All About Bolts

Learn more about bolts so you'll use the right hardware for your projects.

Is there a DIYer who doesn’t keep a stash of miscellaneous bolts to replace ones that get lost, broken or deformed? We’re all guilty of going to the well of mystery bolts and fishing out whatever size works to fix the problem at hand. But we really should be considering whether that random bolt is appropriate for the application.

On the surface, bolts appear to be quite simple. Without researching the topic, most of us don’t know much beyond the fact that a Grade 8 bolt is stronger than a Grade 5. Understanding bolt’s anatomy, how strength is measured and what grades mean will help you to navigate the various sizes, strengths and styles and find the right fastener for your situation.

Bolt basics
In most cases, bolts are made of low- or medium-grade carbon steel, but they can also be made of alloy steel for heavier-duty applications. The process starts with a large coil of steel wire, which is cut into bolt-length pieces; then the head is formed and the shank is threaded.

If the bolt is destined for a heavy-duty rating, it will go through processes known as quenching and tempering (Q&T) to harden and strengthen it. Quenching involves heating the bolt and rapidly cooling it; then the bolt is reheated to temper it.

A bolt’s ability to withstand deformation and breakage is what qualifies it for a specific strength grade or property classification (see table, below). There are four common grades for both standard and metric bolts. In addition, there are three strength categories that represent stress levels the bolts must endure as a part of a given grade or classification:

  • Proof load is the axial tensile load the bolt must withstand without evidence of permanent deformation.
  • Yield strength is the maximum load at which the bolt exhibits a specific permanent deformation.
  • Tensile strength is the maximum load in tension (pulling apart) the bolt can withstand before breaking or fracturing.

Visual cues can help you to identify different bolts. Grade 2 bolts, which are made of low- or medium-grade carbon steel, feature a blank head and are the weakest of all standard bolts. The next strongest are Grade 5 bolts, which are identified by three radial lines on the head. These bolts are made of medium-grade carbon steel and have undergone the Q&T processes for increased strength. The strongest commonly found standard bolt is the Grade 8. These bolts feature five radial lines on the head and are made of Q&T medium-carbon-alloy steel. Rounding out the standard bolts are 18-8 stainless steel bolts, which feature blank heads and are made of steel alloy with 17 percent to 19 percent chromium and 8 percent to 13 percent nickel.

On the metric side, the different bolt classifications have standard equivalents. The weakest of all common metric bolts is the Class 8.8, which is made of Q&T medium-carbon steel. Considerably stronger than the Grade 2, the Class 8.8 is roughly equivalent to a Grade 5 bolt. Similarly, the Class 10.9 bolt, which is made of Q&T alloy steel, is comparable to a Grade 8 bolt. Of commonly found metric bolts, the Class 12.9 is considered the strongest. It is made of Q&T alloy steel (much like the Class 10.9) and features strength characteristics beyond those of a Grade 8 bolt.

Soft vs. hard
Many people mistakenly think that for some applications it’s better to use a “softer” bolt, such as a Grade 5, instead of a “harder” or more brittle bolt, such as a Grade 8, because the bolt will bend instead of break. This is never true.

As evidenced by the strength capacities (see table), a Grade 5 bolt’s minimum yield strength (the point at which it will deform) is 92,000 psi, and its minimum tensile strength (the point at which it will break) is 120,000 psi. In comparison, a Grade 8 bolt’s minimum yield strength is 130,000 psi, and its minimum tensile strength is 150,000 psi. It’s obvious that the soft vs. hard question should not be an issue — a Grade 5 bolt will break before a Grade 8 bolt even begins to bend. Overall strength is what counts, and the stronger bolt will always win. Although a Grade 8 bolt costs slightly more than a similar-size Grade 5, you shouldn’t compromise strength or safety to save a few dollars (or cents).

Fine vs. coarse
Another common misconception is that coarse-thread bolts are stronger than fine–thread bolts because the thread pitch is longer (wider) and the threads are thicker overall. Although both styles offer benefits, fine-thread bolts are stronger in tension and shear strength than similar-size coarse-thread bolts because fine-thread bolts have a larger stress area (spread out over more threads) and a larger minor diameter (measured from inside the threads). They also have less chance of loosening because of their shallower thread incline, and their smaller thread pitch allows for finer adjustments.

The drawbacks are that fine threads are more susceptible to cross threading and require longer thread engagements to prevent galling, which makes them less desirable for applications where high-speed assembly is necessary. Most fasteners in automotive applications are coarse-thread because dirty environments can be destructive to fine-thread bolts.

With all of the options to consider when selecting fasteners for automotive, mechanical or home-improvement projects, keep in mind the strength levels of each bolt grade and the manner in which they will be used. Not all fasteners must be of the highest strength, but choosing high-grade bolts for parts under stress or heavy loads is an excellent way to prevent failure. Spending a few extra dollars on heavy-duty hardware will keep you safe and give you peace of mind.