Metric property classes
Metric bolts carry a two-part number such as 8.8 or 10.9. The first figure relates to nominal tensile strength and the second to the ratio of yield to tensile strength. Higher numbers mean higher strength and usually lower ductility.
Nuts must match bolts
A nut's property class should be at least equal to the bolt's class so the joint fails in the bolt rather than by thread stripping. Mixing a low-class nut with a high-class bolt undermines the whole joint.
Coatings and corrosion
Plain, zinc plated, hot dip galvanised and PTFE coated finishes all behave differently in service. Galvanising affects thread fit, so nuts are usually oversize tapped to suit galvanised bolts.
What to state on an enquiry
Diameter and pitch, length, head type, property class or grade, finish, nut and washer requirement, and quantity. For flange bolting, stating the flange size and class helps confirm the stud length.
Common questions
- Can I use a lower grade nut with a high grade bolt?
- It is not recommended. The nut should be of an equal or higher class than the bolt so that the assembly develops the bolt's full strength.
- How are stud bolts for flanges specified?
- Usually by diameter, length, thread type, material specification and the matching nut specification, along with the flange size and class they are being used on.
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