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 boxer 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. Velocette, Matchless/AJS and Norton motorcycles continued to be pre-unit (the former machines with pressed-steel primary cases) until the end of production in the 1960s and 1970s respectively.
Read more about Unit Construction: Advantages and Disadvantages, Detailed Construction
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