How to Choose Bolts, Nuts and Washers

A hex bolt gets its name from the distinct six-sided hexagonal head shape. Hex bolts are used for wood to wood or metal to wood connections. Some hex bolts (called tap bolts) have threads running the full length of the shank. They provide good grip strength because they distribute the load along the entire bolt. The thicker, unthreaded shoulder portion of a partially threaded bolt provides additional strength to the fastening job. Hex-head bolts are commonly used in construction and automotive applications.

The domed head of a carriage bolt gives a smooth, finished appearance and adds a measure of safety and security for wood to wood connections. These bolts are installed through predrilled holes and secured with washers and hex nuts. Carriage bolts have a square area under the head that helps lock the bolt to the work piece, making tightening the nut easier. Carriage bolts are used for projects such as decks, furniture and outdoor playsets.

Anchor bolts are designed to be embedded in concrete foundations to make connections with other structural pieces. Bolts can be placed in both floors and walls.

L-bolts are used in wet concrete. As the concrete hardens, it holds the bolt with the threads exposed. L-bolts can allow you to secure a deck post to a concrete pad. 

Toggle bolts are anchor bolts that provide support for hanging items when it's not possible to drill into a wall stud.

U-bolts have two threaded shanks with either a round or flattened shape. Rounded U-bolts allow you to secure pipe or conduit to a surface, while squared U-bolts can fasten items to a surface such as a square post. Two nuts and a metal plate secure the U-bolt against the item you're fastening.

Eye bolts have a loop or ring end (eye) and a threaded end, allowing you to secure chain or tie down rope to wood, metal or concrete surfaces. These bolts are extraordinarily strong and are also used for lifting applications. Eye bolts are designed for predrilled holes and include a matching nut. Screw eye bolts are similar but have coarser self-tapping threads and a pointed tip for driving into a pilot hole in wood.

A hanger bolt is a unique headless fastener with machined threads on one end and coarse self-tapping threads with a point on the other. This allows for driving the bolt into a predrilled hole in wood or similar materials. As the name states, these bolts are used for hanging materials or objects like a porch swing from wood beams and boards.

Split bolts are designed for secure electrical connections. They allow splicing of two wires and allow you to secure cables to terminals. Split bolts made of different materials work with different types of wires. Make sure your bolt is correct for the type of wire you're using.

Body bolts are often used in the automotive industry. They're designed to fasten fenders and other parts to a vehicle body. They include a washer and have a hex head and a threaded shank.

Axle bolts help attach wheels to equipment such as lawn mowers and wagons. The threaded part of the shank accepts a locking nut, while the smooth portion of the shank close to the head allows the wheels to rotate freely.

Hex nuts have a six-sided drive surface to match hex bolts. They're a common type of nut used with bolts of matching size and thread type to secure wood and metal components.

Nylon lock nuts have a hexagonal head with a built-in nylon ring. As you install a lock nut onto a bolt, the bolt threading displaces or deforms the ring, increasing friction and helping prevent loosening caused by slipping or vibration.

A castle nut is a type of lock nut that works with a pin or clip inserted through a hole in the bolt. Notches cut into the nut prevent rotation when they stop against the pin.

Wing nuts are designed to allow fastening without the use of tools; the wings allow you to tighten or loosen them with a thumb and finger. These nuts are used to allow quick and easy modifications and removals. You might see them on drum kits, light stands and other items that need frequent adjustments.

Cap nuts or acorn nuts have a domed shape and hexagonal driving surface. You can often install them by hand. They screw onto the exposed threads on a bolt or threaded rod to protect against injury, damage and to add a decorative look. You'll see cap nuts on projects such as outdoor playsets and fences.

Square nuts have four relatively large driving surfaces, making them easy to grip and turn. The design of the nut allows more surface area to contact the piece that its fastening, providing more resistance to loosening and tightening. Use flat washers with square nuts to prevent the edges of the nuts from damaging the work piece.

Flat washers, sometimes called plain washers or standard washers, is the most common washer and is used in general repairs and assembly applications. A flat washer can minimize friction on the work piece caused by installing a bolt or nut. In addition to spreading the force of a nut or a bolt over a larger area, you might also use a flat washer to prevent a nut from digging into the material and weakening the connection. In some applications, flat washers can function simply as a spacer.

Fender washers are shaped similar to flat washers but are thinner with a comparatively large outside diameter relative to the center hole. This design gives a fender washer more surface area to distribute the load from the fastener across a wider area, making them especially effective when working with softer materials. Fender washers get their name from use in automotive work, but they’re also used in electrical and plumbing applications along with sheet metal work.

Rather than distributing pressure on a work piece, lock washers increase friction to help prevent a nut and bolt fastener from vibrating or rotating loose. There are several types. 

Split lock washers or spring washers look like a broken ring. They can help keep a fastener from loosening due to vibration. The ends of a split lock washer are bent in opposite directions to create spring pressure between the work piece and the nut or, if a nut isn’t required, between the work piece and bolt head. The pressure helps hold the fastener in place, preventing it from backing out. The ends on the washer can also bite into the work piece and the bolt or nut, further preventing the fastener from reversing out of the work piece. Note that if it’s tightened too much, a split lock washer will lose the spring action and not lock as well. 

Wave lock washers look similar to a standard washer except, as the name implies, they’re not flat. They have a wave shape that creates a spring action when compressed, similar to that of the split lock washer. 

Toothed lock washers can have external or internal teeth that create friction against a fastener and, once secured, can bite into the nut or bolt to create resistance against turning. You’ll usually see external-tooth lock washers on larger bolt assemblies. An internal-tooth lock washer creates a cleaner appearance. They’re often used with smaller fasteners. Both types work best in materials such as plastic and softer metals.

Finishing washers, also known as countersunk washers, are a type of washer for use with screws. They have a concave surface that allows a flathead screw to countersink against the washer, providing a neater appearance. Some finishing washers are designed for specific types of screw heads.