Bolts are an amazingly simple yet complex fastening method. They’re used everywhere so mostly we take them for granted, but why are there so many different kinds? And what bolts should I use?
The screw concept originated only a few hundred years BC according to the Bolt Science History page. While the concept is so common now, it wasn’t until mass production a little more than a 150 years ago that things began to standardize and become widespread.
I don’t normally think of our modern bolts as a “newer” technology, but in the course of world history, it’s really not that old. The usefulness is evidence everywhere. Hardly a household in the world doesn’t have a screwdriver or a wrench or pliers of some type for tightening or loosening screws.
Now there is the proliferation of sizes, head types, thread counts, lengths, and so much more. So how do we know what to use? Fortunately, we don’t need to be experts to choose the right bolts for DIY projects. However, we do need some basic knowledge so that we don’t botch something important.
What Bolts Grade?
The strength of the bolt material (steel specifically) is the “Grade”. A weak bolt is fine for some applications, but might cause disaster in another. On the other hand, sometimes you want a weak bolt so in a critical situation, the bolt will break instead of . . . say the machine which is much more expensive.
The grade has to do with the material of the bolt. Basically the alloy of steel to make it, as well as the heat treating process to make it strong. I won’t go into detail, but think of it as different alloys (or types) of steel.
The actual strength of the bolt includes the material as well as the size of the bolt (1/4″ or 1/2″ etc.). A low grade 3/4″ bolt is much stronger than a 1/4″ high grade bolt, for instance. So, what bolts you choose must take several factors into consideration.
Here several common bolt grades. These are the ones that are really helpful to know.
For English Bolts:
Grade 2 (or ungraded), Grade 5, & Grade 8. There are lots of others, but these are the basics. Markings on the bolt (hex) heads (radial lines) show the grade. It’s a pretty simple code. In the image below, the middle bolt has 3 radial raised lines. That’s a grade 5 bolt. The goldish one has 6 radial raised lines. That’s a grade 8 bolt. Other markings on the bolt don’t really matter (for bolts 101).
The Grade 2 bolt is on the left. This one has several letters that tell details, but there are no radial marks. That’s a Grade 2 or less.
Interestingly, while working on a project with my son one day, he noticed that they are just the number of marks plus 2 is the grade. I’ve used these codes for years and never noticed that obvious tid-bit.
For Metric Bolts:
Grades < 5.8, Grade 8.8, 10.9 & 12.9 Again, there are lots of variations, more than in English, but the cool thing is most have the marking right on the head. Have a look at the image. This is not all, but it is the most common, and it will give you the idea. For more information, here is a pretty good explanation of what the numbers mean.
Notice that the 12.9 is a Socket Head Cap Screw, rather than a Hex Head like the others. For both English and metric bolts, the socket heads are effectively all made of this higher grade steel. Often they are black, but color is not a good designation. We’ll talk about that below.
Why are Socket Heads stronger? Some say it’s so the sockets don’t strip with the relatively small wrenches, others say it’s to minimize the number of bolts that have to be produced. Still others say it’s because you can’t really mark them very well. Whatever the reason, it’s just a good thing to keep in the back of your mind. Socket head bolts are really strong.
Additionally, socket heads come in several varieties — like the one above, or flat head, or button head, etc..
Rough Equivalence of English v. Metric Grades:
So how do these different grades relate? Is one that much stronger than another? How do the Metrics compare to the English grades? Here’s a simplified chart showing comparison for the bolt grades.
Please Note. There are a lot of specifics missing in the chart above. For instance, tensile strength numbers are rough, and tend to decrease for large bolts (over 1″ or 1.5″ diameter). There are many other caveats and variations which are well documented on many websites. No need to repeat them here. These are the basics, sufficient for DIY.
For DIY, the takeaways from this chart include:
- Grade 5 bolts are roughly twice the strength of grade 2 or lower.
- Grade 8 and the socket heads are even stronger.
- The tensile strength numbers are not so important as the relative perspective.
- Metric bolts have similar strengths, but very different designations.
It’s worth noting that Other Materials like Plastic, Brass and even Stainless Steel don’t use these grades. These types also vary in strength, so be careful using them if strength is important. They do not have markings like above.
Again, these are the basics. For more detail, search the web. There is a ton of great information on bolt and grade variations and differences.
There are so many different Types of bolts it’s enough to make your head spin. While they all have their purpose, we’re only going to cover some of the basics. We’ll start with Head Style.
This image has some examples of bolt head styles. Most of these are 1/4″-20. Here 2 “button head” bolts are shown – one in Stainless Steel, and one Grade 8+. The smaller bolt on the right is a pan head, Philips. A “flat head” is in the center (sometimes called a Flat Head Socket Cap Screw, or FHSCS”. Bent bolts, like the U-Bolt on the left are almost always Grade 2 or less (but, of course, you can get them in higher grades if needed).
Bolt Head Styles:
Most of the common bolt head styles fall into 3 categories:
- Wrench Head where the wrench goes around the head (like a square head, or a hex head, etc.). The gold color one in this image is one example of a Wrench Head.
- Socket Head where the wrench goes into the head (SHCS, Button Heads, Flat Heads, Philips Heads, etc). The middle 2 black bolts are variations of the Socket Head. Sometimes they are hex key sockets like these, yet others, like Torx, are common.
- Retained Head where features on the underside of the head retain it from spinning. The longer silver one is a Retained Head (commonly called a carriage bolt).
