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Pre-unit construction, also called separate construction, is a motorcycle engine architecture where the engine and gearbox are separate components with their own oil reservoirs, linked by a driving chain within a primary chaincase. Mounting plates are usually attached to the frame allowing for chain adjustment by gearbox fore-and-aft movement and via screw adjusters and elongated mounting holes. Even though Singer 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 assembly 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. The Meguro K models were copies of the BSA A7 and BSA A10 design, but although external appearances were similar, no parts are interchangeable with the BSA forebears.
A pre-unit motorcycle would not normally be able to use the engine as part of the frame as a stressed member, since the engine/primary-chaincase/gearbox assembly would not be sufficiently rigid.