Motorcycles produced by Vincent H·R·D at their factory in Stevenage, Hertfordshire, England were renowned for their design innovation, engineering excellence and high performance. Already advertising their existing 110 mph Rapide machine as "The world's fastest production motorcycle", in February 1948 the distinctive Vincent Black Shadow was announced with a top speed of 125 mph. Built in three different Series over the course of its life, the line continued until 1955, after which the company stopped all motorcycle production. Design Engine and transmission The Black Shadow uses a version of the air-cooled four-stroke 50° V-twin engine that powers the Vincent Rapide. Bore and stroke of the undersquare twin are 84 mm and 90 mm respectively, giving a displacement of . Each cylinder's two overhead valves are operated through rocker arms and short pushrods by two gear-driven cams mounted high in the engine's timing case. The valves have both upper and lower guides, and the rocker arms are forked at the valve end to operate the valve by pushing on a metal shoulder part way along the length of the stem. On Series B and Series C Shadows the spark-plug for the front cylinder is on the right hand side while the one for the rear cylinder is on the left. On Series D bikes both plugs are on the right. The engine has dry sump lubrication. The electrics on Series D bikes are by Lucas This included a change to coil ignition and a DKX2A distributor. Engines for Black Shadows were largely built out of selected Rapide production parts. The camshafts, for example, were standard Rapide units; MkIs in early examples and MkIIIs from 1952 onward. Parts were hand-selected for tighter tolerances, and airflow was improved by blending and polishing of the engine's ports. Early Black Shadows have a third inner valve spring not shared with the Rapide, but this feature was dropped from later bikes. Due to the low-octane 'Pool' petrol that was all that was available in the early post-war years, bikes of the period could have only low compression ratios. Although the prototype Black Shadow was built with a compression ratio lower than the Rapide's nominal 6.8:1, production Shadows have a ratio of 7.3:1 due to having different pistons from the Rapide, although still supplied by Specialloid. The Black Shadows have different carburettors from the Rapides, with Series B and C bikes having 1⅛" Amal 289 carburettors and