There are many different kinds of mechanical fasteners on the market, and while many of them look the same, they can have very different properties. The material they are made from, the strength of the fasteners, and the threads on them can all be different, so you must understand how to identify the hex head bolts that are right for your needs.
One of the most common materials used for hex head bolts is mild steel, but even within that one material, there can be many different strengths and properties. You will need to consider all the properties that are important before selecting the fasteners for your job.
Stainless steel hex head bolts are also very common and are a good fit for fasteners that will spend a lot of time outside or exposed to weather, salt, and other corrosive materials. Stainless steel hex head bolts are not always stronger than steel, but they are often more durable in harsh environments.
If you are not sure what you need, you may want to visit a fastener supplier and talk to them about what you are doing with the fasteners. The supplier can show you many different options and help you determine the strength, size, and length of the hex head bolts that you need.
The threads on your fasteners can be a little confusing, and many times you will find the same size fastener with fine and coarse threads. Selecting the right thread could be significant, so if you are going to be using a nut on the hex head bolts, you can choose the one you prefer, but if the bolt is threading into a capture nut or a hole that is tapped to receive it, you need to make sure you have the right thread.
If you have the original bolt, you can match the threads, and any fastener supplier can use a thread gauge to measure the pitch to ensure you get a suitable replacement. The finer threads are typically stronger because more threads are holding the tension, but often, a coarse thread is more than strong enough for the job and can be easier to deal with because they are more common and easier to work with.
Keep in mind that there are also metric threads, commonly fine threads, and some hex head bolts have reverse threads on them. Reverse thread hex head bolts are not as common unless the bolt secures a rotating assembly that would unscrew a standard bolt, but they are out there, so it is essential to recognize them.