Advertisement
2-38
PISTONS
RODS
ENGINE
(60)
dial at "0" and measuring across the gauge con-
tact points with outside micrometer while
gauge is at the same "0" setting.
If
a cylinder bore is moderately rough or
slightly scored but is not
out-of-round
or ta-
pered, it usually is possible to remedy the con-
dition by honing the bore to fit a standard serv-
ice piston, since standard service pistons are of
high limit
diameters.
If
cylinder bore is very
rough or deeply scored, however, it may be
necessary to rebore the cylinder and fit an over-
size piston in order to insure satisfactory re-
sults.
If
cylinder bore is tapered .005" or more, or
is out-of-round .003" or more, it is advisable to
rebore for the smallest possible oversize pistons
and rings. With this amount of bore wear, some
piston wear has usually taken place so that the
total clearance in the ring travel will be suffi-
cient to produce noisy piston operation.
c. Inspection of Pistons, Rings and Pins
Clean carbon from piston surfaces and under
side of piston heads. Clean carbon from ring
grooves with suitable tool and clean out oil
holes in oil ring grooves. Remove any gum or
varnish from piston skirts with suitable
solvent.
Carefully examine pistons for rough or scored
bearing surfaces, cracks in skirt or head,
cracked or broken ring lands, chipping or un-
even wear which would cause rings to seat im-
properly or have excessive clearance in ring
grooves. Damaged or faulty pistons should be
replaced.
The pistons are cam ground, which means
that the diameter at right angle to piston pin
is greater than the diameter parallel to piston
pin. When a piston is checked for
size
it must
be measured with a micrometer applied to skirt
at points exactely 90 degrees to piston pin. See
figure 2-35. Measurements should be made at
top and bottom ends of skirt; the diameter at
Figure 2-35-Measuring Piston with Micrometer
top end will normally be very slightly less than
at bottom end after a piston has been in service
in an
engine.
Inspect bearing surfaces of piston pins and
check for wear by measuring worn and unworn
surfaces with micrometers. Rough or worn pins
should be
replaced.
Test fit of piston pins in
piston
bosses.
Sometimes pins will be found
tight due to gum or varnish
deposits.
This may
be corrected by removing the deposit with a
suitable solvent.
If
piston bosses are worn
out-of-round
or
oversize and the piston is
otherwise satisfac-
tory for service, the bosses and connecting rods
may be honed or reamed for oversize piston
pins which are furnished for
service.
Piston
pins must fit pistons with an easy finger push
fit at 70°F.
Examine all piston rings for scores, chips, or
cracks, and for tensions as compared with new
rings. Place all rings except Flex-Fit rings in
cylinder bores at lower end of ring travel and
check gaps, which are normally .010" to .020".
If
gaps are excessive it indicates that rings
have worn considerably and should be
replaced.
d. Reboring Cylinders and Fitting
New Pistons
If
one or more cylinder bores are
r
ough,
scored, or worn beyond limits prescribed under
Inspection of Cylinder Bores
(subpar.
b)
,
it
will be necessary to smooth or
true
up such
bores to fit new pistons.
If
relatively few bores
require
correction it
will not be necessary to rebore all cylinders to
the same oversize i~
order
t
o
maintain engine
balance, since all
over-size
service pistons are
held to the same weights as standard size pis-
tons. If conditions justify replacement of all
pistons, however, all new pistons should be the
same nominal size
.
Standard size service pistons are high limit
or maximum diameter; therefore, they can
usually be used with a slight
amount
of honing
to correct slight scoring or excessive clear ance s
in
engines having
r
elatively low mileage.
Serv-
ice pistons
are
al
so furnished in .005", .010" ,
.020" and
.030"
oversizes. All service pistons
are diamond bored and selectively fitted with
piston pins; pistons are not furnished without
pins.
No attempt should be made to cut down over-
size pistons to fit cylinder bores as this will
destroy the surface treatment and affect the
Advertisement
