Pre-unit and unit construction
A pre-unit motorcycle is one in which the engine and gearbox are separate ‘units’, connected by a primary chain. They tend to be earlier bikes, whereas a unit construction engine is one made with the crankcase and gearbox in a single casting, yet the gearbox is a separate item within that casting, still driven by a primary chain. Pre-unit motorcycles had largely died out by 1969, though a few manufacturers such as Velocette and Norton persevered.
Primary drive
This is the method by which the engine sends the drive to the gearbox, via
