Reconditioning Cylinder Bores - Cadillac 54-6219 1954 Shop Manual

10-23
ENGINE MECHANICAL
The piston skirt is sufficiently flexible so that
it will contract to overcome this confliction just
as piston rings contract to fit the cylinder.
The above conditions are perfectly normal and
are necessary for long life and quietness of piston
action. No undue friction or scoring will result,
due to the flexibility designed into the casting and
the oval
shape of
the skirt.
b. Feeler Gauge Measurements
Where
no
micrometers are available to measure
piston clearance, the use of a feeler ribbon gauge
will serve satisfactorily, as there is a definite re
lation between feeler gauge pull in pounds and
micrometer clearance in thousandths of inches,
as
shown
in
Fig. 10-26, which is set up
for
.002",
.0025", and .003" feeler gauge thicknesses.
Feeler ribbon gauges for this purpose must be
considered as precision tools and if rusted or
wrinkled are unfit for use. The most convenient
length is about 8", with a cotter pin soldered on
one end to hook onto the pull scale, in addition,
the lower ends should be rounded and any frayed
edges honed to prevent possible scratching of the
cylinder
and incorrect scale readings, Extreme
cleanliness and a little lubrication is also neces
sary for accurate results.
in order to obtain the piston clearance at the
upper end of the skirt, it is necessary to insert
the piston into its extreme upper position, along
with the ribbon, which must be kept located over the
vertical slot and not in excess of 1/2' below the
upper end of the skirt, as illustrated in Fig. 10-23.
It is advisable to mark the ribbon adjacent to
the piston head, so that its proper location on
the piston can be
maintained.
It is helpful
in
get
ting the ribbon started
into the cylinder
along
with the piston to use both thumbs to spring the
piston inward.
Feeler gauges may also be used for measuring
taper in cylinder, as follows:
Using a ribbon which gives between 2 and 6
pounds
pull,
insert the ribbon with the piston,
open end down into the cylinder about one inch,
taking the resulting pull at this point and in in
tervals of each two inches downward, until the
open end of the piston is 1/8" below the bottom of
the cylinder. By keeping a record of results the
clearance difference can be read from the chart.
Checking the clearance with feeler ribbon gauges
should not be attempted without help on the under
side of the engine to push back the piston when re
quired and to prevent it from being damaged if it
should fall through the cylinder,
The clearances will become approximately .0005"
greater at the upper end of the skirt, within a
few hundred miles of operation, and remain so
until increased due to normal wear, The initial In
crease of .0005" should not be mistaken as mis-
fitting or rapid wear as this condition is due to
normalizing of the
piston
casting. If this did not
occur, it would be necessary to fit the piston with
.0005' more clearance.
25 Reconditioning Cylinder Bores
Worn cylinder bores can be reconditioned by
reboring and honing in the service station. Several
precautions are necessary, however, to assure a
perfect fit between piston
and cylinder.
Take
special note of the following:
1. Use the proper grit stone. When honing cyl
inders with only a few thousandths to be removed,
a medium 300 to 400 grit stone will serve for
both roughing and finishing. Where considerable
stock is to be removed, however, 150 grit stones
are best for roughing. To obtain the best results,
the cylinders should be round and straight within
.0007" and, in addition, free from chatter marks
and dug-in spots from the hone.
Fast wearing hones are not recommended, due
to the loss of time in keeping them tight and the
undue amount of loose abrasive material which
may
remain in the engine. It is possible to
obtain free cutting hones sufficiently hard that
wear is negligible.
-.002 -.001 -0+
.O0 .002
Clearance in Inche,
Hg. 10-26 Piston Clearance Chart