- And we shouldn’t forget the No Head custom bolts made from Threaded Rod (aka All-Thread) or even the extra super specials you make yourself with a Thread Die.
Other styles exist, but are not as common. Like trailer wheel bolts, for instance.
Wrench Head bolts are convenient when there is room to surround the bolt head — like with an open end wrench or ratchet socket. They are convenient to grab from the side. And, they come in a variety of heads — like square heads and the flange head (in the small image) — which are very useful for DIY.
Socket Head bolts are convenient because you can place them down into something (countersink hole). They also work on flat surfaces. Buttons (pan heads and others) have a purpose for rounded heads that don’t protrude sharply or have edges to catch. Finally, Flat Heads can go into the work and leave a “Flat” surface.
The image below shows a countersunk Socket Head Cap Screw, abbreviated SHCS, and a Flat Head shown in a cutaway CAD model so you can see how they interact with the work pieces.
Retained Head bolts are great when you don’t want to interact with the head, or you want tamper resistance. For a carriage bolt (like the one pictured above) you place them in a square hole, then you don’t have to hold the head when tightening the bolt.
Your choice for “What Bolts” should reflect the need for access, as well as the aesthetic and clearance desire.
More About Bolt Heads:
I really like this Fastener Type Chart from Bolt Depot. Someone put a lot of work into it with the drawings of many head types, fastener styles — even nuts and washers. That’s a good reference page.
What Bolts Threads?
Again, this is a big bag of options. The traditional tread is a triangle shape using 60 degree angles. That’s by far the most common, but more types like ACME, Spiral Lock, and others exist. Those have a purpose, but we won’t go into them here.
To confuse the issue, there are a bunch of different “Pitches” with each size too. The pitch is the number of threads per unit length. For instance, 18 threads per inch. Or, in metric, pitch reads as the distance between threads — like M10 x 1.5 — where 1.5 is the distance in millimeters between the threads.
Without going into crazy detail, there are typically 2 “pitches” that matter — Fine Thread and Coarse Thread. Those descriptors are good, because if you look at the example photo here, you can see a big difference between the Fine Thread and the Coarse. Fine just means there are more threads, closer together.
While Metric bolts are similar in allowing different thread pitches for each diameter, they don’t usually refer to them as “Fine Thread” or “Coarse Thread”. Ask your bolt supplier or search the web for more information.
In general, Coarse threads are nicer to work with because you don’t have to turn them as much to insert them or put a nut on. Fine threads can tighten more because they have a reduced action angle. Arguably the Fine thread is marginally stronger than Coarse, but not enough to make decisions on strength based only on thread pitch.
One other minor difference, in a vibration environment, fine threads have an advantage. Yet, other methods like Nylock nuts, LocTite, etc., are much more effective.
In the images above we see 3 different finishes. First is the silver — which is a zinc (in these photos). A silver color can also be the result of hot-dip-galvanize, of chrome, of no finish and of Stainless Steel.
The Gold color is “Zinc Yellow-Chromate” which withstands contact stress better, so we sometimes see this on Grade 5 or Grade 8 bolts. Like the Silver, the Gold doesn’t really mean anything.
The other color in the images above is Black. These are a “Black Phosphate” finish. Again, the black color doesn’t really mean anything, but it’s very common in the Grade 12.9 (metric) and English Socket Heads (effectively a Grade 9).
While lots of other finishes and colors exist, these are common.
Most bolts have some kind of finish (except usually Stainless Steel). Different finishes handle different conditions better or worse, so learn what you need. Zinc is a great general purpose protection from rust if the bolts are not submersed or in much direct contact with water, but Hot Dipped Galvanized or Stainless steel do better for wetter and more exposed conditions. The black does well for indoor and dryer conditions.
Descriptive Bolt Names
Bolts are named by their size, head type, thread pitch and finish. For instance, an English bolt designation might be: 1/2″-20 HCS Gr5 in Zinc. For this example, the 1/2″ is the nominal bolt outside diameter (in inches). The actual diameter of the threads will be less than that, and the shank (unthreaded portion if it has it) will be very close to this diameter. The -20 in the name is the threads per inch.
In the name, sometimes letters appear or sometimes the words are spelled out. In this case HCS = Hex Cap Screw, which means it has a hex head. Others like SHCS = Socket Head Cap Screw or FHS = Flat Head Socket or BH = Button Head or PHP = Pan Head Phillips, sometimes appear. If you’re not positive, it’s best to skip the letters and just use the words to avoid confusion.
The bolt grade, Gr5 (from the example above), is often left out of the name, but is needed to define the bolt. In this case, Gr5 is Grade 5. This is important because it says the material and defines the strength. Not all bolts have grades. Some materials like SS (Stainless Steel), or Brass, or plastic, and others don’t have grades like steel. Yet, they do come in different variations and strengths. Stainless Steel, for instance, has many types, but they don’t have a “grade” like above.
Finally, the finish. In our example it’s Zinc.
For defining what bolts you need, include all the options so you get what you want. Sometimes it doesn’t matter, like finish, for instance. That’s OK.
How to Choose Your Bolts
This post is getting unwieldy in length, so we’re going to break it in two. The topic of How to Choose (and use) Bolts is important and very relevant, so let’s consider the above information for background, and go to rules of thumb for making bolt choices. Continue Reading >>
Thanks to Lightning Bolt of Colorado Springs for insight with this article.