How a bolted joint works
A bolt is a threaded rod with a head on one end. It passes through a hole in the parts being joined, and a nut on the other end is tightened until the parts are squeezed together. The squeeze, called clamp load, comes from the bolt stretching very slightly like a stiff spring. That clamp load keeps the joint tight and lets friction between the parts carry some of the sideways force as well.
This is different from a screw, which cuts its own thread into the material. A bolt needs a through-hole and access to both sides, but in return it can be tightened repeatedly and removed without wearing out the material. The wider picture of how bolts compare with other fastener types is covered in the overview article.
Bolts versus machine screws
The line between a bolt and a machine screw is fuzzy in everyday use. Bolts are generally larger, used with a nut, and meant for structural or mechanical joints. Machine screws are smaller and often thread into a tapped hole rather than a nut. Carriage bolts have a smooth rounded head and a square neck that bites into wood so the bolt does not spin. Hex bolts have a six-sided head tightened with a wrench or socket. Lag screws, sometimes called lag bolts, are heavy wood screws with a hex head and are not the same thing as a bolt with a nut.
Thread types
Threads come in a few families, and the nut must match the bolt exactly.
- Coarse threads have fewer, deeper threads per inch. They go together quickly, tolerate dirt and minor damage better, and are the usual default.
- Fine threads have more threads per inch. They allow finer adjustment and can hold more securely against loosening from vibration, but they cross-thread more easily and are more easily damaged.
- Imperial and metric. Inch-based threads are described by diameter and threads per inch. Metric threads are described by diameter and pitch, the distance between threads in millimeters. The two systems do not interchange, even when a diameter looks close. The difference between metric versus SAE sizing causes plenty of confusion in garages.
If you are trying to identify a bolt, a thread gauge or the matching nut is the reliable test. Forcing a nut that does not spin on freely is a sign of a mismatch, and it can ruin the thread. The topic of thread pitch and repairing damaged threads is covered separately.
Strength grades in general terms
Bolt strength is shown by markings on the head. Inch-sized bolts use a system of radial lines: a plain head generally indicates a lower-strength bolt, and more lines generally indicate a stronger one, with the common grade numbers rising in step. Metric bolts instead carry a property class number such as 8.8 or 10.9, where higher numbers indicate higher strength.
Higher is not always better. Harder, stronger bolts can be more brittle, and they cost more. For general household and woodworking use, standard-strength hardware is often enough. For vehicles, lifting equipment, trailer hitches and anything where failure could injure someone, use the grade specified by the manufacturer, never a lower one. Tightening to the specified value matters too, which is where torque settings come in.
Nuts
The most common nut is the hex nut. A few other types solve particular problems:
- Lock nuts have a nylon insert or a deformed section that resists loosening from vibration. The nylon type is usually not reused many times.
- Wing nuts tighten by hand, useful for adjustments that change often.
- Acorn or cap nuts cover the exposed thread for a finished look and to avoid scratches.
- Flange nuts have a built-in washer-like collar.
What washers do
Washers are small, but they do three jobs.
Flat washers spread the clamp load over a larger area. Without one, a nut or bolt head can dig into soft materials such as wood and slowly crush it, which loosens the joint. A plain flat washer also protects painted or finished surfaces from the turning nut.
Fender washers have a much larger outside diameter for the hole size. They are helpful on thin sheet material or when a hole is slightly oversized.
Lock washers are meant to resist loosening. Split lock washers press against the nut and bite slightly; toothed ones grip the surface. Their effectiveness is debated, and opinions vary, so for vibration-prone joints many people turn to lock nuts, thread-locking compounds or other designs the manufacturer recommends.
Washers also matter for corrosion. Using a washer of a different metal can set up a galvanic reaction in damp conditions, so the washer should generally match the bolt and nut. The article on fastener corrosion explains this in more detail.
Using them well
- Match materials. Use the same metal and finish for the bolt, nut and washer when you can.
- Choose the right length. The bolt should extend past the nut by at least a thread or two. Too long, and it can get in the way or reduce the effective strength of the joint where unthreaded shank would be better.
- Drill a snug hole. An oversized hole lets the joint shift. A drill bit sized to the bolt diameter, or just slightly larger, is typical for wood.
- Tighten properly. Tighten until snug and then follow the specification. In wood, over-tightening can crush the fibers beneath the washer. When a spec exists, use a torque wrench.
- Do not reuse worn parts blindly. Stretched, rusted or damaged bolts and nuts, especially lock nuts, are worth replacing.
Safety notes
Wear eye protection when cutting bolts or driving them with power tools. Use a wrench or socket that fits snugly; a loose fit rounds the corners of the head. Never substitute a bolt of unknown strength for a critical part. This is general information, so consult the manufacturer’s instructions or a qualified professional for safety-critical work.