The Nuts and Bolts of Nuts and Bolts

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

The most common fastener on most motorcycles is the familiar “hex cap screw,” which has a six-sided head. Also often encountered is the Phillips socket-head cap screw, which has a cross-form socket into which a Phillips screwdriver fits (there are different standards here, such as the JIS, so you’ll find some drivers fit better than others). Another common fastener is the stud, which is a length of rod with threads on each end. In many engine designs, a set of long studs with one end screwed into the crankcase with the opposite end projecting upward through the cylinder and head.

Hand wrenches for turning hex heads are of two kinds, open end and box. The box wrench encircles the nut or bolt head completely, giving this wrench type great strength. Clearly, the chance of damaging the fastener’s shoulders is less with a box wrench, which, unlike an open-end wrench, is not being pried open by the applied torque.

And this brings us to the crux of the matter: The torque you apply to a fastener is delivered to two or more of the head’s six flanks, and clearly, the more flanks your wrench engages, the lower the pressure on each flank, reducing the possibility of damaging or rounding-off the hex head. For some high-torque applications, so-called “12-point” fasteners may be used, whose heads have double the number of driving shoulders.

The usual box wrench is made with 12 shoulders anyway, just to allow for those tight situations in which there isn’t room to swing the wrench a full 1/6 of a turn before it (or your tender knuckles) hits something solid.

Fasteners on European or Asian vehicles, as well as a growing number of US products, are sized in metric. In former times, British bike fasteners were sized in the Whitworth system, so a well-equipped mechanic needed three sets of wrenches and sockets to service all varieties of motorcycle.

In early days, so-called “stove bolts” were made with square heads. Such square drives continue in use to this day as the socket-wrench drive system. Each tubular socket has one end into which fits a hex- or 12-point fastener, but the other end has a square hole that fits a bar, ratchet handle, “speed handle” (a crank), or some electric or pneumatic driving device. Such square drives are made in 1/4 inch, 3/8 inch, 1/2 inch, and for giant fasteners such as those you find in the oil patch, 3/4 inch. A 3/8-drive socket set covers most motorcycle work.

Thread Pitch

The pitch of a thread is the distance the nut or bolt advances or retreats per revolution. Very common on metric motorcycles are the 6 x 1.0 and 8 x 1.25mm sizes. The first number gives the diameter of the fastener’s threads while the second is the pitch. In the SAE system used in the US, the similar sizes are given differently, as diameter followed by the number of threads per inch—for example, 1/4-20 and 5/16-18. SAE specifies both a coarse and a fine pitch in each size, so you can also find 1/4-28 and 5/16-24. Both metric and SAE thread-pitch gauges are valuable additions to any toolbox.

Related Content: Threaded Fasteners on Your Motorcycle

Fastener Materials

Damaging? Rounding off? Aren’t metals really strong? The wrench multiplies the force of your hand many times, such that the pressure against the shoulders of a bolt head can easily be great enough to damage it (especially if the fastener has become a bit undersized from rusting).

In fact fasteners use a wide range of materials. Lowest in price are fasteners made from mild steel, which yields at 58,000–60,000 psi. (Yield strength is determined with a tensile test machine, which pulls a specially shaped test piece until it breaks, noting the change in force required as the piece stretches.) That seems like a lot, and compared to our bones, it is. But it’s easy to find fasteners made in much stronger materials. So-called “Grade 8” bolts, indicated by six radial lines on their heads, yield at a stress of 150,000 psi. And super fasteners for special applications are made of materials giving nearly twice this strength.

Fastener Strength

How can two fasteners of the same dimensions, both made of steel, have such different yield strengths? Metals yield by a process of planes of atoms in one metal crystal sliding over another. Alloying, adding other elements to the steel, can in various ways block this sliding of atomic planes, thereby raising the material’s yield point. We have all heard of “chrome-moly steel”; such steel is alloyed with small amounts of chromium and molybdenum. Premium hand tools are made of a steel alloyed with chromium and vanadium.

Elastic vs. Plastic Deformation

Speaking of strength, it’s important here to mention that when you tighten or “torque” a fastener in place, what you are doing is tensioning the shaft of the fastener as a very stiff spring. The tiny degree of elastic stretch you apply to a fastener by tightening it produces the powerful clamping force that holds the parts you are bolting together. When you elastically deform metal, the change of length or shape is not permanent. You are stressing the metal within its elastic range. When you apply even more force, and some of the deformation becomes permanent, the process becomes plastic deformation. Normally, threaded fasteners are used within their elastic range, which brings us to…

The Torque Wrench

Because it’s easy for an inexperienced person to over- or under-tighten critical fasteners, service manuals usually specify some way to directly measure installation torque. This generally means using a torque wrench. We don’t want head gaskets to leak or connecting-rod cap bolts to loosen, and we certainly don’t want the Mad Torquer breaking parts with the usual excuse, “Well, I didn’t want ‘er to come loose, did I?” More is not better. Correct torque is better.

