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# What the Numbers on Metric Bolts Mean

Have you ever looked closely at a metric bolt?

Did you notice that it has numbers on it? Many people think the numbers represent the thread pitch or the size of the bolt. The numbers do not represent either.

Numbers on metric bolts represent the strength of the bolt.

The head of the bolt has some numbers on it. These are two separate ratings that have a period between them. (Keep in mind that you will find bolts that do not follow this standard. Some numbers are also located on the side of the bolt head.)

The first number represents the tensile strength. This is the pressure it takes to break the bolt. Tensile strength is rated 4-14; with 14 being the toughest. Let’s take a closer look and convert that number into something useful.

Identify the first number on the bolt. Multiply it by 100 (this gives the tensile strength in N/mm2). Next, multiply it by 145 (this gives the PSI bolt rating). Example --- A metric bolt with a 14 for the first digit, for example, would have a tensile strength of 174,000 PSI (14 x 100 = 1,400 N/mm2 x 145 = 203,000psi).

Time to move on to the second number. This the number after the period.

The second number is the bolt’s yield strength. Yield strength is the pressure required to stretch the bolt into permanent deformity.

How is this number decided? Multiply the number (including the decimal point) by 1200 to get N/mm2. Then multiply that number by 145 to get psi. Example --- A metric bolt with a second number of .8 for example is rated to withstand pressure, without permanent damage, to 139,200 psi.