Norton Model 50
The pre-war version of the Norton Model 50 started production in 1933 and used a smaller version of the engine in the Norton Model 18 and ES2. During the '30s, the open diamond frame and cycle parts were usually the same as on the Model 18 whereas the ES2 used the full cradle frame.
In 1934, it gained a second drive side main bearing and the timing gears were revised, reversing the direction of magneto rotation in the process. The Norton/Burman gearbox was used from 1936 and in 1938 there was a new cylinder head with through studs to retain the head and barrel. The original exposed hairpin valve springs were superseded that year by enclosed coils; a neater looking arrangement but the facility to change the springs without removing the head was lost.
The silencer fitted onto the 1938 OHV and SV machines was a truly hideous affair, a sort of rectangular box with two short outlet pipes that quickly got labeled the 'cow's udder'.
The Model 55 seems at first sight to be a twin cylinder machine but is in fact a twin ported variant of the Model 50 which was only made in the pre-war years and is now very rare. It was first offered to customers in 1933 but quite whether the extra exhaust equipment made any difference to the performance is questionable as the engine did not have an extra exhaust valve to go with it. In 1939 the price margin between the Model 50 and the Model 55 was £2. The frame is of the open diamond type, using the engine to provide a rigid joining section. Machines produced to 1937 had exposed hairpin valve springs, but these were changed to enclosed coils in the following year. Twin port machines were just a fleeting fad of the '30s, production was not restarted after World War II though the Model 50 did carry on.
The Model 50 was re-introduced in 1955 using the ES2/19S cycle parts but with engine dimensions of 71 x 88mm and a slightly higher compression ratio of 7.3 to 1. Many changes occurred in 1957, most notably a new cylinder head with integral push rod tubes, new cams, an improved frame and a new rear hub and brake back plate. 1959 saw a change to alternator electrics, coil ignition and the Wideline Featherbed frame with 'short' Roadholder front forks. In 1960 the gearbox internal ratios were changed and 1961 saw the introduction of the Slimline Featherbed frame, and a revised shape petrol tank with new tank badges which allowed the use of two-tone paintwork. Production ceased at the end of 1963.