Unit construction is a single casing for the engine and the transmission in machines like motorcycles or farm tractors. It is common in many Japanese and European motorcycles, but the American Harley-Davidson Sportster model also utilizes this construction in its particular power plant. Many farm tractors use unit construction in their designs, primarily for strength. In many cases, the machine using unit construction design principles uses the engine case as a primary component in the frame or chassis, eliminating the need for frame rails. This high-strength aspect of the unit construction allows the manufacturers to eliminate unneeded components while retaining the strength required for durability in a more compact package.
By using unit construction methods in the manufacture of an engine and transmission, some of the more vulnerable components can be eliminated from the design. Weaker chains and belts can be converted to more durable gears and sprockets when the transmission resides within the same case as the power plant. It is also often easier to consolidate an entire engine and transmission into a smaller, single case as opposed to designing two individual cases for the components. This leads to a more compact and space-saving design.
In most cases, motorcycles using unit construction methodology are built with a split main case. Once the cylinder head is removed, the engine's case can be split or taken apart, and the internal components, such as the transmission gears and the crankshaft, can be removed. When reassembling a unit construction engine, one half of the engine case can be laid on its side and all of the components and internals can be put into place in the single side case. Once complete, the other half of the engine case can be placed over the components. They are then sandwiched into place within the two case sides.
While unit construction has the advantage of giving designers a much smaller package to incorporate into the overall bike, one disadvantage to this type of engine lies in the replacement of an entire case assembly in the event of a broken gear or damaged gearbox. A broken gear in this type of engine is usually responsible for the entire engine being torn apart and the internals placed into a new engine case. Instead of repairing a single component such as a transmission, the mechanic must now completely disassemble and reassemble the entire power train in order to complete the repair. This can also be very costly and expensive.