Mystery Of Metric Bolts
Ever wonder what those numbers on metric bolts mean? I always thought that the numbers were either the size of the bolt or the thread pitch. They are actually neither. Those number are actually the strength of the bolt. The number on the head is actually two separate ratings separate by a period. (Well..... supposed to be anyway. Some of the imported bolts don't seem to always follow this standard.)
The first number is the tensile strength or the pressure required to actually break the bolt.. This number can be between 4 and 14 with 14 being the toughest. To convert that number into something useful, take the first number on the head, multiply it by 100 (this give you tensile strength in N/mm2), then multiply it by 145 to get you the PSI that the bolt is rated at. A metric bolt with a 12 for the first digit, for example, would have a tensile strength of 174,000 PSI (12x100=1,200N/mm2 x 145=174,000psi).
The number after the period is the Yield strength of the bolt. This is the pressure that is required to ultimately stretch the bolt into permanent deformation. This number is reached by multiplying the number (including the decimal point) by 1200 to get N/mm2 then multiply that number by 145 to get psi. A bolt with a second number of .9 for example is rated to withstand pressure of up to 156,600 psi. Without permanent damage. These numbers can be anywhere on the head of the bolt including the side of the head. I hope this helps clear up any confusion.
Did you find this tip useful? You'll find plenty more like this in our new, easy to read, 35 page booklet "Guide To Imported Tractors". Click here to find out about the book. Don't forget to sign up for our free monthly newsletter with more articles like this by entering your email address in the box at the top left of the page