Cobra machined each of these two Valkyrie radiator covers from solid pieces of billet aluminum measuring 2.5 x 4 x 9 inches and weighing 9.5 pounds. These slender parts now tip the scales at less than a pound. Photo by Fran Kuhn.
Billet is a term tossed around frequently and loosely in cruiserland, but many people are confused about what billet actually is and why it's usually expensive. We recently heard of a proud cruiser owner who (when responding to a question about a bolt-on accessory) said, "That's not aluminum; that's billet." Clearly, some confusion exists here.
A quick trip to the dictionary tells us that a billet is simply a "bar of metal." Billets can be made of magnesium or steel or iron or lead, but because of its light weight and relatively low cost (compared with other light metals such as magnesium and titanium), aluminum works best for cruising applications. However, since pure aluminum is a fairly soft metal, the aluminum most billet accessories are machined from is actually an alloy of aluminum and other metals.
The folks at Jardine tell us that one of the most popular alloys found in motorcycle parts is 6061-T6 aluminum. In the 1950s the Aluminum Association adopted a four-digit numerical classification system for aluminum. The first digit, the six, states that this alloy contains aluminum, magnesium, and silicon; giving the alloy good formability, corrosion resistance, and strength. (If the first digit were a one, the metal would be more than 99 percent pure aluminum.) The zero means that special controls to limit particular impurities within the alloy were not incorporated (not a concern in motorcycle applications). The final two digits identify the particular alloy within the aluminum, magnesium, and silicon group. The -T6 designates aluminum alloys that have been thermally treated and artificially aged for additional hardness. So, to sum it all up: 6061-T6 aluminum is a light, strong, corrosion-resistant alloy that is ideally suited for motorcycling.
Since billet aluminum parts start their lives as blocks of aluminum, the finished parts must be carved from these blocks. Cobra's manufacturing engineer took us through the process of turning a boring block of aluminum into a part that is ready for the chromer or polisher. When Cobra develops a new part, all of the measurements are taken from the OE part that is being replaced or from where the new part will be mounted. The measurements are then turned into a 3-D wire-frame model on a computer. At this point, styling touches that influence the look and feel of the part can be previewed without cutting a single piece of aluminum. Once the part has been completely designed, the engineer creates the cutter tool list and the carving order that will result in the completed part (like a sculptor progressively removing aluminum from the billet). Cobra's new Valkyrie radiator covers shown above are a good example.
Yes, much of a piece of aluminum billet ends up in the recycling bin after a milling machine has extracted a part from within it. Unfortunately, aluminum chips return only pennies on the dollar of the cost of billet aluminum. That fact and the expensive, computer-controlled machinery required to precisely cut each part explain the higher cost of billet accessories. So, if billet parts are expensive to produce why not just die-cast the parts? Simply put, die-cast parts are made from aluminum poured into a mold, therefore it's difficult to achieve the uniform structure, strength, and flawless finish found in top-quality billet parts. Billet-look or billet-like parts are usually cast items dipped in chrome.
Now, when someone refers to a part as billet, you'll know what they think they're talking about.
For more articles on custom bikes and articles about how to customize and modify your motorcycle, see the Custom section of MotorcycleCruiser.com.