Pre-unit construction

BSA motorcycle showing typical pre-unit construction engine and gearbox design.

Pre-unit construction, also called separate construction, is a motorcycle engine architecture where the engine and gearbox are separate casings, with their own oil reservoirs, and usually attached to parallel plates that also attach to the frame. Even though Singer^[1] offered an integrated engine and gearbox in a single casing in 1911, it was not until the 1950s that technical advances meant it was possible to reliably construct engines with integral gearboxes in one unit, known as unit construction. Another variant is semi unit construction, where the gearbox is bolted directly to the engine.

The term pre-unit is particularly applied to BSA and Triumph vertical twin motorcycles as a consequence of the strong publicity attached to their change to the unit construction of vertical twins in the early 1960s. Norton and Royal Enfield kept producing separate construction engine and gearbox motorcycles.

The 1969 'Isolastic' frame Norton Commando had the engine, gearbox, and swingarm bolted together on plates bolted to the frame with shimmed rubber bushes.

Among the Japanese manufacturers, Kawasaki produced a separate construction 650 cc vertical twin, inherited as a consequence of taking over Meguro in 1964,.^[2] The Meguro K models copied the BSA A7 and BSA A10 design, external appearance was similar although no parts are interchangeable.^[3]

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