The Basics of the Beguiling Bolt

October 3, 2022

Have you ever found yourself wandering through the fastener aisle at Wilco? Hundreds of little snapping drawers with odd descriptions and illustrations. How in the world are you going to find the right one for your project? This article will not only introduce you to the language of fasteners but also give you a good place to start when choosing one for yourself.

stainless bolts and washers on gray surface


First things first. Bolts are classified by grades. The bolt grade refers to the grade of steel used to make the bolt. The higher the number, the stronger the steel. The most common bolt grades you’ll find are these:

Standard Measurement

  • Grade 2
  • Grade 5
  • Grade 8

Metric Measurement

  • Grade 5.8
  • Grade 8.8
  • Grade 10.9

You might be tempted to always buy the highest grade to ensure your project stays together. But there are considerations to be made when choosing a grade. What are you holding together? And how heavy is the load? Bear with me as we look at a little physics.

We’ve learned that the larger the number, the stronger the bolt. Stronger bolts can handle more stress. They can also handle more torque – a fancy word for how much you can tighten them.

Problem is, the stronger a bolt is, the more brittle the steel, meaning it might break. And the softer the steel means it might bend. Most jobs require a grade 5 (8.8) or below. If you’re spending the money to buy a grade 8 bolt, you probably know why.

crescent wrench adjusting painted washer on bolt


Markings are pretty simple. Lines or numbers are placed on the head of each bolt. Metric grade goes with the obvious numbers that correspond with the grade – 8.8 is 8.8. But for standard grades, radial lines are used: no lines for grade 2, three lines for grade 5, and six lines for grade 8. I know what you’re thinking…why not use the same number of lines as the grade. Honestly, I think it’s to keep a sense of mystery to bolts. I mean, come on, they aren’t super exciting, to begin with.


When you choose your bolt, you also need to consider the type of material used to make it. The two most common materials are steel and stainless steel. They are inexpensive and can be used for most projects. Steel is a dull gray. Stainless steel is shiny. They are both susceptible to rust, although stainless steel is less so.

Aluminum is lighter and softer than steel but is more corrosion resistant. Brass and bronze are usually used for their coppery shine. Brass doesn’t rust at all so although it’s softer than steel, it beats steel for aesthetics and rust resistance. Bronze is stronger than brass but it will corrode like copper. Choosing between brass and bronze comes down to the environment.

Coating and Plating

Now that you know the main materials, zinc, galvanized, chrome, or copper can cover the bolts to either make them prettier or corrosion resistant. Zinc plating gives the steel a shiny silver or yellow hue and helps to add corrosion resistance. Hot dip galvanized is a dull gray but because it’s a thicker layer than zinc plating, it’s better at resisting corrosion. Chrome and copper are mainly for aesthetics.

The basic composition of a bolt comes down to the head, the shank, and the thread. The head is the top. The shank is the smooth section of the stem. And the thread is the lines on the stem


While most bolts you find are straight with a flat top, each head helps determine use. Two obvious ones are the eye bolt head – a closed or almost closed circle used for threading material through, and the u-bolt – a curved shape with threads on either end used to secure something to a surface.

There are three common heads. The square head is an older style and one that is easy to grip with a wrench. The hex head (six-sided) is a more modern style perfect for tight places because of the easier wrench rotation. When tightening, you don’t have to turn as far to get the wrench to the next flat edge. The carriage head is a dome-shaped top with a square bottom. It is most often used with a metal plate and a square hole. It locks the bolt in place without letting it turn.

Zinc bolts on gray woodgrain surface


Threads are the lines you see on the end of the bolt, swirling around the shank. If you look at a variety of bolts, you will see that some threads are fine – close together – and others are coarse – wide apart. Determining the pitch is important for bolts to fit together with nuts. For standard bolts, the thread pitch is how many lines in an inch. For metric bolts, the thread pitch is how far the lines are apart. I find the latter to be difficult and usually do it by sight comparison.

Putting It Together

Now that you know more about bolts than you ever thought possible, let’s put it all together so you can sound like a pro. Answer these questions and you’ll get exactly what you want.

  1. What material – steel, stainless, aluminum, brass, or bronze?
  2. What coating, if any – zinc plated, galvanized?
  3. What is the diameter of the shank (the straight part)?
  4. What is the thread pitch?
  5. What is the length from below the bolt head to the end of the bolt?
  6. How long do you want the unthreaded portion of the bolt – all thread or industry standard?
  7. What grade – 2, 5, 8, 5.9, 8.8, 10.9?
  8. What head type – square, hex, carriage?

Now you can put it all together and ask your Wilco hardware specialist for exactly what you want. “I would like a steel galvanized ¼ inch x 20 x 3-inch grade 2 hex bolt head with all thread.” Whew. That’s a mouthful. But it’s clear and will help you find the right bolt for the job.

A Note About Nuts

All of the information above relates to nuts as well. Matching the materials, coatings, thread, and diameter are very important. A grade 8 nut may strip the threads of a grade 2 bolt. And a steel nut may corrode on a brass bolt. Even better than reading the box, try fitting the bolt and nut together. That’s a surefire way to get a matching set.

I bet you didn’t realize how complicated something as simple as a bolt could be. But when you get down to the nuts and bolts, it’s quite riveting. So whether you’re heading to the store to buy parts for a new project or you’re rummaging through the bucket of random fasteners in the back of the shed, you should now be able to identify the type of bolt for all your needs.