The 1921 French Grand Prix proved a successful testing
ground for Marston's prototype, long-stroked engine of 492cc. It had a
77mm bore by 105.5 mm stroke. The 'Longstroke' model went into production
the following year, winning the Senior T.T. It was the last side-valve
engined motorcycle to do so.
The Sunbeam Longstroke was the fastest production,
single cylinder, side-valve motorcycle in its day. The model stayed in
production for the life of the company and the subsequent AMC take-over in
late 1937. It underwent many stylistic changes; notably from flat tank to
saddle tank in 1929. Known as the Model 6 prior to ICI's takeover of the
company in 1929, it was marketed as 'the Lion' during the 1930s - the tank
emblazoned with ICI's trademark lion emblem.