There are two types of crankshafts, the assembly, or "built up," type, which is made up of right- and left-hand crankshaft halves assembled onto a crank journal or pin, and the unit, or one-piece, crankshaft, which is forged or machined out of a single chunk of iron or steel. As a rule, built-up cranks use roller bearings and one-piece rods, while unit cranks use plain bearings and two-piece, or "split," rods. Harley-Davidson FL and XL motors use built cranks (as they have since their inception), while more, uh, current designs tend to go with unit cranks.
Because you've got a lot of parts thrashing around at high speeds, the crank assembly needs to be accurately balanced-if it isn't, it'll shake itself and the rest of the motorcycle, including the rider, to pieces in short order. Modern engines incorporate a balance shaft to counteract the crankshaft forces and reduce vibration.
Connecting Rod
The connecting rod connects the piston to the crankshaft. Because the rod swivels in relation to the piston and the crankshaft, it's fitted with a bearing at either end. The small-diameter end of the rod connects to the piston and normally has a plain bushing, while the larger-diameter end is mounted to the crankshaft and utilizes either a roller bearing (H-D X and F motors) or a plain-insert-type bearing (everyone else)
Rod materials include aluminum, steel billet and, in some high-performance applications, titanium. In most cases, the rods are either forged or machined from stock, although the latest technology uses a procedure called "powder metallurgy, " which is a sintering process that uses high pressures and temperatures to create extremely strong, light and expensive rods out of metal powder.
Roller bearing rods are built in one piece and installed during the crankshaft assembly process, while split rods have removable end-caps that allow them to be installed over the crankshaft. When sintered rods are used, the end-cap is separated from the rod body by fracturing, which results in an uneven mating surface. When the rod is installed, the slightly jagged ends ensure a perfect cap-to-rod alignment, compared to the minor misalignments that can occur if the mating surfaces are both flat. As I recall, the late BMW CL models used fractured rods, but I can't think of any cruisers currently using them.
Piston, Rings And Wrist PinThe piston is used to compress the mixture in the cylinder and to transfer the energy of the expanding gases to the crankshaft. Since they're subject to extremes in heat, pressure and acceleration, pistons need to be extremely durable and constructed to very high standards. Consider a Honda VTX1800 that's turning 3,000 rpm. At those speeds, its pistons travel 34 feet per second, with a piston crown temperature that averages 600 degrees Fahrenheit.
Most pistons are either cast or forged out of aluminum alloy, although some high-performance versions are machined out of aluminum billet. For years, cast pistons were at the bottom of the performance food chain, but the introduction of the "Hypereutectic" cast piston (look it up on Wikipedia) has changed that perception. Currently, most production motor-cycles use some form of Hypereutectic piston, and they hold up extremely well.
The piston is fastened to the connecting rod by the wrist pin, which in turn is held in place by spring steel clips or, sometimes in high-performance applications, by aluminum or Teflon buttonsz
Cylinders
The cylinder's function is rather obvious: It acts as a guide for the piston and contains the expanding combustion gases. What's less obvious is how it's put together. Unless they're for a very special application, cylinders are made from one-piece aluminum castings that have either a steel sleeve called a liner pressed into it or utilize special hard-metal "chrome" plating.