Common Fastener Screw Terms explained

Understanding common fastener terms

What does M8-1.25x16 MM,(FT) FLAT SOCKET CAPS 12.9 COARSE ALLOY DIN 7991 THERMAL BLACK OXIDE mean?
It helps to break it down into smaller blocks like this

• M8-1.25 x
• 16 MM,(FT)
• FLAT SOCKET CAPS
• 12.9  ALLOY 
• BLACK OXIDE

(disregarding the following for the time being - DIN 7991, COARSE, and THERMAL)

So putting it all back together we get -

M8-1.25 x 16mm Flat Head Socket Cap Screw, Grade 12.9 Alloy Steel with Black Oxide coating.

I intentionally picked a product title that had more info than we needed for this example.  The unused info here being ‘DIN 7991’, ‘Coarse’ and ‘Thermal’.  Beyond the basics, these three extra words are very valuable once you know them. 

Metric fasteners generally revolve around two standards, DIN (Deutsches Institut für Normung or roughly translated to ‘German institute for Standardization’ for our non-German speaking friends) and ISO (or International Organization for Standardization).

In the case above, DIN 7991 explains that this screw is exactly the same screw your manual or whatever is referencing when it notes DIN 7991.  This is incredibly helpful once you know to look for either DIN or ISO verbiage.  DIN 7991 is the exact same thing as FLAT SOCKET CAPS, a Flat or countersunk screw that takes an Allen (Hex) key to drive.

Reference list of Common DIN and ISO numbers for screws and bolts:

The second word in that list was Coarse.  This again is a duplicate in this case because the thread pitch was already listed if you knew it when listing the screw size. In the screw above, the first bit of that, M8-1.25, breaks down the screw outside thread diameter and Thread Pitch, or M8 (screw size) and 1.25 (thread pitch or for Metric screws, the distance in mm between threads).  M8-1.25 denotes a coarse thread standard, so listing it as Coarse is more of a ‘checks and balances’ for you to be aware of. 

List of common Metric screw diameters and thread sizes:

From that table you can see that most metric screws are only made in a coarse thread option.  Fine thread and ‘super fine’ thread really are seen in the metric world as rare.

Lastly, ‘Thermal’ refers to a specific method for applying Black Oxide coating or commonly shortened to just Black Ox.  Black oxide generally is either applied Thermally or Chemically.  Outside of Black Oxide, Zinc plated is the common place where you will find additional info provided.  Our zinc plating is fasteners are designated as Cr+3 and are RoHS compliant.  You can also still find Cr+6 products elsewhere which are not RoHS compliant but have similar ‘salt spray life’ finish durability test ratings.  Zinc coatings further breakdown into how they are applied again just as black oxide did (Mechanical or Electroplated ).