Norton 1939 Model 20 500 cc 1 cyl ohv

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 Norton  1939  “Model 20”  490 cc OHV twin port single frame# 9894  engine # 90675

In 1923 the first OHV model Norton was brought out, the 490 cc  Model 18 with bore and stroke dimensions of 79×100 mm. 
A slightly more expensive twin-port variant – the Model 20 – joined the line-up for 1930.
 In 1931 the side valve and overhead valve models were extensively re-designed, dry-sump lubrication and a rear-mounted magneto being standardised. Further improvements  were made to the engine throughout the 1930s; Norton’s own four-speed foot-change gearbox  replaced the old Sturmey Archer design in 1935.  
In 1938 complete enclosure of the valve gear was effected.
Some of the new features for 1939 were: improved oil control ring,  front brake moved to the right of the machine, tyre inflator moved from tank to rear chain guard, polished instead of sandblasted gearbox cover and magneto  chain cover.    
Compression ratio of the powerful sports engine is 6.5:1.
The Model 18 was revived when Norton re-commenced production in 1946, but the deleted twin-port Model 20 didn’t make a comeback after the war. This attractive Norton has been restored to a high standard.