The 500 c.c. version has a bore size of 86 mm. and 85.5 mm. stroke, as
opposed to 596 c.c. engine, which has a bore size of 89 mm. and 96 mm.
stroke. The peak r.p.m. of the 500 c.c. engine is 6,200, the larger capacity
engine peaks at 5,500 r.p.m.
A number of engines were fitted with a Monobloc type carburetter with
a bore size of 1
x".
Where a sudden change in diameter between the carburetter outlet, the
carburetter spacer and the inlet port occurs, the parts or whatever part
is affected should be flared out or blended, to overcome any abrupt
change in diameter.
The use of G.P. type carburetter with a bore size of 1
a" with a parallel
inlet tract will increase the volumetric efficiency of the engine. Attention
is drawn to the use of late type camshafts in early type engines, see para-
graph on 'valve timing'.
Port polishing. If, during the process of tuning, the ports are polished,
the metal removed must be of the smallest amount possible to avoid
drastic changes in the port shape. This applies particularly to the metal
immediately below the valve inserts, which could cause the inserts to
collapse. It is usually considered that a nice bright polished finish to the
sphere of the cylinder head and piston crown is essential for best results.
This is not so, for the ideal condition of the combustion chamber is when
it has reached a nice black or ebony-like finish. This probably explains
why it takes a little time for the engine to settle down before coming on
'full song'. Therefore the sphere of the head should not be disturbed,
other than to remove soft carbon formed near the exhaust valve. If the
engine is 'set up' correctly and under race conditions, the carbon forma-
tion should be negligible.
Compression ratio. The normal compression ratio on the CS and TCS
engines is 8.7 to 1. An alternative piston giving a ratio of 12 to 1 is
available, With this piston the ignition is put back to a maximum of 34°.
Straight petrol can be used, octane 100 (research method). A suitable
spark plug with a high heat factor is essential.
1957-1959 engines. The cylinder head and piston introduced for the
1960-61 season is more efficient that the earlier type. These new parts can
be used together on earlier engines, but not separately.
The big end is robust in construction, the crankpin is materially and
dimensionally identical to the G50 race model Matchless, As with the
250 c.c. Scrambler Model, it is of paramount importance that the big end
is rigidly assembled, to avoid power loss, apart from the risk of breakage.
In consequence, if renewals are made, both flywheels must be forced
firmly against the two shoulders of the crankpin, by using a press designed
for this work. In fact a 12-ton press is used in the factory to assemble the
flywheels. An attempt to drive the flywheels together and rely on nut
tightness can only lead to mechanical trouble.
Big end wear. Wear on both edges of the crankpin is due to inertia wear,
caused by the crankpin cage rubbing on the crankpin. This becomes most
acute when the engine is run at high r.p.m. with little or no load. This
wear does not affect or cause movement in the big end assembly. Wear on
the roller path of the crankpin can be due to abrasive impregnated in the
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