pistons and the face of th'e cylinder block adjacent to the cylinder bores.
With a clean rag damped with paraffin clean off every trace of foreign matter
remaining, but do
not
attempt to polish things up with emery cloth or other
abrasive, or you will do far more harm than good. When th,ese two pistons
have been properly cleaned give the starting handle half a turn and clean
the other two in the same way.
Attention should now be given to the cylinder head. Remove the
sparking plugs and turn the head upside down, thus exposing the combustion
chambers, in each of which will be observed the circular heads of two valves
-one inlet
and one exhaust.
With
a
blunt screwdriver carefully scrape away the carbon deposit
adhering to the surface of the combustion spaces, taking particular care to
go round each valve with a small screwdriver in order to remove all trace of
carbon. Clean the combustion chambers and valve heads carefully with rag
moistened with paraffin.
Removing
the
Valves.
Having thoroughly cleaned the combustion
spaces and v:llve heads, place the cylinder head on the bench the right way
up. To obtain access to the valve springs,
it
is necessary to remove the
camshaft. This is easily achieved by unscrewing the four nuts holding the
camshaft bearing caps in position. These should he given half a turn in
rotation, in a similar manner to the cylinder head retaining nuts, until they
are
errentually removed. The camshaft can then be lifted from its bearings
and removed by passing
it
through the valve cover saddles. Removal of the
camshaft enables all the valve rockmer-arms to be swung clear of the valves.
A small wood block slightly thicker than the depth of the combustion
spaces and an easy fit within them should now be prepared.
Slip this block
into the combustion space so that the valve heads are resting upon it, in
ordtr
that the valve spring can be compressed with a suitable tool such as
a
screwdriver, without forcing the valves open. Depression of the springs will
expose two small conical cotters engaging in a groove in the pencil-like end
of
the valve stem. Removal of the cotters will release the valve springs cap
from the
halve
stem, permitting its removal and releasing the valve spring.
When the springs of both valves have been removed, the head may be raised
from the bench and the wood block withdrawn, thus allowing the valves to
be drawn from their guides. Repeat this operation on the remaining valves
until
they are all removed.
Grinding-in
the
Valves.
Examination of the valves will show
that the
edges of their mushroom-like heads are bevelled off at an angle to correspond
with the similar bevelled edges of the valve ports in the cylinder head and
thus provide a gastight joint when they :Ire in contact. Obviously, gastight-
ness is not attained
if
these bevelled surfaces are dirty or
"
pitted," and in
order to clean them up so that they make perfect contact over the whole of
their surfaces it is ntecess:lry to grind them in. When grinding-in the valves
the utmost care should be taken to sec that they are inserted into the correct
port. Hach valve is clearly numbered on its head, and on the lower face of
the cylinder head adjacent to the valve port will be found its corresponding
number.
[)/l
I'AGF
SEVENTEEN
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