Metric Bolt Identification

Fig. 1:

An ISO metric bolt or screw, made of steel and larger than 6 mm in diameter can be identified by the symbols ISOM or M embossed on top of the head (Fig. 1). In addition to marks to identify the manufacturer, the head is also marked with symbols to indicate the strength grade e.g. 8.8, 10.9, 12.9 or 14.9. The first figure gives the minimum strength of the bolt material in tens of kgf/square mm. Zinc plated ISO metric bolts and nuts are chromate passivated and colored greenish-khaki to gold-bronze.