Bolts
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Frequently asked questions
What does 8.8 or 10.9 on a bolt head mean?
It is the property class under ISO 898-1. The first number times 100 gives nominal tensile strength in MPa, and the product of both numbers times 10 gives yield strength: class 8.8 is about 800 MPa tensile and 640 MPa yield. Classes 10.9 and 12.9 are stronger but less ductile, so use them where the design calls for it.
How do I measure bolt size and length correctly?
Nominal diameter is the major diameter of the thread, so M12 is 12 mm, and pitch is checked with a thread gauge. Hex bolt length runs from the underside of the head to the end of the shank, excluding the head. For countersunk screws, length includes the head.
Fully threaded or partially threaded: how do they differ in use?
A partially threaded bolt has a plain shank, which is better in shear when the shear plane falls on the shank. A fully threaded bolt is convenient when clamping varying thicknesses. ISO 4014 covers partially threaded and ISO 4017 fully threaded hex bolts.
Can high-strength bolts be reused after removal?
Not if they were tightened into yield or by the turn-of-nut method, because the shank has permanently stretched. Hot-dip galvanized class 10.9 structural bolts are also commonly specified as single-use. For critical joints, replace the bolt, nut and washer together.
What is the risk with electroplated class 10.9 bolts?
High-strength steel from about class 10.9 upward is prone to hydrogen embrittlement after electroplating if it is not baked afterwards. This causes sudden fracture hours to days after tightening. For high classes, prefer zinc flake coatings or specify hydrogen relief baking.
How much larger than the bolt should the clearance hole be?
ISO 273 defines three clearance hole series: fine, medium and coarse. For M12 these are about 13, 13.5 and 14.5 mm. The medium series is the usual choice in steel structures, while the fine series is used where accurate location matters.
What minimum thread engagement does a bolt need in a tapped hole?
In a steel tapped hole, engagement of about 1 times the diameter usually lets the bolt break before the internal thread strips. In cast iron increase this to about 1.25 to 1.5 times, and in aluminum to about 2 times the diameter. Where soft material lacks depth, a wire thread insert (helicoil) can reinforce the hole.
Should bolt threads be lubricated before tightening?
Lubrication lowers friction, so the same torque produces higher preload and can exceed yield if a dry-thread torque table is used. On the other hand, it gives more consistent preload across a bolt pattern and prevents seizing on removal. What matters is that the torque table matches the actual thread condition.
