What's the difference between 4.6 and 8.8 bolts

Bolts are available in standard size and a range of lengths.

Bolts can act in two ways, namely in tension and shear. The individual parts of a bolt comprise a hexagonal head and a circular section shank, which may be fully or partly threaded. They come in shank diameters from very small (6mm) up to 36mm. Larger diameters may be available to special order.

Two grades of bolt are commonly used. These are grade 4.6 and 8.8. The first digit relates to the ultimate strength of the material, whilst the second is the ratio of yield stress to ultimate strength. Thus grade 4.6 bolts have an ultimate material strength of 400N/mm2 and the uield (or proof) stress is 60% of the ultimate strength. Simlarly grade 8.8 bolts have an ultgimate strength of 800 N/mm2 and a ratio of yield/proof stress to ultimate strength of 80%. Simple grade 4.6 bolts without corrosion protection are commonly called 'black'bolts'