What is "unit construction"

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 their own 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 3 separate oil reservoirs for engine, gearbox and primary drive.

Triumph and BSA were already using cast 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. Only in the 1960s did Japanese motorcycles introduce the now- familiar horizontally-split clam-shell which have become almost universal even today. The horizontally-split engines used 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 (while the norm for bikes) is generally unacceptable for cars and trucks.