Douglas 1921 4hp 600cc 2 cyl sv

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Douglas 1921  4 HP      596 cc   side valve flat twin       frame# 14949      

This Bristol-based firm developed the horizontally opposed fore-aft unit in about 1907 and continued utilising this engine layout  well into the thirties. 
The earliest Douglas models had direct belt drive, but in 1910 a two-speed gearbox was added, with the control on top of the petrol tank. Douglas machines became very popular because of their lightness,  ease of starting and speed: in the 1912 Junior TT races Douglas took first, second and fourth places. 
In the First World War  thousands of  2 ¾ HP machines were commissioned for use by despatch riders and  performed splendidly. 
In 1915 a 596 cc model was introduced to cater for  the need for a more powerful sidecar puller. 
A notable difference in engine design with the     2 ¾  HP model is the wet sump oiling system.  
A year later the 4 HP model was improved in a number of ways. 
To mention a few: other front forks, new design of exhaust, frame  and tank alterations, front and rear brake changes, many  small engine design improvements. 
To sum it up: quite another machine! Between 1916 and 1921 hardly any changes were made. 
At the 1921 Olympia Show the company remarked: “ The 4HP Douglas remains practically unaltered for the experience gained during the war under extremely unfavourable conditions has made the machine eminently satisfactory in its present form.” The 4 HP disappeared from the range in 1924. 
Note the connection pipes between inlet manifold and exhaust, a feature to preheat the  incoming air. 
This  4 HP is an older restoration with a nice patina.

Douglas 1921 4hp 600cc 2 cyl sv1
Douglas 1921 4hp 600cc 2 cyl sv1 Douglas 1921 4hp 600cc 2 cyl sv1
Douglas 1921 4hp 600cc 2 cyl sv1 Douglas 1921 4hp 600cc 2 cyl sv1
Douglas 1921 4hp 600cc 2 cyl sv1 Douglas 1921 4hp 600cc 2 cyl sv1
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