What drill do I use for an M8 tap?

About 6.75 mm, from diameter minus pitch: 8 less 1.25. That gives roughly seventy-five percent engagement, which is the practical target.

Why not tap for full engagement?

It adds a few percent of strength and a great deal of torque on the tap. Seventy-five percent is where the trade-off sits for almost every application.

Conversionstechnical

Thread and Bolt Size Converter

Thread sizes between metric pitch and imperial threads per inch, with tapping drill sizes. Metric states the distance between threads and imperial states how many fit in an inch, so the two are reciprocals.

Also called: tap drill size chart, metric to imperial thread.

Designation

M8 x 1.25

M8 x 1.25: 8 mm diameter with a pitch of 1.25 mm, which is 20.32 threads per inch. Tapping drill is about 6.75 mm and clearance 8.64 mm. Metric states the distance between threads and imperial states how many fit in an inch, so the two are reciprocals through 25.4 mm.

Diameter

Pitch

1.25

Threads per inch

20.32

Tapping drill

6.75

Clearance drill

8.64

Thread depth

0.77

On the two conventions

Metric states the distance between threads and imperial states how many fit in an inch, so the two are reciprocals through 25.4 mm.

How this is calculated

Method and background

How this is calculated

Metric threads are specified by pitch, the distance between adjacent crests, and imperial threads by how many crests fit in an inch. They are reciprocals through 25.4 mm. The tapping drill rule of thumb is diameter minus pitch, which gives around seventy-five percent thread engagement: enough for nearly full strength while leaving the tap able to cut without breaking. Chasing a hundred percent engagement gains a few percent of strength and greatly raises the chance of a snapped tap.

the tapping drill rule of thumb subtracts one pitch from the diameter, which gives about 75 percent thread engagement

d: Diameter
p: Pitch

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

an M8 coarse thread

Nominal diameter: 8
Diameter in: Millimetres
Pitch or threads per inch: 1.25
Thread: Metric, pitch in mm

DesignationM8 x 1.25

8 less the 1.25 pitch; 25.4/1.25

Open this example

imperial converts to the same pitch

Nominal diameter: 0.32
Diameter in: Inches
Pitch or threads per inch: 20.32
Thread: Imperial, threads per inch

Designation0.315in - 20.3 TPI

boundary: the reciprocal relationship

Open this example

Method and limits

What it assumes

Coarse thread series unless a pitch is entered.

What it deliberately does not model

Fine, extra-fine and pipe threads use different pitches at the same diameter.
Tapping drill sizes vary with material; the rule of thumb suits steel and aluminium.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

What drill do I use for an M8 tap?: About 6.75 mm, from diameter minus pitch: 8 less 1.25. That gives roughly seventy-five percent engagement, which is the practical target.
Why not tap for full engagement?: It adds a few percent of strength and a great deal of torque on the tap. Seventy-five percent is where the trade-off sits for almost every application.