Mercury 305 CID, Service Manual, Page: 264 / 967

3A-72 - ENGINES 90-823225--1 1096
CYLINDER BORING
1. Before using any type boring bar, file off top of cyl-
inder block to remove dirt or burrs. This is very im-
portant to prevent boring bar tilt, with result that
rebored cylinder wall is not at right angles to
crankshaft.
2. Measure piston to be fitted with a micrometer,
measuring at center of piston skirt and at right
angles to piston pin. Bore cylinder to same diam-
eter as piston and hone to give specified clear-
ance.
NOTE: Hone cylinders as outlined under “Cylinder
Honing” and “Piston Selection,” following.
3. Carefully observe instructions furnished by man-
ufacturer of equipment being used.
CYLINDER HONING
1. Follow hone manufacturer’s recommendations
for use of hone and cleaning and lubrication dur-
ing honing.
2. Occasionally, during the honing operation, thor-
oughly clean cylinder bore and check piston for
correct fit in cylinder.
3. When finish-honing a cylinder bore to fit a piston,
move hone up and down at a sufficient speed to
obtain very fine uniform surface finish marks in a
crosshatch pattern of approximately 30 degrees
to cylinder bore. Finish marks should be clean but
not sharp, free from imbedded particles and torn
or folded metal.
4. Permanently mark piston (for cylinder to which it
has been fitted) and proceed to hone cylinders
and fit remaining pistons.
IMPORTANT: Handle pistons with care and do not
attempt to force them through cylinder until cyl-
inder is honed to correct size, as this type piston
can be distorted by careless handling.
5. Thoroughly clean cylinder bores with hot water
and detergent. Scrub well with a stiff bristle brush
and rinse thoroughly with hot water. It is extreme-
ly essential that a good cleaning operation be
performed. If any abrasive material remains in
cylinder bores, it will rapidly wear new rings and
cylinder bores in addition to bearings lubricated
by the contaminated oil. Swab bores several
times with light engine oil on a clean cloth, then
wipe with a clean dry cloth. Cylinder should not be
cleaned with kerosene or gasoline. Clean re-
mainder of cylinder block to remove excess ma-
terial spread during honing operation.
PISTON SELECTION
1. Check used piston to cylinder bore clearance as
follows:
a. Measure cylinder bore diameter with a tele-
scope gauge 2-1/2 in. (64 mm) from top of cyl-
inder bore as follows.
72386
b. Measure piston diameter at skirt across cen-
ter line of piston pin as shown.
72624
c. Subtract piston diameter from cylinder bore
diameter to determine piston-to-bore clear-
ance.
d. Determine if piston-to-bore clearance is in ac-
ceptable range shown in “Specifications.”