What Do The Markings On The Bolt Heads Mean?

The most common bolt performance grade currently available is the metric grade set by ISO, which consists of two numbers separated by a dot and is usually marked on the top or side of the bolt head or nut. The more common ones are 8.8, 10.9 and 12.9.

Multiply the number before the decimal point by 100 to give the approximate minimum tensile strength of the bolt. The number after the decimal point multiplied by 10 gives the approximate percent yield strength relative to the minimum tensile strength. For example, the tensile strength of grade 8.8 is about 800MPa, and the yield strength is about 800*80%=640MPa. You may have also seen the two logos below.

This is the marking for stainless steel bolts or nuts, A2 stands for 304 stainless steel and A4 stands for 316 stainless steel. The following number represents the tensile strength and will vary depending on the tensile strength of the bolt. For example, 70 means that the tensile strength is equal to 700MPa general stainless steel. Both stainless steels have good corrosion resistance, 316 is better than 304, and is suitable for places with higher requirements.

You may still have questions, why are there no marks on some bolt heads? That usually means that the performance level of the bolt is relatively low, such as level 4.8 or level 5.8, which is usually not marked.

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