FEATURE ARTICLES

The Nuts and Bolts About Nuts and Bolts

Machinery components — and all devices and equipment assembled from multiple parts — are held together by fasteners. The only other way to do so is by welding or the use of special adhesives, both of which are generally more expensive methods that render the component difficult to disassemble. Manufacturers can choose from hundreds of thousands of standard fasteners, not to mention millions of specialized types. Materials for fasteners range from metal to plastics, ceramics to special alloys, all of which can be made in an endless array of shapes and sizes.

A detailed discussion of fasteners (less than several million words) can only cover the common, everyday types encountered by mechanics and hobbyists. By far, the most common fastener is the threaded type, and the most common of those are nuts and bolts and screws and studs.

Bolts are technically described as externally threaded fasteners designed for insertion through holes in assembled parts. Specifically, a bolt is normally tightened and released by turning a mating nut*.

Types of Bolts

The nomenclature of bolts is confusing, to say the least, but we'll list the most common ones you are likely to encounter while working on automobiles.

Stove Bolts are very common and were named for their original purpose, the assembly of cast-iron stoves. They are available in diameters from 5/32 to 1/2 inch and in lengths from 3/8 to 6 inches. They have slotted heads for screwdrivers (some more modern versions can have Phillips heads) and 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 and square necks. 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.

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 that 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.

T-Head Bolts are specially 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 utilize these bolts often.

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 in that they mate with an internal thread into which it is tightened or released by turning its head with the appropriate driver*.

*Alert! Obviously, nearly everyone calls a threaded fastener larger than about 5/16ths inch in diameter a "bolt." Since that's the case, we won't be finicky about the technical descriptions. From here on we will assume bolts can sometimes be inserted directly into threaded holes and screws can be used with nuts, but you get the idea.

Screws vary widely in design, size and purpose. Hex cap screws are typically found throughout automobiles and are frequently considered the same as bolts (cylinder head bolts are actually hex cap screws). Screws are found with hex socket caps, Torx socket caps and even slotted and Phillips heads. There are wood screws, sheet-metal screws, self-tapping screws and many, many others. Other than those holding engines, transmissions and drivegear together, the most common found on automobiles are self-tapping and trim screws.

Machine screws are used for the assembly of metal parts, so virtually every one of these on a car is a machine screw (not to be confused with "machine threads," a common description of fine threads but not technically correct.

Studs are simply 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 automobiles are on intake and exhaust manifolds.

Washers

Where would bolts and screws be without washers? Loose, that's where they'd be! 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 act as helical springs. 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.

Note: The softer the surface under the teeth, the better the locking washer works.

And Now, The Nuts!

How could any proper article on fasteners avoid a discussion of the nut? After all, bolts wouldn't be worth much if they didn't have a nut holding them on, would they?

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 wrenching, strengths to suit applications and provision for locking. Most nuts are hexagonal in shape to accommodate wrenches, but some square nuts are found in automobiles. The most important thing to remember is: the nut used must be the same grade of metal as the bolt!

Common nuts used in automotive applications are:

Machine screw nuts are fine-threaded and used to hold brackets and other assemblies.

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. Dash trim and radio assemblies use jam nuts frequently.

Thick nuts are typically used for coarse-threaded bolts and in applications where great holding strength is necessary, for instance — bumpers.

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 taller, usually slotted, nuts used for high-hold situations. Castle nuts are frequently available with plastic or metal inserts that exert high friction while turning.

Flange nuts are made with a wide bottom flange to spread loads over a large surface. They are frequently found securing fenders to bodies.

Cap nuts typically have solid tops that are acorn-shaped. They are used in situations where they will be seen in the finished product and are frequently chrome-plated. These nuts require the use of a specific-length bolt.

Are There Any More Variations?

Yes, there are. All of the stuff you've been reading about so far can come in a variety of finishes. The most common are: nickel; brass; copper; cadmium; electro-galvanized; Parkerized and zinc-plated. A good rule-of-thumb is to try to use the same finish of fastener that was there in the first place, but many restorers these days tend to use stainless hardware for its bright, non-rusting finish and overall strength. Now, let's discuss thread types...

Coarse or Fine?

Fasteners are threaded according to how many threads* there are to the inch (or centimeter, in the case of metric). Most threaded fasteners are available with either coarse threads conforming to Unified National Coarse (UNC) standards, or Unified National Fine (UNF) threads. Fine threads have more threads per inch than coarse.
*Since a thread is simply an inclined plane cut along the surface of a fastener, the angle of that plane can be increased (coarse) or decreased (fine) to provide the desired number of threads per inch.

