Home Improvement : The Nuts and Bolts of Fasteners
What could be simpler than going to a local hardware store and picking up a handful of bolts? Well, programming your VCR, blindfolded, with mittens on, for one thing. You’ve probably experienced this scenario before:
You walk into the hardware store and head straight for those little boxes of bolts, hoping you can find what you want before a salesman can “help” you with a long list of questions.
“Did you want a carriage bolt, machine bolt or stove bolt? May I show you something in a stainless machine screw. What size? Fine or coarse threads? Need a specially hardened grade?”
Hopefully, you’ve brought along an example of what you want. Otherwise you can get lost after the first round of questions.
Arm yourself with a few basic facts, however, and next time you go bolt shopping, you can be the one in charge. Let’s take a look at the basics.
Sizes:
Bolts are sized according to diameter, thread and length in that order. In most cases, diameter and length are listed in inches, and thread in threads per inch. Thus a one-fourth-inch diameter bolt, with 20 threads per inch, two inches long, would be listed as a one-fourth inch x 20 x 2 inch or quarter/20 x 2 inch.
Sometimes, threads are listed as UNF (fine) or UNC (coarse) instead of per inch. Just a little variation to keep things interesting.
Why have different types of threads? Fine threads make a stronger bolt that can be torqued up tighter than a bolt with coarse threads. They are also better on smaller bolts. On the other hand, coarse threads are tougher. They’ll hold better in high-wear situations, or when you may have to remove or replace the bolt frequently. They also resist cross threading better than fine threads.
If you have to buy a bolt to fit a specific nut, take the nut with you to the store and make sure you can thread it onto the bolt you buy. Thread it all the way on. Sometimes a nut will go on a turn or two even if it doesn’t match the thread of the bolt.
More on sizing: Surprise! Not all bolts have diameters listed in inches. Machine screws are sized according to number. The smallest readily available size is No. 6 (one-eighth inch); the biggest is No. 14 (5/16 inch).
And don’t forget metrics. You may run into these if you work on an imported car or bike, for example. Metric hardware is sized according to a whole different set of standards and it won’t interchange with other hardware. You’ll even need a set of metric wrenches to fit the metric heads.
Speaking of heads, that opens a new group of variables. Some bolts have square heads, others have hex heads. Which are better? If you have a set of socket or box wrenches, you’ll like hex better. Your wrenches will fit them, and you’ll be able to work faster, and in tighter places.
If you work mostly with adjustable wrenches, you might prefer square heads. A sloppy adjustable wrench can slip and round off a hex head in short order but it should have no trouble at all gripping a square head.
OK, enough about sizes and heads. Let’s look at some of the basic types of bolts.
Stove bolts: These have slotted, screw-type heads, either round like the head on the top left in the sketch, or flat. They come with course threads and often with square nuts. Diameters run from around one-eighth up to 5/16 inch. Lengths run from about three-eighth inch up to three inches.
Stove bolts are fairly crude, made from carbon steel and somewhat roughly finished. Use them wherever appearance, rust resistance and super strength are not critical. Remember that since they are tightened with a screwdriver, you can’t normally crank them down as tight as hex or square-head bolts.
Machine screws: These have slotted heads, too. But they are more finely made and available in metals other than steel.
You can find them plated, in solid brass, bronze and even stainless steel at marine shops. Machine screws come with hex-type nuts. Use them for fine to medium work, wherever you want a screw-type head. Where might that be? Anywhere there isn’t enough space to use a wrench, or where a protruding hex head might get in the way or be an eyesore. You might use a flat-head machine screw to secure hinges or other hardware with countersunk holes, for example.
Machine bolts: These are also called cup screws and they are the classic, workhorse bolt. They are sized in inches and almost always have hex heads. If you need high strength and resistance to wear, machine bolts are made in special grades and the grades are coded by markings on the heads. Ordinary steel bolts have no marking at all.
High strength bolts will have radiating spoke-like markings, and the more “spokes” the stronger the bolt. The bolt at the bottom of the sketch, for example, has three marks and it’s a grade five.
High-strength bolts are not necessary for most work, but you might need them for certain automotive applications. If the bolt you are replacing needs markings, make sure the bolt you buy to take its place has them, too.
Carriage bolts: These are used on wood-to-wood or metal to wood connections. A carriage bolt has a round, unslotted head with a square shoulder beneath it (like the bolt at top right in the sketch). When you tighten the nut on a carriage bolt, the square shoulder under the head sinks into the wood and grabs, preventing the bolt from turning.
This lets you install the bolt with a single wrench, and it also makes the bolt good for security applications like fastening hinges, lock hasps, latches and so on. Since the head is smooth and has no slot, you can’t remove the bolt unless you can reach the nut.
The smooth head also makes the carriage bolt a good choice where a protruding hex or square head could be dangerous. Outdoor furniture and play equipment are two good examples of this kind of work.
Los Angeles Times Syndicate
