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What Are Bolts

A bolt is a form of threaded fastener with an external male thread (a fastener being something that mechanically joins objects together) requiring a matching pre-formed female thread such as a nut. Bolts are very closely related to screws.

 

If shown the above image, most people would be able to point out which one is a screw, and which one is a bolt. But the actual distinction between a bolt and a screw isn’t very clear. The academic distinction, per Machinery's Handbook - pretty much the bible of the USA’s metalworking industry since 1914, is in their intended design:

• Bolts are designed to pass through an unthreaded hole in an object and be fastened with the aid of a nut. However, a bolt can be used without a nut to tighten into a threaded object such as a nut-plate or thread tapped housing.
   
• Screws are used in objects which contain their own thread, or to cut its own internal thread into them (think: wood screws).

The vagueness in these descriptions means most people will identify a bolt or screw depending on what it’s used for, and the terms screw and bolt are mixed and matched by people in different countries all around the world.

So What Are Bolts

• Bolts are often used to make a bolted joint.
• This is a combination of the nut applying a clamping force and the shank of the bolt acting as a rod, preventing sideways movement shear forces. For this reason, many bolts have a plain unthreaded section on the shank as this makes for a better, stronger bolt.
• Note: The unthreaded shank has often been given as the defining trait of a bolt compared to a screw, but this is the way only some bolts are designed, and other bolts have a thread their entire length.