Standards

Metric threads: practical standards notes

A practical view that separates coarse/default pitch, fine pitch, tap drill diameter, and clearance hole diameter.

Coarse (standard pitch)

Default when pitch is omitted. For example, M8 by itself means M8x1.25.

Uses a smaller pitch at the same nominal diameter, so the tap drill diameter differs from the coarse series.

The pitch must be called out, such as M8x1; typical tap drill for M8x1 is 7.0 mm.
Works well in thin-wall sections.
Improves clamp adjustment sensitivity.

Practical starting values for coarse/default and fine pitch. Tap drill means the pre-hole before threading.

Size	Coarse P (mm)	Coarse tap drill	Fine P -> drill
M3	0.5	2.5 mm	0.35 -> 2.65 mm
M4	0.7	3.3 mm	0.5 -> 3.5 mm
M5	0.8	4.2 mm	0.5 -> 4.5 mm
M6	1	5 mm	0.75 -> 5.2 mm
M8	1.25	6.8 mm	1 -> 7 mm
M10	1.5	8.5 mm	1.25 -> 8.8 mm / 1 -> 9 mm
M12	1.75	10.2 mm	1.5 -> 10.5 mm / 1.25 -> 10.8 mm
M16	2	14 mm	1.5 -> 14.5 mm
M20	2.5	17.5 mm	2 -> 18 mm / 1.5 -> 18.5 mm
M24	3	21 mm	2 -> 22 mm

Mini Tool

Shows the common workshop drill first; the calculated value below changes with selected thread engagement.

Nominal metric sizePitch seriesPitch P (mm)Thread engagement (%)

Common workshop value

6.8 mm

This is the pre-hole for tapping; bolt clearance holes are separate in the table below.

Starting point for hard materials: 6.95 mm

M8x1.25 - 6H/6g

Formula note

D_tap ≈ D_nominal - P x (engagement/77)

The table value is a practical standard starting point; the calculated value helps trade thread height against tap load.

Example: M8 = M8x1.25 coarse/default with a 6.8 mm tap drill; M8x1 fine pitch typically uses a 7.0 mm tap drill.

Close / normal / loose diameters are for free bolt passage; do not mix them with tap drills.

Size	Close	Normal	Loose
M3	3.2	3.4	3.6
M4	4.3	4.5	4.8
M5	5.3	5.5	5.8
M6	6.4	6.6	7
M8	8.4	9	10
M10	10.5	11	12
M12	13	13.5	14.5
M16	17	18	19
M20	21	22	24

Good for early design loops. Validate by standard/tolerance and critical-load checks.

Material	Guideline	Note
Steel base material	1.0 x D	Typical starting point for general machine design.
Cast iron (EN-GJL / EN-GJS)	1.2 x D	Slightly longer engagement helps brittle base materials.
Aluminum alloy	1.5 x D	Longer thread reduces stripping risk in softer alloys.
Brass / Bronze	1.2-1.5 x D	Use upper range for frequent assembly/disassembly.
Plastics (PA6, POM, etc.)	2.0 x D + insert	Prefer metal threaded inserts for durability.