The Nuts and Bolts of Threaded Fasteners, Part 1
Threaded fasteners literally keep your Corvette together. Without these little wonders, the engine would just be a pile of metal components, your suspension wouldn't stay on, and so on. There's so much to discuss that this edition of Garage Talk is Part 1 of 2 parts so we don't gloss over any of the important stuff. That being said, let's not screw around and get right down to the nuts and bolts of it all.
The most common threaded fasteners are bolts, screws, and studs. Technically, bolts are externally threaded fasteners designed for insertion through holes in assembled parts. Specifically, a bolt is normally tightened and released by turning a mating nut.
The nomenclature of bolts can be somewhat confusing at first, so here's an explanation of what's what:
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Stove bolts are very common and named for their original purpose, which was the assembly of cast-iron stoves. They are available in diameters from 5.32 inch to 1/2 inch and in lengths from 3.8 inch to 6 inches. They have slotted heads for screwdrivers (some more modern versions can have Phillips heads); the heads themselves are available in round (half a sphere), flat (flat on top, tapered to threads), or pan (wide, rounded head).
Carriage bolts are a class of bolts that generally have round heads with small square sections underneath. They usually fit into square holes, thereby "locking" themselves in place so that a nut can be turned tight without spinning the bolt itself. Sometimes carriage bolts have ribbed necks rather than square ones.
Step bolts are carriage bolts that have enlarged heads that "step down" to the square neck. The flanged heads spread loads over a very large area. Bumper bolts are typically step bolts so the chromed round head gives a better appearance.
Joint bolts are tapered at the tip to act as a "self-aligning" device to center itself in a captive nut and start threading straight. They are found in many areas on automobiles and other vehicles, as well as some machinery.
T-head bolts are made with a T-shaped head instead of the usual hex or square heads. They are used in situations where there is no easy way to put a wrench on the head. Engine blocks and transmission cases frequently utilize these bolts.
Countersunk bolts have heads that are flat on top and tapered to the shaft. They usually have hex sockets (Allen), but sometimes have large Phillips sockets. They are used in applications where there is no space for the head to protrude.
Screws differ from bolts because they mate with an internal thread into which they are tightened or released by turning with the appropriate driver.
A little nomenclature caveat is in order here. Lots of folks call any threaded fastener larger than about 1/4-inch in diameter a "bolt." While that's not technically correct, I won't split hairs here about the absolute technical definition; instead, from this point on, let's just assume bolts can sometimes be inserted directly into threaded holes, and screws can be used with nuts. Even though that's not the usual scheme of things, I think you get the idea.
Screws vary widely in design, size, and purpose. Hex cap screws are frequently found throughout automobile construction and are frequently considered the same as bolts (in actuality, cylinder head bolts are really hex cap screws). Screws are found with hex socket caps, Torx socket caps, and even slotted and Phillips heads. There are wood screws, sheetmetal screws, self-tapping screws, and many, many others. Other than those used to hold engines, transmissions, and drive gear together, the most commonly found screws on automobiles are selftapping and trim screws.
Machine screws are used for the assembly of metal parts, so virtually every one of these on a car-and most other metal machinery-is a machine screw.
Studs are rods that are threaded on either one or both ends, and the thread type is often different on opposing ends. The most common studs found on automobile and other engines are on the intake and exhaust manifolds.
What would you call a bolt or screw without a washer? Probably loose, for starters! Most fasteners are installed where vibration and temperature changes occur. Mechanical motion, over time, causes fasteners to back off and loosen, hence the need for something to help prevent that effect: the washer. The primary function of a washer is to provide a surface against which the head of the fastener or surface of a nut can bear. Flat washers do this very well and spread loads, but normally don't help to keep the fastener tight. Lock washers are designed to keep fasteners from loosening. They come in many forms, the two most common being the split (helical) ring washer and the toothed washer. Split washers function in much the same way that helical springs do; when the bolt is tightened sufficiently, the ends of the washer come together under compression, resisting movements of the bolt by creating a certain degree of friction against which the bolt would have to overcome. Toothed washers work very well because their many teeth bite into the surface against which the head or nut bears, creating large amounts of friction. They come in external, internal, and internal/external tooth forms. The softer the surface under the teeth, the better the locking washer works.
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Essentially, a bolt can't do its job if it doesn't have a nut holding it on. Therefore, any threaded fastener that doesn't screw into a mating surface needs a nut to hold it in place. There are as many different nuts as there are washers and other hardware items because they are designed to accommodate different methods of tightening, strengths to suit varied applications, and some also have provisions for locking. Most nuts are hexagonal in shape to accommodate wrenches, but there are also some square nuts out there as well. If there's one cardinal rule when it comes to nuts, it's this: The nut used must be the same grade of metal as the bolt or failure will surely and eventually occur.
Here are some of the more common nuts you'll probably be dealing with in your garage, aside from the human ones.
Machine screw nuts are fine threaded and frequently used to hold brackets and other assemblies.
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Track bolt nuts are usually square and frequently found holding fixtures in place. These are often held "captive" in some sort of housing that allows some degree of movement and adjustment, such as door latch mechanisms.
Jam nuts are typically thin and used for applications where space is critical. You'll frequently find them used for automotive dash trim and radio assemblies.
Thick nuts are typically used for coarse-threaded bolts and in applications where great holding strength is necessary -for instance, securing bumpers to their mounting brackets.
Slotted nuts are cut in several places to accommodate the use of cotter pins or other devices that act as safety systems to prevent loosening. Axle nuts are typically slotted.
Castle nuts are also known as castellated nuts. They are also slotted, taller nuts used for high-hold situations. Castle nuts are frequently available with plastic or metal inserts that exert high friction while turning as added insurance to keep them tight.
Flange nuts are made with a wide bottom flange to spread loads over a large surface. They are frequently found securing fenders to car and truck bodies.
Cap nuts are typically nuts with solid tops that are acorn-shaped. Cap nuts are used in situations where they will be seen in the finished product and are often chrome-plated. These nuts require the use of a specific-length bolt.
As I said at the beginning, there's a lot to know about threaded fasteners, so we'll continue this discussion next month in Part 2 where we will cover course and fine threads, metric and fractional standards, hardness, torque considerations, and head markings. Don't miss it!

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