A fastener is fully described by a string that looks like DIN 912 M6x30-8.8-A2A. Each element constrains a different property, and dropping any one of them leaves an order ambiguous. Here is what each part does.
Standard code
DIN 912 — the standard the fastener meets. This fixes the shape: head form, drive, whether there is a shank, and the dimension table. It is the first thing to check, because a socket head cap screw and a hex head bolt are not substitutes regardless of thread size.
Standards are grouped into equivalent pairs — see DIN vs ISO interchangeability for how far that equivalence actually goes.
Size: diameter, pitch and length
M6x30 — a 6 mm metric thread, 30 mm long.
- If no pitch is shown, the coarse series applies. Fine threads must be stated:
M10x1.25. - Length is measured differently by product form, and this is the most common source of wrong orders:
- Screws and bolts: length under the head, excluding the head. - Countersunk screws: length including the head, because the head sinks flush. - Set screws: the full length, since there is no head. - Nuts and washers: no length; they are specified by thread or by bore.
The size index lists every diameter with the standards available in it, which is the quickest way to confirm what length range a given standard covers.
Property class
8.8 — steel property class. For stainless this becomes A2-70 or A4-80, a different numbering scheme with a different meaning. See how to choose a bolt property class for what the numbers encode and when upgrading is the wrong move.
Coating or finish
A2A and similar suffixes describe surface treatment: plain, zinc plated, hot-dip galvanised, zinc-nickel, and so on.
Two things to know:
- Coating changes friction, and friction changes how much preload a given torque produces. A torque figure specified for plain steel is not correct for a lubricated or heavily coated fastener.
- Plated class 10.9 and above needs baking to avoid hydrogen embrittlement. The finish code is therefore also a process requirement, not only a cosmetic one.
Checking an incoming part
A quick sequence that catches most errors:
- Read the head marking — it should give the class and a manufacturer's mark. An unmarked head on a fastener that requires marking is a red flag.
- Measure the key dimensions — for a screw, thread diameter and length under the head; for a nut, thread and width across flats; for a washer, inside and outside diameter.
- Confirm the standard-specific feature — countersink angle, chamfer count, or shank length, depending on the product form.
- Compare the property class to the requirement, not to the previous delivery.
The specification index gives the nominal dimensions for every size in the database, so a dimension found in inspection can be checked against the published table directly. Each standard page also carries selection and fitting notes for that product form.