Unit construction

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For the vehicle design where the vehicle's skin is used as a load-bearing element, see Monocoque.
1921 ABC opposed twin unit construction engine / transmission
1923 BMW R32 unit construction engine / transmission

Unit construction is a term used to describe motorcycle or motor car engine design where both the engine and gearbox are an integrated unit within the same casing. Engine and transmission may or may not share a common oil supply. Prior to unit construction, the engine and gearbox had their own separate casings and oil reservoirs and were connected with additional components, usually a chain drive, either exposed or in an oil bath primary chaincase. Early engine design had been of Pre-unit construction where engine and gearbox were in separate casings. The primary chaincase was usually made of pressed metal, but some were cast.

The advantages of unit construction are:

  • fewer external oil lines to leak/fracture
  • no external drive connecting engine and transmission, thus simplifying maintenance
  • mounting in the frame is simpler - no need to align engine and transmission within frame
  • the combined casting may be stronger or lighter - hence more easily used as a stressed member of the frame.

The chief disadvantage is that the engine and transmission have different lubrication requirements. In a design where the engine and transmission use a common oil supply, the oil used must take the requirements of both into account. (eg 1972 Ducati 750) Also the gearbox cannot be changed to that of another manufacturer. If different gears are required, they have to fit in the existing unit construction casting.

The chief advantage of separate construction is the ease of removing gearbox or engine separately for maintenance, or for complete gearbox replacement, with stronger gears, or more gears, or better spaced gears.


Early History

Alfred Angas Scott, of Scott motorcycles fame, patented an early form of caliper brakes in 1897 (Patent GB 1626 of 1897),[1] designed a fully triangulated frame, rotary induction valves, and used unit construction for his motorcycle engine. Scott started making boat engines in 1900. He patented his first engine in 1904 (Patent GB 3367 of 1904)[1]and started motorcycle production in 1908 with a vertical two-stroke 450 cc twin, with patented triangulated frame, chain drive, neutral-finder, kick starter (Patent GB 27667 of 1908),[1] and two-speed gearbox. His patented two-stroke engine designs are still the basis of modern two-stroke engines.[2]

In 1911 Singer offered motorcycles with unit-construction 299 cc and 535 cc engines.[2]

In 1914 ABC founder Granville Bradshaw designed a unit-construction horizontally-opposed ('flat') twin for Sopwith Aircraft, who, at the time, also made motorcycles.[3]

In 1921 an expanding Bianchi (Italy) showed its first unit-construction side-valve 600 cc V-twin.[4]

In 1923 Rover introduced a 250 cc unit-construction model, followed by a 350 cc in 1924, but production ended in 1925.[5]

In 1923 the advanced three-speed Triumph single cylinder 346 cc sv unit-construction Model LS appeared, but did not sell well, and ended production in 1927.[4]

In 1923, BMW released its own unit construction shaft drive boxer twin of 498 cc. BMW has never built a motorcycle with a separate gearbox.[6]

From 1924 FN single cylinder engines changed from semi unit construction (as seen in the last semi-unit single, the 1922 FN 285TT, in its last year of sale in 1924,) to unit construction engines (as seen in the new-for-1924 M.60).[7]

In 1928 BSA made their first and only two-stroke, a 175 cc unit construction bike, for only one season, otherwise four stroke twins became unit construction in 1962.[8]

The 1930 Triumph 175 cc Model 'X' two-stroke, two-speed is their first "all-unit construction" two stroke single cylinder engine.[4]

From 1932 New Imperial was known for pioneering innovations in unit construction on motorcycles . They made the Unit Minor 150 and Unit Super 250 in this manner and by 1938 all of their machines were unit construction.[9][10]

In 1938, Francis-Barnett offered a 125 cc unit-construction Snipe.[11]

In 1946 the Series B Vincent employed unit construction and used the engine-gearbox as a stressed member of the frame.[12]

The 1947 Sunbeam S7,an advanced overhead-cam, longitudinal twin, unit construction motorcycle, designed by Erling Poppe, used shaft drive.[2]

In 1957 the Royal Enfield Clipper was replaced by the unit-construction Crusader. [5]

In 1957 the first unit construction twin cylinder motorcycle made by Triumph, the 350 cc (21 ci) 'Twenty One' 3TA , designed by Edward Turner and Wickes, was introduced for the 21st Anniversary of Triumph Engineering Co. Ltd. Unfortunately it also had the first "bathtub" rear enclosure, which proved a sales failure.[13]

The 1958 Ariel Leader used unit construction.[3]


Triumph Bonneville T120 engine

Triumph Motorcycles produced its first single cylinder unit construction model with the 149 cc Terrier launched in 1952. It was quickly followed by the more popular 196 cc Tiger Cub in 1953.[14] They made the first twin-cylinder unit construction model in 1957 with the release of the 350 cc 'Twenty One' 3TA introduced for 21st Anniversary of Triumph Engineering Co. Ltd. This model was a failure in the United States because of its bathtub rear enclosure that many dealers removed to sell the bikes.[15] Several models, like the Triumph Bonneville and other large capacity twins, appeared first in a pre-unit construction design and by 1963, when unit construction was a common motorcycle engine design, these models appeared as unit construction models.[16] The 1963 - 1969 unit construction Triumph Bonneville is now one of the most sought after models by enthusiasts.


