How to read a fastener grade
The number stamped on the head is a property class, not a material. It tells you what the bolt will carry, and nothing about the coating.
8.8, 10.9, 12.9
The first number is tensile strength in hundreds of MPa, the second is the yield ratio. 8.8 means about 800 MPa tensile and 640 MPa yield. 10.9 means 1000 and 900.
A2-70 and A4-80 are not stronger
A2 and A4 are stainless steel groups in ISO 3506, usually supplied as 304-type and 316-type material. The exact grade comes from the mill certificate, not from the group label. Both sit below property class 8.8. On a load-bearing joint stainless is picked for corrosion resistance; it does not add strength.
Coating decides the service life
Zinc plating is the cheap option and the first to go red. Zinc flake and dacromet last longer and cost more. Ask for the salt-spray report number, not the word anti-rust.
What to write on a PO
Property class, thread pitch, length, coating and the standard it is made to. A drawing without the standard is an argument waiting to happen.
