• Bolts are identified by the markings on the head or top of the bolt.bolts image by Petr Gnuskin from Fotolia.com
  
  Bolts are graded based on the hardness of the metal used in their construction. Using bolts that aren't strong enough for their intended purpose often leads to damaged equipment or even injury. There are three types of grade markings that are used to display the grade markings of bolts. Standard hex bolts use patterns of lines that run from the outside of the bolts head to its center while metric hex bolts use a system of numbers to define bolt strength.
  
  

Standard Hex Bolts

• Standard bolts use the American system of measurement and have distinct grade markings. There are four main grades of standard bolt including grades 1, 2, 5, 7 and 8. Lower grade numbers indicate that the bolt has less strength and heat tolerance than bolts with higher numbers. Hex head bolts with no markings on the flat part of the head are grade 1 or 2 bolts. Bolts with three lines running from the outside of the bolt head to the center are grade 5. There are three types of grade 5 bolt including grade 5, 5.1 and 5.2. Grade 5.1 bolts have three lines in the shape of the letter "T" while 5.2 bolts have three lines pointing from the center to three adjacent points of the hex head. Bolts with 5 lines pointing from the outer edge of the bolt head to the center are grade 7 and bolts with 6 lines pointing to the center are grade 8.
  
  

Metric Hex Bolts

• The dimensions of metric bolts are based on the metric system. They are usually not interchangeable with standard bolts and have 6 commonly available grades. Metric hex bolts are graded using numbers that are stamped on the flat part of the bolts head. Metric bolts with low numbers indicate weaker bolts while higher numbers indicate a tougher and more heat resistant bolt. The numbers used in the metric rating system include 4.5, 4.8, 5.6, 8.8, 9.8 and 10.9 with 4.5 being the weakest bolt and 10.9 being the strongest.
  
  

Metric Stud Bolts

• Metric stud bolts use a different system of markings due to the shape of the bolt. Since these bolts do not have a head the grade markings are stamped into the end of the bolt. Metric stud bolts are rated at 8.8, 9.8 and 10.9. Metric stud bolts with a grade of 8.8 are marked with a circle stamped into the end of the bolt. Stud bolts that are graded 9.8 have a plus sign stamped into the end and grade 10.9 bolts have a diamond stamped into the end of the bolt.