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A
THE ENGINE
be rc:faced with
a
suitable grinder
or new valves should
be fiued.
If
valve scats show signs of pitting or unevenness they
should be trued by the usc of a suitable ginder or
special cutter.
When
usmg
a
cutler care
must be exercised
to
remove only as little m¢lal
as
is necessary
to
ensure a
true
surface.
When
grinding a valve onto its seating
the
valve face
should
be smeared lightly with fine- or
rnedium-gade
carborundum
paste and then lapped in
with a
suction
grinder
(special tool 18G29).
Avoid
the
usc of excessive
quantities
of grinding paste and sec that it remains in
lbe region
of
the
~-alve
seating only.
A
light coil spring placed under the
vah-e
head
will
assist
considerably in
the
process of grinding.
The valve
should be
ground
to
its scat
l'i
th
a semi-rotary
motion
a nd occasionally
allowed
to
rise
by the
pressure
of the
light coil spring. This
assists
in
spreading
the paste
evenly over
the valve face
and
scat.
It
is necessary
to
carry
out
the grinding
operntion until a
dull, even, matt
surface, free from
blemish,
is
produced
on
the
valve
seat
and
valve
face.
On
completion
the
valve seat
and ports
should
be
cleaned
thoroughly
with
a
rag soaked in
paraffin
(kero-
sene), dried, and then thoroughly cleaned
by compressed
a
ir. The valves
should be
washed in paraffin (kerosene)
and all traces of
grinding paste
removed.
After
replacing the valves test them by pouring a small
quantity
of paraffin
(kerosene)
through the ports. If
there is any leakage
of
paraffin
(kerosene)
past the
valve
seatings
the valve
should be removed and examined and,
if
necessary,
rc:gound.
Section
A.18
TAPPET
C LEARANCES
If the engine
is
to
give its best
perfor1.1ancc
and
the
valves are
to retain
their ma.timum
life
it is essential
to
maintain
the correct tappet clearances.
Accordingly it
is
recommended
that
the
clearance be checked at
regular
intervals
and any
necessary adjustments
made.
The
clearance figure for both
inlet and exhaust valves
is
shown in the 'GENERAL DATA' section.
The engine
is
designed
to operate with this clearance and no depar-
ture from
it
is
permissible.
The
valve clearances are adjusted bymeansofhardened-
stecl shims interposed between
the under
side
of each
tappet and the top
of
the
valve stem.
Turn the
engine and check
the clearance of each
tappet
l'ith
a
feeler gauge. Care
must be
taken
to ensure
that
the clearanee is measured on
the
back of
the cam,
i.e
. opposite
the
peak. Take
note of
the cleara nce figures.
To adjust the tappet clearances disconnect the cam-
A
.8
sbaft
driving sprockets
(Section
A.JO)
and
remove
the
camshafts.
Withdraw
one of the tappets
which
requires
adjustment, using a valve grinding suction tool
(sec
Fig.
A.S), and rcmo¥e the
shim.
Insert a new
shim
of
suitable
thickness to
corn:ct
the valve
clearance
and replace
the
tappet. Correct the olber valve
clearances in
a
similar
manner and
replace
the camshafts.
Re-check
the
valve
clearances after finally
tightening
the camshaft bearing
nuts,
and carry
out
any further
adjustments if
necessary.
Variatioas in the tappet
clearances have
a
marked
inJiuence upon the
valve timing
and
it
is
therefore
advisable to
check
the
timing
(sec Section A.J9) after
adjusting lbe tappets.
The shims are
a
vailable in 16 sizes and the thickness of
a shim is indicated by a stamped number.
The
n
umbers, with corresponding
sizes,
are detailed
in
the
table below:
Number on
shim
Siu
I
·086 in.
(2·182
mm.)
2
·088 in. (2·233 mm.)
3
{)9()
in. (2·284 mm.)
4
·092
in.
(2-335
m
m.)
5
·094
in.
(2·386 mm.)
6
-()96
in.
(2-437
mm.)
7
·098
in. (2·488
mm.)
8
·100
in.
(2·S40
mm.)
9
·102
in.
(2·S91
mm.)
10
·104 in. (2·642 mm.)
II
·106
in.
(2·693 mm.)
12
·108
in.
(2-744
mm.)
13
·1
10
in.
(2·795
mm.)
14
·112 in. (2·846 mm.)
15
·
114
in. (2-897
mm.)
16
·116 in.
(2·948
mm.)
MOA
(Twin Cam).
Issue
2. 23026
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