Use a 5.0 mm Drill for a Standard M6 × 1 Cutting Tap

Use a 5.0 mm drill bit for a conventional M6 × 1 cutting tap. This is the standard coarse-pitch M6 thread and the usual meaning of “M6” when no pitch is stated.

Do not apply that answer to every M6 tap. Fine-pitch threads need different holes, while a thread-forming or roll tap needs a substantially larger hole.

M6 thread and tap type	Starting drill size
M6 × 1 cutting tap	5.0 mm
M6 × 0.75 cutting tap	5.25 mm
M6 × 0.5 cutting tap	About 5.5 mm; verify against the tap data
M6 × 1 forming/roll tap	Commonly 5.5 mm; verify the specified range

Why 5.0 mm is the standard M6 tap drill

The quick calculation for a metric cutting tap is:

nominal diameter − pitch = tap-drill diameter

For M6 × 1:

6 mm − 1 mm = 5 mm

Dormer Pramet gives this exact M6 example and warns that forming taps require a larger pre-drilled hole. It also notes that drill tolerance and workpiece material can justify adjustments, so production work should use the selected tap’s data rather than the rule alone (Dormer Pramet).

The same calculation gives 5.25 mm for M6 × 0.75 and 5.5 mm for M6 × 0.5. Guhring’s cutting-tap chart lists 5.25 mm as its 75% thread choice for M6 × 0.75 (Guhring tap-drill chart). Treat calculated sizes as starting points when the drawing, required thread class or tap maker specifies something else.

The “M6” marking identifies a 6 mm nominal thread diameter, not the pilot-hole diameter. ISO’s metric-thread designation rules state that omission of the pitch indicates a coarse pitch; M6 × 0.75 is therefore a different thread from plain M6 (ISO/R 261:1969). Check the full marking on the tap before drilling.

Can an inch drill substitute for 5.0 mm?

There is no exact inch-series equivalent:

• 5.0 mm = 0.19685 in
• No. 9 = 0.1960 in, or 4.978 mm
• No. 8 = 0.1990 in, or 5.055 mm
• 13/64 in = 0.203125 in, or 5.159 mm

A No. 9 is dimensionally closest, but it is slightly undersize and can increase thread engagement and tapping torque. A No. 8 is a practical slightly oversize substitute when an exact 5.0 mm bit is unavailable. A 13/64-inch bit removes still more material and produces a shallower thread.

Guhring’s chart puts 5.0 mm at about 75% theoretical thread for M6 × 1, 13/64 inch at about 65%, and 5.25 mm at about 55%. Guhring separately recommends roughly 60% to 70% thread for many applications rather than treating maximum engagement as universally better (Guhring). That does not make every substitute acceptable: for a specified or safety-critical joint, follow the drawing, required thread class and tap manufacturer’s limits.

A forming tap does not use the 5.0 mm hole

A forming tap displaces material into the thread profile instead of cutting chips, so it requires a larger starting hole. Haas lists 5.5 mm for an M6 × 1 roll tap and calls its chart a starting point that may require adjustment for tolerance, material and hardness (Haas Tooling).

The acceptable window can be narrow. One OSG M6 × 1 forming tap specifies a pilot-hole range of 5.49 to 5.592 mm (OSG). Use the data for the actual forming tap rather than drilling the 5.0 mm cutting-tap hole, which can sharply increase forming torque.

Before tapping the hole

1. Confirm the tap says M6 × 1 and determine whether it cuts or forms threads.
2. Drill straight with a sharp bit suited to the workpiece. For ordinary cutting-tap work, start at 5.0 mm unless the tool data specifies otherwise.
3. Lightly chamfer the opening so the tap can start cleanly.
4. Keep the tap square and use a cutting fluid suitable for the tap and material.
5. For a blind hole, allow extra depth for the drill-point cone, tap chamfer and chip space. The drilled depth must exceed the required full-thread depth.
6. Stop if torque rises unexpectedly. With a cutting tap, clear chips rather than forcing the tool.

For adjacent metric sizes, see the M5 tap-drill guide and M8 tap-drill guide.