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4 1/4 DB Series M All-Weather Tourer by Maberly
The Bentley 3½ Litre (later enlarged to 4¼ Litre) was presented to the public in September 1933, shortly after the death of Henry Royce, and was the first new Bentley model following Rolls-Royce's acquisition of the Bentley brand in 1931.
Bentley sold only the drivable bare rolling chassis with engine and gearbox, scuttle and radiator, ready for coachbuilders to construct on it a body to the buyer's requirements. Many distributors ordered their preferred bodies as showroom stock to enable them to stock finished cars ready for immediate sale.
Bentleys of this era are known as Derby Bentleys because they were built in the Rolls-Royce factory located in Derby, England. Those of Bentley's previous independent era are Cricklewood Bentleys.
Chassis series G to L (excluding I) were 4¼ Litres, and the M series was the 4¼ Litre Overdrive chassis. Each series consisted of 100 chassis numbers, either odd or even. The numbers 13 and 113 in each series were not used, to avoid upsetting superstitious customers.
Beginning in March, 1936, a 4¼ Litre version of the car was offered as replacement for the 3½ Litre, in order to offset the increasing weight of coachwork and maintain the car's sporting image in the face of stiff competition. The engine was bored to 3½ in (88.9 mm) for a total of 4.3 L (4257 cc/259 in³). From 1938 the MR and MX series cars featured Marles steering and an overdrive gearbox. The model was replaced in 1939 by the MkV, but some cars were still finished and delivered during 1940-1941.
Designated the "K" series, the new chassis was built in two consecutive runs, the "KT" and KU", the Derby works producing one hundred of each, with the KTs being numbered evenly and the KUs being assigned odd numbers. The new engine was introduced as a bored-out version of Bentley's original 3.6-liter motor. The K chassis offered a number of interesting technological changes over their predecessors, although some enthusiasts remained a bit skeptical of items such as the new "de-turbulated" cylinder head design. This alteration involved changing the form of the combustion chamber. As one marque expert explains, "The earlier cylinder heads' combustion chamber was smaller at the entrance with a kind of edge or lip on the fire face of the head—kind of bulb-shaped. It was to create turbulence of fuel/air mixture upon intake. This lip at the leading edge partially sealed off the chamber in an attempt to slow fuel flow into the cylinder and enable all of the fuel to ignite." Bentley engineers found if they reduced the "swirl" of the incoming fuel/air mixture, or "de-turbulated" it by removing that lip edge and making it straight- sided, it helped the cylinder fill and ignite more effectively." However, it also effectively reduced the compression ratio. Whether that was reflected in the engine's performance is arguable, but it is said that those who raced Bentleys - and there were many – preferred the "turbulated" combustion chamber.
1234 4¼ Litre cars were built, with Park Ward remaining the most popular coachbuilder. Many cars were bodied in steel rather than the previous, more expensive, aluminium over ash frame construction.
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