Unit singles

BSA Bantam
125 cc engine on model D1

The BSA Bantam range of two stroke engines introduced the unit construction concept to BSA since its introduction in 1949. BSA produced their first four stroke unit construction singles in 1959 when they introduced the C15 to replace the venerable C10 and C11 singles. The unit construction (in contrast to the separate engine and gearbox of the C100/C11) gave the family of motorcycles started by this model its familiar name.

The C15 was intended as a utility "get to work" model, and served this purpose faithfully for many thousands of users. It was a simple and reasonably robust design.

Along with the C15 came the B40, the 350 cc version. This was no faster than the C15, but had a little more lugging power. A version of the B40 was also produced (in considerable quantities) for various branches of the military. These motorcycles (known as the "Ex-WD B40") were more rugged than the vanilla version (in particular, the timing-side main bearing was over- rather than under-engineered and an oil filter was fitted), slightly de-tuned and given a version of the competition frame. For these reasons, these bikes can make very good buying, and are often used as the basis for competition machines.

Several minor changes were made to the C15 in 7 years (with some variations on the theme - the "warmer" SS80 and SS90, plus competition versions).

In 1967 the model underwent some revisions and a name change to B25. The model then continued with little variation until BSA collapsed in the early seventies.

The BSA unit single was an affordable introduction to motorcycling for many young men in the Sixties and Seventies. The simple design meant that inexperienced and under-equipped home mechanics could keep them running under most circumstances. The effects of such inexperienced maintenance led to a slightly undeserved reputation for unreliability - a well maintained and regularly serviced unit single will chug along for a very long time with no problems.

The warmer versions (such as the much-loved Starfire) were generally less robust, but their light weight, enjoyable handling and peppy engines meant that many people considered the hours of necessary maintenance a worthwhile trade-off.

Many BSA unit singles were built, meaning there are few Sixties motorcycles with such a large supply of readily available spares. The tunability and ready supply of these motors, combined with their compact and light(ish) construction has also made them a popular choice for modern "Classic" competition.

The BSA design was based on the Triumph Tiger Cub, first produced in 1952. The continuation of the model until 1973 speaks well for the popularity and utility of this design, but also reflects badly on the forward-thinking and investment of the BSA management. By 1967 unit singles were looking slow and rattly and the "charm" of the traditional British oil-leak was wearing thin. The new breed of Japanese motorcycles arriving on the scene were fast and exotic in comparison, and the buying public can certainly not be blamed for their eventual shunning of the entire British motorcycle industry.


  1. 1.0 1.1 1.2 [1] DiscoveringYorkshire Scott's early motorcycle developments (Retrieved 25 November 2006)
  2. 2.0 2.1 2.2 [2] IanChadwick Brit Bikes S (Retrieved 25 November 2006)
  3. 3.0 3.1 [3] IanChadwick Brit Bikes A (Retrieved 25 November 2006)
  4. 4.0 4.1 4.2 [4] IanChadwick Triumph Time2 (Retrieved 25 November 2006)
  5. 5.0 5.1 [5] IanChadwick Brit Bikes R (Retrieved 25 November 2006)
  6. Title: The Illustrated Encyclopedia of Motorcycles, Editor: Erwin Tragatsch, Publisher: New Burlington Books, Copyright: 1979 Quarto Publishing, Edition: 1988 Revised, Page 89, ISBN 0-906286-07-7
  7. [6] FN History 1924 - 1945 (Retrieved 13 December 2006)
  8. [7] IanChadwick Brit Bikes B (Retrieved 25 November 2006)
  9. [8] IanChadwick Brit Bikes N (Retrieved 25 November 2006)
  10. [9] Is-it-a-lemon New Imperial (Retrieved 25 November 2006)
  11. [10] IanChadwick Brit Bikes F (Retrieved 25 November 2006)
  12. [11]Is-it-a-lemon Motorcycle: Vincent(Retrieved 25 November 2006)
  13. [12] IanChadwick Triumph Time3 (Retrieved 25 November 2006)
  14. [13] Triumph Tiger Cubs & Terriers History (retrieved 1 August 2006)
  15. [14] Triumph Motorcycles timeline: 1946-1962
  16. [15] Triumph Motorcycles timeline: 1963-1972: The Glory Years

See also

  • ABC Motorcycle
  • BMW motorcycles
  • List of BSA motorcycles
  • List of Triumph motorcycles