Reading a screw specification: what M2×0.4×6 leaves out

Checking the size with a caliper is the obvious verification

The box says M3. You have a caliper. Measuring across the threads to confirm takes five seconds, and for most dimensions in a factory that is exactly the right instinct.

It comes back under 3 mm — a conforming M3×0.5-6g measures anywhere from 2.874 to 2.980 mm across the crests. Nothing is wrong with the screws, and nothing is wrong with the caliper.

What a designation contains

Take M2×0.4×6. It settles three things. A drawing that can actually be quoted needs at least these:

The product grade in that designation is a tolerance class, not a quality class: product grade is not quality grade.

So measure it and check — and it will not measure what it says

The common misjudgement: measure an M2 across the crests, read 1.96, reject the lot.

Nominal diameter is a design datum, not a target. Of the usual external positions, 6g has a negative upper deviation and 6h has an upper deviation of zero — so the major diameter is expected to come out equal to or under nominal. That is the design, not a defect.

Pitch: establish coarse or fine first

This section assumes you already know the screw is metric. If that is not yet settled, start with the pitch will not separate #10-32 from M5 — that pair differs by 0.006 mm per thread, so a pitch reading carries no information about it.

ISO 261 defines the general purpose metric series. Across the range this site covers, the coarse pitches are:

Fine pitches are a separate set, and a drawing that says only M3 is read as coarse by convention. A fine thread has to be written out in full, for example M3×0.35 — the two look almost identical in the hand, and the difference shows up when one of the two threads is already damaged.

Measuring pitch without a gauge

• A pitch gauge beats technique. A set of gauges settles it
• Without one, span several crests and divide. Measure across ten crests and divide by ten, so the error is averaged. A single pitch measured with calipers is unreliable at these sizes
• ⚠️ Whether the caliper jaws reach the thread root is itself a source of error

Length: the datum moves with the head

The same “6 mm” is a different length on a different head. A countersunk head sinks into the hole, so it counts and length is overall. A pan head stands on the surface, so length is measured under the head.

Tolerance class: the chart has no column for plating

6g / 6h is the field most often left off, on the grounds that everything is standard anyway. It decides something specific: whether the thread has room for a coating. The number and the letter answer different questions, and it is the letter that carries the allowance.

A single-side coating of thickness t adds roughly 4t to pitch diameter, and the upper deviation at 6h is zero — there is no allowance to spend. This bites hardest between M1 and M1.4, and it does not appear at sample stage. It appears on the first production lot.

Where the charts stop

The last field is the chart's own range of validity. The smaller the part, the less there is to copy — the standard method for torque and clamp force testing begins at M3, and some material tests call for specimens a small screw cannot yield.

Fill these in before you send it out

• Thread system and pitch (fine pitches written out in full)
• Length, with the datum stated — overall or under head
• Tolerance class, and whether it applies before or after plating
• Head style and drive; for countersunk heads, the angle of the hole as well
• Material, property class, finish
• Which grade of inspection document, and against which standard

For a standard screw called out against a product standard, that list is most of what the callout needs and about half of an enquiry. A drawn part needs more — its own dimensions and datums, geometric tolerances, surface texture — because the product standard is no longer supplying them. The other half is what a supplier prices rather than makes — quantity, schedule, and who verifies what — and leaving it out is what turns a quotation into three rounds of questions.

References

• ISO metric screw thread — Wikipedia

Standards referred to above, listed by number for verification:

• ISO 68-1　basic profile for metric screw threads
• ISO 261　general purpose metric screw threads — general plan
• ISO 262　selected sizes for screws, bolts and nuts
• ISO 965-1 / ISO 965-2　tolerances and their deviations

This page explains what the fields mean and how to read them. Values are governed by the standards themselves and by your drawing, not by this site.