Hex Bolt (GB/T 6173 / ISO 8675) are specified where the joint requires structural and machine joints that must be tightened to a controlled preload. The sections below cover the typical end-uses by industry, the size and material decisions for each, assembly practice, and the applications this fastener type is not suited to. Stocked across roughly M3 to M36, they cover general to heavy-duty work.
Typical Applications for Hex Bolt — GB/T 6173 / ISO 8675 (Series 057)
The most common settings where these are specified:
- Maintenance & repair (MRO): a stocked size for servicing existing plant where the original fastener spec must be matched.
- Fabrication & metalwork: bench and on-site assembly of steel frames, enclosures and weldments.
- Structural steel: bolting beams, base plates and bracing where a defined preload and property class are required.
- Automotive & transport: chassis, suspension and powertrain mounting points subject to vibration.
- Heavy machinery: securing gearboxes, frames and guarding on plant that sees shock loads.
How to Specify Hex Bolt — GB/T 6173 / ISO 8675 (Series 057) for Your Application
- Size: Match the nominal size to the mating thread or hole. This product spans M3–M36; check the full table below for the exact dimensions of each size.
- Strength class: Choose the property class (e.g. 8.8, 10.9, 12.9 or A2/A4 stainless) for the load and environment, and tighten to the matching torque so the joint relies on preload, not friction.
- Environment: For damp, coastal or chemical exposure prefer A4/316 stainless or a suitable coating; indoors, plated steel or A2 is usually sufficient.
- Standard: This product is supplied to GB/T 6173 / ISO 8675. Quoting the standard on your order guarantees interchangeable dimensions between suppliers.
Installation & Assembly Practice
- Tighten to the torque for the size and property class with a calibrated wrench — the joint is held by preload, not by the thread friction alone.
- Ensure at least one full thread projects beyond the nut after tightening, and never re-use a bolt that has been tightened into yield.
- Under slotted or oversized holes, fit a hardened washer so the preload does not relax into the hole edge.
Common Selection Mistakes with Hex Bolt — GB/T 6173 / ISO 8675 (Series 057)
- Specifying class 12.9 "to be safe" in a corrosive environment — high classes are more sensitive to hydrogen embrittlement and stress corrosion; 8.8 hot-dip galvanized often outlasts them outdoors.
- Choosing a length that puts the shear plane through the threaded portion when a plain shank (partially threaded bolt) was available.
- Mixing stainless bolts with plated-steel nuts: the joint loses its corrosion logic and can gall or seize.
Where Hex Bolt — GB/T 6173 / ISO 8675 (Series 057) Are Not the Right Choice
Not for permanent joints that will never be opened (a rivet or weld is cheaper), and not where the head must sit flush — use a countersunk screw instead.