HOME IMPROVEMENT : Handy Tips for Nuts, Bolts--in a Nutshell

Nuts and bolts are useful when you need a strong joint, when you might be taking the item apart later and when joining two pieces of metal or other thin materials. As with any fasteners, it’s important to choose the right ones and install them properly. Here’s how:

Choosing nuts and bolts

There are three major types of bolts:

* Machine bolts are made with unslotted square or hexagonal heads that accept a wrench and are used with either square or hex nuts. These bolts are available with fine threads, but coarse threads are more common.

* Carriage bolts have unslotted oval heads with a square shoulder beneath the head and are designed to be used with wood. The square shoulders sink into wood to keep the bolt from turning when a nut is tightened onto it. Carriage bolts have coarse threads.

* Stove bolts have slotted heads that are round, oval or flat and accept screwdrivers. They have coarse threads and are sold as general utility bolts. In smaller sizes, they are called machine screws and may have either fine or coarse threads.

* When buying bolts, you have to give not only the diameter and length you want but the number of threads per inch as well. This is designated by a number following the bolt diameter. Thus a 1/4-20 bolt would be a quarter inch in diameter and have 20 threads per inch. The bolt’s length is usually given after the thread-pitch number.

* When replacing damaged or lost fasteners, always use nuts and bolts of the same size and strength as the originals. A plain bolt head with no markings indicates a hardware grade bolt, which should not be used on automobiles or machinery.

The heads of bolts suited for use with machinery are marked to indicate their tensile strength--how much turning force, or torque, the bolt will withstand before breaking. Tensile strength is expressed in pounds per square inch or psi. The quality or grade is given as an SAE number set by the Society of Automotive Engineers.

Lines on the head of a bolt indicate its SAE grade. The grades generally range from SAE 3 (2 lines and a tensile strength of 100,000 psi) to SAE 8 (6 lines and a strength of 150,000 psi).

Installing nuts and bolts

To install a bolt, slide it into place and screw the nut on clockwise. Once the nut is partly on, hold the bolt head stationary with a wrench or a screwdriver (if the bolt head is slotted). Turn the nut by hand until it is snug.

Then tighten it with another wrench, but do not over-tighten. Use wrenches that fit correctly.

You can place washers next to the outer surfaces of the parts being joined to protect them from the pressure of the bolt head and nut. Lock washers or thread locking solutions, available at hardware stores, prevent nuts from loosening.

Removing nuts and bolts

If a nut is too worn to grip with a wrench, apply locking pliers as tightly as possible. If this fails, file two sides of the nut flat, then try the locking pliers again or an adjustable wrench.

If a nut or bolt is rusted or frozen, apply penetrating oil. Let it sit for a few minutes, then try the wrench again. Repeat if necessary.

To remove a nut and bolt that resists all these efforts, you have to damage the fastener.

Use a hammer and cold chisel to cut a groove into the nut near a corner. Place the chisel in the groove and pound it counterclockwise. If the nut still doesn’t turn, cut through it with the chisel or a hacksaw.

If you have to cut the nut loose or if the bolt snaps off while you are trying to remove it, use a bolt extractor, sold at hardware stores.