Coarse threads are easier and faster to use. They provide an easier "start" of the fastener, with less possibility of cross-threading. Nicks and burrs from handling are less likely to affect assembly, they are less likely to seize in temperature applications and in joints where corrosion is likely. Coarse threads are less likely to "strip" and are more easily tapped into brittle materials.

Fine threads provide superior fastening (typically 10% stronger holding power than coarse) in hard materials. They can be adjusted more precisely due to their shallower helix angle. They are better in situations where length of engagement (depth) is limited and where wall thickness is limited, again because of their smaller thread cross-section (coarse threads are cut deeper into the shaft of the fastener than fine.)

None of this is of any consequence if the fastener isn't properly installed, and that leads us to the "heart" of the matter, a fastener's proper torque levels...

Torque

All fasteners have to be correctly tightened in order to perform the job for which they are intended. If, for instance, two steel plates need to be tightened together with a force of 100 pounds, several bolts (or a single 3/4 inch diameter bolt) might be used to create the binding force. Once tightened, the two plates are held together just as if a 100 pound weight were sitting on top. Any force less than 100 pounds would fail to pry them apart.

However, if the bolt(s) were loose, external loads, vibration and temperature change would eventually cause the plates to come apart because they would fatigue (the simplest form of fatigue is that of metal being bent back and forth, but in this case the bolts would be elongated enough for the joint to loosen).

The strength of any joint is dependent upon the two factors: 1) the strength of the fastener itself, and 2) the degree to which it is tightened. Tightness can be accurately controlled by the measurement of the torque (twisting force, measured in Foot Pounds*) to which it is tightened. Torque applied to a fastener creates inner tension (stretching) that, in turn, creates the holding power desired.

Special tools called torque wrenches perform this task. The strength of the fastener is determined by the raised markings (called Grade Markings**) on the head of the bolt or screw. These head markings were developed by the SAE (Society of Automotive Engineers) for automotive applications, and the ASTM (American Society of Tests and Measurements) for structural applications.

*A one pound weight or force applied to a lever arm one foot long is equal to one foot-pound, or twelve inch-pounds, of torque. Technically, the expression is known as a pound-foot, but conventional usage favors "foot-pound."
** The way to read the marking system is to add two to the number of marks. No marks indicate a grade 1 or 2, three marks indicate a grade 5, four marks a grade 6, and 6 marks a grade 8.

The first thing to know about grade markings is that no markings must be considered to mean the fastener is made of mild steel. Otherwise, the number of marks on the head, the higher the quality and strength. Therefore, bolts of the same diameter will vary in strength depending upon the material and number of threads per inch.

The tables below show typical torque values for the most common bolts you are likely to encounter on an automobile. While other factors affect the torque (discussed later) these general levels indicate a safe "starting point" or level of torque to be applied:

TYPICAL TORQUE VALUES FOR GRADED STEEL BOLTS

But What About Grades 3 and 7?

Glad you asked. The reason we didn't cover those two grades is because you don't see them very often in the real world. Grade 3 bolts are little different from Grades 1 and 2, in that all these bolts are generally made from cold-worked steel with either a low or medium carbon content. It isn't until you get to Grade 5 that bolts are made of medium carbon steel that is quenched and tempered (harder and stronger). Grade 7 bolts are sort of specialized too, in that they are medium carbon alloy steel, quenched and tempered, with "rolled" threads rather than cut threads. Rolled threads are relatively expensive to produce and such bolts are mostly used in aircraft and military applications. They are very hard to find at local auto parts suppliers and, if encountered, can be replaced by a Grade 8 bolt.

Not So Fast! There's More to Nuts and Bolts, And It's Important!

There are several factors that affect overall torque on a fastener: the type of lubricant used, if any; the material from which the fastener is made; the type of plating on the fastener; the type of washer used; and the finish of the thread surfaces, as well as others.

Because of so many variables it is not possible to create a one-size-fits-all torque chart that will tell you the exact tightness for every bolt and screw for every situation. It simply isn't possible to establish a "perfect" relationship between the torque applied and the internal tension that results. Therefore, we must present this most-important rule:

The manufacturer's specifications must be followed for your specific vehicle. General torque tables are only useful as "starting points" or for non-critical applications (non drivetrain-related) on the car. The tables above show maximum torque values based on the bolt or screw retaining a quantity of oil from the manufacturing process and do not apply where special lubricants are used. These special lubricants can reduce the friction in the fastener assembly, so the torque applied may produce far greater tension than desired.