The markings on a bolt head are a promise about the metal inside it. Reading them takes one minute and prevents the two most common purchasing mistakes: paying for strength you do not need, and buying stainless where it will fail.

The Two-Number System


Grades like 8.8 and 10.9 are two numbers multiplied. The first times 100 gives the tensile strength in N/mm². The second times the first times 10 gives the yield strength. So an 8.8 bolt holds about 800 N/mm² tensile and 640 N/mm² before it permanently stretches.

  • 8.8 - the workhorse. General machinery, frames, jigs. Most of our DIN931 assortment boxes are 8.8 because it covers the majority of maintenance and assembly work.
  • 10.9 - high load, limited space. Lifting points, clamping, press frames. Harder also means more brittle: it does not forgive overtightening.
  • 12.9 - special cases only. It is a tool steel more than a fastener; keep it out of general stock.

When Stainless Is the Wrong Answer


A2 stainless (our M10 DIN933 line is A2-70) buys corrosion resistance, not strength - it sits near 8.8 in load but costs more and galls when run against stainless nuts without lubricant. Two honest cases against it:

1. Chloride environments. A2 pits in marine and pool-water conditions; A4 (316) is the correct upgrade, not a thicker A2.

2. Hot bolting. Above 400 °C stainless loses preload faster than plated alloy steel.

For dry indoor assemblies, yellow-zinc 8.8 at 96 hours of salt-spray protection does the same job for a third of the price.

One Box That Covers Most Jobs


Maintenance buyers do not need fifty part numbers. A sorted box of M6/M8/M10/M12 partial-thread bolts with matching nuts and washers handles roughly nine out of ten workshop calls. The rule of thumb: full thread (DIN933) for grip through plastics and timber; partial thread (DIN931) for steel-to-steel, where the unthreaded shank carries shear.

Heat numbers are printed on every box we ship, so a failed bolt can always be traced back to its melt. If you are rebuilding a machine instead of restocking a shelf, send the drawing - the same desk quotes custom fasteners within 24 hours.