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Should I use Coarse or Fine Threaded Bolts?


What is the difference between fine and coarse threaded bolts?

Bolts with coarse threads have a larger pitch (fewer threads per axial distance) compared to fine threads. A coarse threaded bolt is specified for most applications unless there is an overriding reason to use a fine threaded bolt (e.g. thorough thread adjustment is crucial for the application). Furthermore, fine thread fasteners are more difficult to obtain.

Negatives of using fine threaded fasteners:

  • Fine threads are more prone to galling than coarse threads.
  • They need longer thread engagements and are more prone to damage and thread fouling.
  • They are also less suitable for high-speed assembly since they are more likely to seize when being tightened.

Positive benefits of fine threads are:

  • Size for size, a fine thread is stronger than a coarse thread. This is both in tension (because of the larger stress area) and shear (because of their larger minor dia­meter).
  • Because of the smaller pitch, they allow finer adjustments in applications that need such a feature.
  • Fine threads can be more easily tapped into hard materials and thin-walled tubes.
  • Fine threads require less torque to develop equivalent bolt preloads.
  • Fine threads have less tendency to loosen since the thread incline is smaller and hence so is the off torque.

WA Fasteners stock a huge range of both Coarse and Metric Fasteners.