 mm

M8 holes in a 3D print

Printed holes usually come out undersize. Model the hole at the drill size plus what your printer loses, then tap or drill to size. Why printed holes shrink.

Print a test hole, measure it with calipers, enter the difference. 0 models the nominal size.

Hole to tap
6.80 mm
Clearance hole
8.40 mm
Heat-set insert
9.70 mm

M8 thread repair (Helicoil)

Stripped M8 thread? Drill out with the STI drill, tap with a M8 × 1.25 STI tap, and fit a wire thread insert. The drill in your repair kit’s instructions wins over any chart.

ChartSTI drillinmm
MISUMI8.3 mm0.32688.30

Related sizes

M6next size downM10next size up5/16-18closest inchFull chartmetric coarse

Frequently asked questions

What size drill do I use for an M8 tap?
Use a 6.8 mm (6.80 mm / 0.2677 in) drill. It leaves about 74% thread, the standard for general-purpose tapping.
What inch drill can I use for an M8 tap?
17/64" (6.75 mm / 0.2656 in) is the closest, giving about 77% thread. Anything from about 65% to 80% thread taps well in most materials.
What size clearance hole for an M8 screw?
8.4 mm (8.40 mm) for a close fit, 9.0 mm (9.00 mm) for a normal fit.
What hole size for an M8 heat-set insert?
Between 9.7 mm and 10.2 mm depending on the brand: CNC Kitchen 9.7 mm, Generic (Albany County) 10.2 mm. Use your insert's own number.
What is the inch equivalent of M8?
5/16-18 is the closest size (7.94 mm vs 8.00 mm major diameter). The pitches differ, so the screws and tapped holes are not interchangeable.

Sources

Spotted a wrong number? Email contact@clove3d.com and it gets fixed.

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% thread = (major − drill) ÷ (1.299 × pitch) × 100. The standard drill comes from published charts and is checked against this formula on every build.