Pre-unit construction, also called separate construction, is a motorcycle

Motorcycle

A motorcycle is a single-track, two-wheeled motor vehicle. Motorcycles vary considerably depending on the task for which they are designed, such as long distance travel, navigating congested urban traffic, cruising, sport and racing, or off-road conditions.Motorcycles are one of the most...

 engine

Engine

An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...

 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

Singer (car)

Singer was an automobile company founded in 1905 in Coventry, England. It was acquired by the Rootes Group of the United Kingdom in 1956, who continued the brand until 1970...

 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

Unit construction

Unit construction is a term used to describe the design of larger motorcycles where the engine and gearbox components share a single casing. The term is sometimes applied to the design of automobile engines and was often loosely applied to motorcycles with rather different internal layouts such as...

. Another variant is semi unit construction, where the gearbox is bolted directly to the engine.

The term pre-unit is particularly applied to BSA

Birmingham Small Arms Company

This article is not about Gamo subsidiary BSA Guns Limited of Armoury Road, Small Heath, Birmingham B11 2PP or BSA Company or its successors....

 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

Norton (motorcycle)

Norton is a British motorcycle marque, originally from Birmingham, founded in 1898 as a manufacturer of "fittings and parts for the two-wheel trade". By 1902, they had begun manufacturing motorcycles with bought-in engines. In 1908, a Norton-built engine was added to the range...

 and Royal Enfield

Royal Enfield

Royal Enfield was the name under which the Enfield Cycle Company made motorcycles, bicycles, lawnmowers and stationary engines. This legacy of weapons manufacture is reflected in the logo, a cannon, and their motto "Made like a gun, goes like a bullet". Use of the brand name Royal Enfield was...

 kept producing separate construction engine and gearbox motorcycles.

The 1969 "Isolastic" frame Norton Commando

Norton Commando

The Norton Commando is a British motorcycle with an ohv pre-unit parallel twin engine, launched by the Norton Motorcycle company in 1967. Initially a nominal 750 cc displacement, actually , in 1973 it became an 850 cc, actually ....

 had the engine, gearbox, and swingarm bolted together on plates bolted to the frame with shimmed rubber bushes.

Among the Japanese manufacturers, Kawasaki

Kawasaki Heavy Industries

is an international corporation based in Japan. It has headquarters in both Chūō-ku, Kobe and Minato, Tokyo.The company is named after its founder Shōzō Kawasaki and has no connection with the city of Kawasaki, Kanagawa....

 produced a separate construction 650 cc vertical twin, inherited as a consequence of taking over Meguro in 1964,. The Meguro 650 was a copy of the BSA A7

BSA A7

The BSA A7 was a British motorcycle manufactured by Birmingham Small Arms Company at their factory in Armoury Road, Small Heath, Birmingham in 1946. There were actually two versions of the A7, the original 495cc version and an improved 497cc version launched in 1950...

 and BSA A10 design, external appearance was similar although no parts are interchangeable.