with suitable guide pin.
(2) Wire-wound pistons fitted to 500 c.c. models.
(3) 500 c.c. type high crankcase used for 350 c.c. models after engine
number 8000.
(4) 500 c.c. flywheels used for the 350 c.c. model.
In addition to these engine design changes;
(5) Larger brakes (7" diameter) were fitted.
1949 MODELS
(1) New type cylinder head with hair-pin valve springs with rocker box
to suit.
(2) Valve lifter transferred from crankcase to rocker box.
(3) Wire-wound pistons for all models.
(4) Longer valves, hardened valve end caps discarded.
(5) New rockers for longer valves, also new valve guides.
1950 MODELS
(1) Alloy cylinder heads and barrels used on Competition models only.
Steel crankpin washer in place of bronze type.
1951 MODELS
(1) Alloy cylinder head used on both touring type engines. Crankpin
washers discarded, flywheels altered.
1952 MODELS
(1) Open tray valve spring seat, prongs for valve springs increased in
length.
(2) Cylinder barrel lengthened
8" on 500 c.c. touring engine, compression
plate discarded.
(3) Recess for driving side bearings in crankcase with two diameters, for
close and easy interference fit to avoid “end loading” of these
bearings.
(4) Top compression ring chrome plated.
1953 MODELS
No change.
1954 MODELS
(1) Oil feed in rocker box modified to increase oil supply to rocker end
of inlet valve and stop valve spring wear.
(2) New rockers for valve ends with groove in side for oil duct.
(3) High-lift cams.
(4) Larger diameter timing side shaft, with flywheel to suit.
(5) Two diameter timing side bush, steel sleeve discarded.
(6) Automatic ignition control on 500 c.c. model.
1955 MODELS
(1) New crankcase to use one small and one large driving side bearing.
(2) New driving side flywheel (keyways at 180°),
(3) Circlip fitted to exhaust valve guide,
1956 MODELS
(1) Cylinder wall oil feed discontinued.
(2) Compression ratio increased to 7.5 for 350 c.c. models, 7.3 for 500 c.c.
models.
(3) Oil tank felt filter deleted and magnetic filter fitted in crankcase.
15
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