page_title: Unit construction
section_title: Triumph
hierarchical_section_title: Unit construction / History / Triumph
caption_attribution_description: Vintage Triumph Bonneville
context_page_description: Unit construction is the design of larger motorcycles where the engine and gearbox components share a single casing. This sometimes includes the design of automobile engines and was often loosely applied to motorcycles with rather different internal layouts such as the flat twin BMW models.
  Prior to unit construction, the engine and gearbox had their own separate casings and were connected by a primary chain drive running in an oil bath chaincase. The new system used a similar chain drive and both had 3 separate oil reservoirs for engine, gearbox and primary drive.
  Triumph and BSA were already using cast alloy chaincases and started converting to unit construction in the 1950s. A driving factor behind the BSA/Triumph change was that Lucas had declared an intention to abandon production of motorcycle dynamos and magnetos, and instead produce only alternators. By contrast, Velocette, Matchless/AJS and Norton motorcycles continued to be pre-unit construction until the end of production in the 1960s and 1970s respectively. In reality, the casings were not really "unitary", as the crankcase section was vertically divided in the middle and no oil was shared between the three portions.
context_section_description: 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. They made the first twin-cylinder unit construction model in 1957 with the release of the 350 cc Twenty One 3TA (so named because it was approximately twenty-one cubic inches capacity). The 500 cc Triumph 5TA followed, and the 650 cc models were made unit construction in 1963. The 1963—1969 unit construction 650 cc Triumph Bonneville has become sought- after models, partly as the 1970-onward oil-in-frame chassis was considered inferior.
caption: Triumph unit-construction engine
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