Use a 3.3 mm Drill for an M4 × 0.7 Cutting Tap

Use a 3.3 mm drill bit before tapping M4 × 0.7 with a conventional cutting tap. That is approximately 0.1299 inch. Haas specifies a 3.3 mm tap-drill diameter for its M4 × 0.7 spiral-point cutting tap, and Carbide Depot’s metric tap chart lists the same size.

Check the pitch and tap type before drilling: an M4 × 0.5 fine-pitch tap needs a different hole, and a forming tap needs a larger one. Treat the chart sizes below as starting points; follow the exact tap’s hole-size instructions if they differ.

M4 tap drill sizes

Thread and tap type	Starting drill size	When to use it
M4 × 0.7 cutting tap	3.3 mm	Standard coarse-pitch M4 thread
M4 × 0.5 cutting tap	3.5 mm	Fine-pitch M4 thread
M4 × 0.7 forming/roll tap	3.7 mm in Haas’s chart	Starting point only; verify the exact tap’s data

The cutting-tap sizes appear in J C Gupta & Sons’ metric drill-and-tap chart. Haas’s roll-tap chart lists 3.7 mm for M4 × 0.7, but says hole size may need adjustment for tolerance, material and hardness. It recommends drilling and reaming for roll taps.

Why 3.3 mm—not 4 mm?

M4 describes the thread’s nominal major diameter, not the hole you drill before cutting it. The useful metric cutting-tap rule of thumb is:

Tap-drill diameter ≈ nominal diameter − pitch

For M4 × 0.7: 4.0 − 0.7 = 3.3 mm. For M4 × 0.5: 4.0 − 0.5 = 3.5 mm. Dormer Pramet explains this rule and distinguishes the larger holes needed for forming taps. It also notes that drill tolerance and workpiece material can require adjustments to the usual pre-drill size.

A 4 mm hole is not the normal pilot for an M4 cutting tap. If the screw should pass through the part without threading into it, you need a clearance hole, not a tap-drill hole.

Can you use an inch drill instead?

For M4 × 0.7 cutting taps, No. 30 is a documented inch alternative, listed in Carbide Depot’s chart. Its diameter is 0.1285 inch, or 3.2639 mm—slightly smaller than 3.3 mm, not an exact equivalent.

Bit	Nominal diameter in mm	Difference from 3.3 mm
No. 30	3.2639	0.0361 mm smaller
1/8 inch	3.1750	0.1250 mm smaller
9/64 inch	3.5719	0.2719 mm larger

Metric equivalents for the fractional bits above are calculated using 25.4 mm per inch. Neither nearby fraction is a close replacement: the smaller hole increases the tap’s cutting load, while the larger hole leaves less material for the thread. Dormer warns that an undersized hole risks tool damage and an oversized hole can produce nonstandard threads. If you only have fractional bits, get a 3.3 mm bit rather than choosing the nearest fraction.

Before you tap

• Clamp the workpiece securely and keep the drilled hole and tap aligned.
• Use lubricant suitable for the workpiece and tap.
• For a blind hole, check depth before starting. Allow for the tap’s lead and tip; do not let it hit the bottom.
• If resistance rises sharply, stop and investigate. Check hole size, alignment and chip congestion rather than forcing the tap.

Guhring’s tapping troubleshooting guide identifies undersized holes, alignment errors, inadequate lubrication, chip congestion and bottoming as application problems.