The number 8.8 on a bolt refers to its property class, which indicates its strength. Specifically, a Property Class 8.8 bolt is a medium-strength fastener with a nominal tensile strength of 800 MPa and a nominal yield strength that is 80% of that nominal tensile strength, so 640 MPa. The first number in the property class designation is 1/100th of the part’s nominal tensile strength in megapascals (MPa), while the second number is 1/10th of the percentage of the part’s nominal yield strength compared to the part’s nominal tensile strength. The approximate tensile strength is right there on the bolt head marking, which is a nice feature of the ISO 898-1 property class designations.