Bolt torque & thread specs, decoded.

How do I read a metric bolt designation like M8 x 1.25?

The “M” indicates an ISO metric thread, the number after it is the nominal diameter in millimetres (8 mm), and the value after the × is the thread pitch in millimetres (1.25 mm). A coarser default pitch is used unless a fine pitch is specified.

What does property class 8.8 or 10.9 mean?

ISO metric property classes encode strength: the first number is roughly one-hundredth of the minimum tensile strength in MPa and the product of the two numbers gives roughly one-tenth of the minimum yield strength. Class 8.8 is ~800 MPa tensile; 10.9 is ~1040 MPa; 12.9 is ~1220 MPa.

How is max torque different from shear strength?

Max torque is the recommended tightening torque that develops the correct preload without over-stressing the bolt. Shear strength is the load the fastener can carry perpendicular to its axis before failing. A joint may be limited by either, depending on whether it is primarily a clamped tension joint or a shear joint.

Can I substitute a stainless bolt for a grade 8 steel bolt?

Only if the reduced tensile and shear capacity is acceptable. A2-70 stainless is roughly 700 MPa versus 1035 MPa for Grade 8. The trade-off is far better corrosion resistance, at the cost of strength and a higher risk of thread galling.