The Nuts and Bolts of Screws and Bolts

Grain bin bolts

In the building and manufacturing world, screws and bolts are two of the most ubiquitous fasteners — in fact, you’ve probably come across them just assembling a flat-packed piece of furniture. But what are the differences, and how should they be used? Here are the basics of what you should know:

  • Are They Bolts, or Screws?

    There’s actually no universally accepted distinction between a bolt and a screw, and there’s little reason to draw a fine line between them in most cases. But if you want to be as technically correct as possible, Machinery’s Handbook defines a screw as “an externally threaded fastener … tightened or released by torquing the head” and a bolt as “an externally threaded fastener designed for insertion through holes in assembled parts … tightened or released by torquing a nut. Basically, while a screw is tightened when the flat head on the top is turned, a bolt stays still while a separate nut shortens the distance between the head of the bolt and the working material. This means that while a screw generally is threaded for the majority of its length, a bolt may be threaded only near the end where the nut will be attached.

  • Bolt Manufacturer Markings

    Both screws and bolts come in various diameters that can help you determine what size you need for any given project. But you’ll also need to consider the materials these fasteners are made of. When you buy bolts, you will probably notice markings on their heads. This will give you information on the composition of the bolt, whether it is corrosion resistant, and similar details. Bolt manufacturer markings can also help you identify the maker of the bolt. You can find explanatory charts online if you’re trying to replace a bolt and want to match it to a new product.

  • When to Use Bolts

    For around-the-house projects, you’ll probably be able to find any fasteners you need at a standard home improvement store. For heavier-duty projects or anything that presents an engineering challenge, however, you may need specialty screws and bolts ordered directly from a manufacturer. Common applications include the automotive, aeronautic and heavy machining industries. You may also want to consider custom made bolts; custom bolts will, of course, cost a bit more, but can provide superior performance.

What other questions do you have about screws and bolts? Join the discussion in the comments.