But there are other tightening schemes which also allow us to achieve accurate fastener tension. One, practical only when both ends of the fastener are accessible, is to use a micrometer to directly measure how much tightening has stretched the fastener. This gives highly accurate results. Another is to spin in the fastener until firm initial contact is made, then turn the head through a specified angle. The idea here is that, say, a quarter turn on a 1.0mm pitch fastener will stretch it by 1/4 of a millimeter, which is 0.25 x 0.039 inch = 0.00975 inch.

Yet another method, more accurate still because it is based not on friction but on the properties of the material itself, is torque-to-yield. In this system, typically applied to critical fasteners such as head bolts, a torque-sensing wrench is used to tighten the fastener until it begins to yield (that is, the torque peaks and then begins to fall). Engineers know what force corresponds to the onset of yield for a particular fastener size, material, and heat-treat, so this method gives high accuracy, especially in automated assembly. Needless to say, fasteners tightened in this way cannot be reused.

The High-Strength Pitfall

When you’re just starting out in the mechanical arts and discover the existence of higher strength fasteners, it’s tempting to replace every fastener on your bike with such parts. I fell into this pitfall myself. The problem comes when you torque the stronger fasteners to the higher tensions of which they are capable (otherwise why would you use them?). All too often, such an increase in fastener torque just pulls the threads out of softer aluminum parts, crushes gaskets, or squeezes the soft metal out from under screw heads fastening side covers or crankcases. The engine, its fasteners, and its recommended fastener torques were designed together. Doubling the original clamp forces with higher-tensile fasteners does no good, and often does harm.

Related Content: Ups and Downs of Taking Things Apart

Socket Heads and Cam Out

Anyone who works with Phillips-head screws has experienced the so-called “cam-out.” As you apply increasing torque, the shape of the cruciform socket tends to lift the driver out, allowing it to slip. If you are using an impact wrench, the driver can spin and damage the screw’s socket beyond use. We all learn to press the driver into the Phillips recess with all our might (sometimes I add pressure from my forehead as well) to keep this from happening. Why were these diabolical Phillips screws invented in the first place? They exist to speed production! Unlike a slotted-head screw, they are self-centering. On the production line, you jab your screw gun into the Phillips head, pull the trigger, and you’re done.

An alternative is to toss the Phillips-head screws into your miscellany drawer and replace them with Allen-head screws. These have a six-sided straight-walled recess in the head that fits a corresponding six-sided key without any cam-out. Other socket-head varieties exist as well; spline drive, Torx screws, and so on.

Not seen as often in motorcycles are flathead socket screws. My Kawasaki two-stroke triples had a couple of these threaded land mines in the gearbox, and, because you don’t want anything unscrewing in there, they were always Loctited (glued to prevent loosening). Because this fastener’s heads are conical, there is plenty of room for the roughly conical Phillips recess, but not for the straight-sided hex socket of an Allen screw. I’ve had the experience of not being able to transmit enough torque to a Loctited Allen flathead to loosen it, resulting in rounding off the key or socket. In either case, I had to drill out the fastener on the milling machine. After that, I used Phillips flatheads in that application, and was very mindful not to allow cam-out to wreck them.

The Impact or Hammer Driver

Great power requires great responsibility. A hammer driver or impact driver is a kind of screwdriver containing internal ramps. When struck on one end with a hammer, very large torque pulses are produced at the other (such drivers are generally reversible). I keep a hammer driver in the box because when Phillips-head fasteners won’t come loose any other way, the hammer driver gets them out. The responsibility part comes when you are tempted to tighten with the hammer driver. Best not to; torque measurement is impossible with an impact driver, and banging away with the hammer can easily beat things out of shape.

The Air or Electric Impact Wrench

These are great for disassembly (especially for breaking loose those big nuts retaining sprocket primary gear, or clutch). Care is necessary during assembly for, like computers, if misused they just allow you to make more bad mistakes per unit time.

In addition to the role of the torque wrench as a means to obtain correct fastener torque, its use will eventually give you “torque sense” that can prevent you from becoming the social outcast we all know—the aforementioned Mad Torquer.

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