If you are like me and like to wrench on things, whether it’s fixing a car or repairing the snowblower, you probably have a set of inch and metric wrenches. Sometimes you can determine which set you need by the age or brand of whatever it is you are fixing. Anything made in the United States prior to 1980 is probably not using metric fasteners, while most other parts of the world have never used inch fasteners. If your project was manufactured in the US in the 1980s or 90s, you may see both inch and metric, which may drive you crazy unless you learn a bit about head markings.
Standard headed fasteners, both inch and metric, are required to have a grade or property class marking on the head if it has been heat treated. Also, on the head should be the manufacturer’s identification marking like the triangle in the pictures below. Inch fasteners define their strength (grade) by slash marks as indicated below for grade 5 and grade 8.
Metric fasteners define their strength (property class) with two numbers separated by a decimal point such as 8.8 or 10.9 which are the most common.
Property Class 8.8
So, next time you grab a wrench for a project, take a look at the head of the bolt to see which type of wrench you’ll need!Inch or Metric? Check the Head Marking! by Bossard