Series D bikes having 1⅛" Amal Type 389/10 units. While other contemporary motorcycles tended to be polished and chromed, founder Philip Conrad Vincent (PCV) decreed that the Black Shadow engine itself be black. The engine's signature colour was achieved by applying a "pyluminising" coating of chromate anti-corrosion primer over which a layer of Pinchin & Johnson black enamel was added. The part was then baked in an oven for 2 hours at . Although some have claimed that the black finish increased the rate of heat rejection from the engine, its main real benefit seems to have been its visual impact. Unit construction combines the bike's engine and 4-speed Vincent transmission in a single housing. The ratio of the bottom gear was raised from 9:1 to 7.2:1 Frame and bodywork As in the Series B Rapide, the engine is mounted as a stressed member, eliminating the need for a downtube cradle. Series B and C Black Shadows use an upper frame member (UFM) consisting of a fabricated steel box that doubles as an oil tank. On the Series D bikes the revised UFM is a simple tubular member with brazed lugs at each end bolted to a slightly modified version of the Series C malleable iron steering head casting. This revised UFM no longer carries oil and so a separate oil tank is fitted. Due to extensive use of aluminium alloy, weight for the complete bike is kept to a relatively light . The standard handlebar on all models of HRDs and Vincents is the black enamelled `HRD straights'. Series B and C bars are 7/8 inches in diameter and 25 inches long with the ends bent back at 20°. Series D bars are 1½ inches longer. The Black Shadow could be had in Touring specification, and a sidecar could be fitted on either side of the bike. The Series B and C machines do not carry any indication of the model name in their transfers, lining or badges. Beginning in 1955 on the Series D Black Shadow the figure of Mercury is joined by transfer text reading "Black Shadow". Suspension The rear suspension of the Black Shadow is a cantilever system, with a rear frame member (RFM) of two parallel upright triangular members pivoting from heavy plates on the back of the transmission case at the bottom and attached at the top to two spring boxes angled out from