🔩 What do the parameters of M thread annotation mean? (Essential for procurement/ordering)

The labeling of metric ordinary threads may seem complicated, but it only consists of a few sections. Simply follow and disassemble them:

1、 Complete annotation example

M10 × 1.25 × 40-8.8 grade – Zn (coating)
│ │ │ │
│││└ – Performance level/surface treatment (optional)
││└————— nominal length 40mm
│└——————————————————————–
└———————————————————————–

2、 Explain each parameter one by one

Explanation of the meaning of the annotation section

M10 metric ordinary thread, nominal diameter large diameter=10mm M3/M4/M5/M6/M8/M10/M12

The default pitch for M10 coarse teeth is 1.5mm, and coarse teeth are usually not marked; It’s marked as fine teeth

40 bolt nominal length=40mm external hexagonal bolt: excluding head; Countersunk head/hexagonal countersunk head: including the head

8.8 grade bolt performance level corresponds to a tensile strength of 800MPa/yield of ≈ 640MPa

Zn/Galvanized/Dacromet surface treatment (to be specified in the contract or remarks) is often separately indicated during procurement

✅ Common simplified writing methods (all referring to coarse teeth):
M8 × 30-8.8 Zn → M8 coarse tooth (1.25), length 30mm, grade 8.8, electroplated zinc coating

M10-8.8 → Default coarse teeth, length to be specified separately

3、 Coarse teeth vs fine teeth (a confusing point in procurement) ⚠️）

Specification: Coarse tooth pitch (default), commonly used fine tooth pitch

M6 1.0mm 0.75mm

M8 1.25mm 1.0mm

M10 1.5mm 1.25mm ← Regular fine teeth

M12 1.75mm 1.5mm

M16 2.0mm 1.5mm

❗ Coarse teeth and fine teeth cannot be exchanged!

Example: If the M10 fine tooth (1.25) bolt cannot be screwed into the M10 coarse tooth (1.5) nut, it is necessary to check the pitch or indicate “coarse tooth/fine tooth” when purchasing/leaving the warehouse.

4、 Difference in internal and external thread labeling (by the way)

-External thread (bolt): M10 × 40-8.8 (as above)
Internal thread (nut): M10-8 (grade 8 nut, equipped with 8.8 bolt) or M10 × 1.25 (fine thread nut)

Tapping bottom hole: M10 coarse teeth ≈ Φ 8.5mm, M10 fine teeth ≈ Φ 8.7mm (for reference only)

5、 It is recommended to write this when inquiring about procurement ✅

1000 pieces of M10 × 40 fine tooth 1.25 hexagonal bolts with 8.8 grade blue white zinc

Or rough tooth abbreviation: “M12 × 50 hexagon bolt 8.8 grade Dacromet 500 pieces (rough tooth)”