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Pistons
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Tcppcti
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RECOMMENDED REBORE OVERSIZES ARE AS FOLLOWS:-
Cast lron Block +.010"/+.020"/+.040" 500
C.C.
Cast Iron Block +.010"/+.020" 650
C.C.
Alloy Block +.Olo"]+.020" 500
C.C.
It
w~li be noted that the alloy block can only be re-bored to a maximum of
+
.020°.
this
is
due to the fact that cylinder liners are fitted. Further re-boring would thin
down the liner beyond safety limits.
it
is pos's~ble
ta
change the liners, but work
such as this should only be entrusted to a competent engineering concern, who
will have the necessary eqqipment.
Tappets.
The base of the tappets have a "Stellite" tip fitted
;
this material has
grear resistance to wear and under general running conditions the tappets will not
require changing until a considerable mileage has been covered. The centre of the
tip may show signs of indentation which
is
caused by the peak of the cam. This does
not however indicate wear and the tappet can be re-insraiied.
Tappet Blocks.
It
is
not necessary to remove the tappet blocks from the cylinder
base for inspection
;
the amount of wear can be estimated by rocking rhe tapper
head. The tappet stem should be a sliding
fit
in the tappet block bores and the
tappet base must fig snugly in the block base. Slackness at these points wili cailse
excessive mechanical clatter. To remove the tappet blocks from the cylinder base
flange, place the cylinder block downwards on the bench, remove the locking screw
and drift the tappet block out of the cylinder flange. When fitting the new tappet
block, grease the outer surface and if possible support the cylinder flange (See
Fig. 14). Do not forget to line up the locking screw hole In the tappet block to
that in the flange. Replace the locking screw.
Pistons. If the pistons are to be further employed, the rings and gudgeon pins
when removed from each piston should be kept separate to ensure correct replace-
ment. Carefully clean away the carbon deposit from the piston crown, taking
particular care not to scratch the
metal surface. The light deposit of burnt
oil
in
the piston skirt should be removed by rubbing with a rag which has been dipped
in pettol.
Never in any circumstances use an emery cloth.
To clean the ring
grooves,
it
is advantageous to use an old broken ring by inserting the broken end
into the groove and working
it
around the circumference. Clean out the oii drain
holes in the scraper ring groove and thoroughly wash the piston. Or, the irkside
surface of the piston rings wili be found
a
light deposit of carbon which must be
removed if the rings are to be re-fitted.
Now roll each ring around
its
respective groove to ensure that
it
does not stand
proud of the piston after being fitted correctly and compressed. Before fitting new
rings, the gap must be checked in the lowest part of the cylinder bore. The rirry
must lie square to the bore for checking purposes and to ensure this, place the
bottom1 of the piston skirt onto the ring and ease
it
about
f"
(13 mm.) down the
bore. Check the gap by feeler gauge.
Min.
Max.
Compression Ring Gap
...
0.010" (0.25 mm.) 0.014" (0.35 mm.)
Scraper Ring Gap (500 c.c.)
...
0.007" (0.18 rnm.)
0.011" (0.28 mm.)
,,
..
,,
(650 c.c.)
...
0.010" (0.25 rnm.) 0.014" (0.35 mm.)
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