For the vehicle design where the vehicle's skin is used as a load-bearing element, see Monocoque.

Unit construction is the design of larger motorcycles where the engine and gearbox components share a single casing. This sometimes includes the design of automobile engines and was often loosely applied to motorcycles with rather different internal layouts such as the flat twin BMW models.

Prior to unit construction, the engine and gearbox had separate casings and were connected by a primary chain drive running in an oil bath chaincase. The new system used a similar chain drive and both had separate oil reservoirs for engine, gearbox and primary drive.

Triumph and BSA were already using cast non-ferrous alloy chaincases and started converting to unit construction in the 1950s. A driving factor behind the BSA/Triumph change was that Lucas had declared an intention to abandon production of motorcycle dynamos and magnetos, and instead produce only alternators. By contrast, Velocette, Matchless/AJS, and Norton motorcycles continued to be pre-unit construction (the former machines with pressed-steel primary cases) until the end of production in the 1960s and 1970s respectively. In reality, the casings were not really "unitary," as the crankcase section was vertically divided in the middle and no oil was shared between the three portions. In the 1960s Japanese motorcycles introduced the now-familiar horizontally split clamshell which has become almost universal.

Modern horizontally split four stroke engines invariably use single oil reservoir (whether wet- or dry-sump) but, while this simplifies matters, it is arguable that the previous system of having different types of oil for engine and gearbox is preferable. The BMC Mini was an early example of a car with the "gearbox-in-the-sump;" but this practice of using a single oil reservoir, which has become the norm for motorbikes, is generally undesirable for cars and trucks. Two stroke "total-loss" bikes always have separate oil for the gearbox, as engine oil is burned along with the fuel.