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ENGINE
PISTONS-RODS
2-39
PLACE FEElER GAUGE HERE
Figure
2-36-Position of Fee
ler
Gauge for Checki
ng
Fit
of
Piston
sufficient to produce a variation of .0005".
The high and low limits on clearance of
pis-
tons in cylinders at 70° F. are given in the fol-
lowing table. The clearance should be measured
with two feeler gauges of "Go" and "No Go"
thicknesses specified in the table:
)
Series
Occasionally an engine which
has
been
r
e-
bored will continue to use oil excessively
after
sufficient mileage has accumulated to
break in
new
rings.
This indicates that
cylinder bores
were
not
finished
as smoothly as
requi r ed. The
r
ings
will polish the cylinder wall but
will be
worn excessively in doing so.
Installat ion of a
new set
of
rings will frequently correct
such
ca
ses;
however, the cylinder bores
should not
Feeler Gauges
Clearance Limits
uGo"
"No
Go"
.0018" to .0024"
.0015"
.002"
.0020" to .0026"
.0015"
.002"
Feeler gauges should be approximately 12"
long and
1!2"
wide, except that the "Go" gauge
should
be %," wide, if obtainable, since
this
width gives a more sensitive test of
clearance.
Feeler gauges mu
st be
free of dents, burrs and
rough edges.
Wipe pistons and cylinder walls clean and
dry, then wet pistons and cylinders with pene-
trating oil, which will materially aid in
deter-
mining fits with feeler gauges.
Suspend the specified "Go" feeler
gauge f
ull
length in cylinder bore at point 90 degrees
t
o
centerline of engine, then insert piston
into
bore with head downward and piston pin paral-
lel to centerline of
engine.
See figure 2-36. The
piston should move downward the length of
cylinder by its own weight when tested with
the "Go" feeler
gauge.
Repeat the test
using
the specified "No Go"
feeler
gauge; the
piston
should fit closely enough so that it will not
move downward by
its
own weight.
40-50
70
weight. The smallest possible oversize service
pistons should be used and the cylinder bores
should be honed to size for proper clearances.
Before the honing or reboring operation is
started, measure all new pistons with microm-
eter contacting at points exactly 90 degrees to
piston pin (fig. 2-35) then select the smallest
piston for the first fitting. The slight variation
usually found between pistons in a set may
provide for correction in case the first pist on is
fitted too free.
If
a rebore job is required, a boring ba r of
the fly cutter type is recommended. When
r
ebor -
ing the cylinders, all crankshaft bearing caps
must be in place and tightened to proper torque
to avoid distortion of bores in final
assembly.
After reboring it is always advisable to polish
cylinder bores with crocus cloth, to eliminate
excessive wear on pistons and rings.
If
cylinder bores are to be honed, use clean
sharp stones of proper grade for the amount of
metal to be removed. Dull or dirty stones cut
unevenly and generate excessive heat. When
using coarse or medium grade stones use care
to leave sufficient metal so that all stone marks
may be removed with the fine stones used for
finishing. The final honing must be done with
the finest grade of stones and the cylinder walls
must be polished with crocus cloth to produce
a very smooth finish.
It
is of the greatest importance that re-
finished cylinder
bores
are trued up to have not
over .0005"
out-of-round
or taper. Each bore
must be final honed to remove all stone or
cutter marks and provide a smooth
surface.
During final honing, each piston must be fitted
individually
to the bore in which
it
will be
installed
and should be marked to
insure
cor-
rect inst allation .
Aft er
final
honing and
before
the piston
is
che cked
fo
r fit , ea ch cylinde r
bor e
must
be
thoroughly wash ed
to remove all trac es
of ab-
rasive and then
dr ied
thoroughly. The dry bore
should then be
br ushed clean with a
power-
driven fibre brush. If all traces of abrasive are
not removed, rapid wear of new pistons and
rings
will result.
Both t
he
piston and the cylinder block must
be at the same temperature of approximately
70
°
F
.
when
the
pist on
is
checked for fit in
cylinder bore; t
heref or e
the
cylinder should
be
allowed
t
o
cool
after boring
or
honing
and be-
fore
the piston fit
is
checked . This
is impor tant
because
a
difference of 10°
F
.
between parts is
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