Chrome Bolts and Fasteners - Hardware Fundamentals

Rules of EngagementIn general, when you thread a fastener into ferrous metals (or its nut), once you have male-female thread engagement that equals the diameter of the bolt or screw, it's as strong a connection as you'll get. That rule changes when dealing with aluminum, brass, or other non-ferrous materials. In those situations, you'll need roughly double the diameter of the fastener engaged to have maximum tension and a secure connection.

In other words: the 1/4-inch bolt you use to hold on your license plate doesn't need anything more than a nut 1/4-inch deep to work. However, the 1/4-inch screws that hold your primary cover on will need at least 1/2 inch of depth in their inner primary holes to stay put.

Even though we think in terms of a torque value when it comes to tightening a fastener, what we are really doing is trying to get the proper amount of clamping pressure by stretching the bolt or screw in question. Yes, bolts and screws are elastic, and it's not uncommon for a fastener, at correct preload, to wind up several ten-thousandths longer than its original dimensions.

Although there are six main methods used to control the preload of a threaded fastener, we realistically only need to deal with two of them:

Torque-Control Tightening:This the most common means of setting preload, and it works OK, especially with enough practice to get a sense of touch about it. But the fundamental problem is that because the majority of the torque applied is used to overcome friction (usually between 85 and 95 percent of the reading on your wrench), slight variations in the frictional conditions can lead to huge variations in the bolt preload. When uncontrolled variables (such as whether or not the threads are dry, oiled, or coated with anything from crud to threadlocker) are involved, it becomes virtually impossible to be 100-percent sure things are buttoned down correctly.

Angle-Controlled Tightening:This is popular for such applications as head bolts, compensating sprockets, and drive pulleys on Harleys; you could also call it the "turn of the nut" method. The basic idea was introduced shortly after World War II for manual assembly when a certain tightening angle was specified. Handy for use with either hand tools or power wrenches, the fastener gets tightened to a predetermined angle beyond the elastic range. Basically, this means that you torque to a set value, then twist another set number of degrees. Getting into the act keeps variations in preload reasonable due, in part, to the yield stress tolerance. The disadvantage of this method is the necessity for determination of the precise angle. (For instance, Harley determined that head bolts need to be turned 90 degrees, whereas the latest factory spec for a compensator nut is only 40-50 degrees.) Also, the fastener involved can only do this trick a couple of times before it needs replacing-or fails in service.

Here's a lineup of the usual suspects. From top to bottom we have 1/4-inch, 5/16-inch, 3/8-inch, 7/16-inch, and 1/2-inch bolts. Fine threads are on the left, coarse on the right. The missing man in this formation is the 7/16-inch fine, because with the single exception of some pulley-mounting setups, there is no application for this particular thread on a Harley. Oddly, most of the time, fine threads are used in high-stress applications regardless of the material they work in, whereas low stress but potentially high-shear situations are tackled with coarse threads. All of the ones you see here are chrome and as high-quality as you can get. That means thick plating on the heads and thin on the threads without loss of strength or function.

Here you can see the markings that indicate the grade of the fastener. No marks = lowest grade and a tensile strength of 60,000 psi. Three hash marks = Grade 5 and tensile strength in the 90,000-120,000-psi range. Five hash marks = Grade 8 and a minimum tensile rating of 150,000 psi. Typically, unless otherwise noted, the unmarked shiny chrome bolt on the right will be somewhere in the first two categories, probably at least Grade 5 if it's from a quality manufacturer.

If you have qualms about tensile strength in your application, be aware that the solution is to use an Allen-head bolt, since by definition all Allen bolts are at least Grade 8. (Some are even higher, such as the 8.8 bolts used on the front-brake calipers on '84-'99 models.) What you shouldn't do is confuse tensile strength with shear strength, since the higher the grade, the more brittle the bolt, even before it's been chromed. Make sure the hardware you choose to use is up to the task by checking the grade of the OEM fastener. Standard hardware specs and dimensions (along with their grades) are listed in the back of the parts books for most Harley-Davidson models, so check there or with your dealer.