Check Crankcase; Checking Cylinder Head Mounting Surface; Check Cylinder Bore; Cylinder Finish (Cross Hatch) - Generac Power Systems GTH760 Service Manual

CHeCk CrANkCASe

Check crankcase for cracks, stripped threads or broken fins.
Check cylinder bores for damage or scoring.
1. Check cylinder head mounting surface for distortion with
a straight edge, Figure 10-1.
If mounting surfaces are distorted more than 0.1 mm (.004"),
the crankcase must be replaced.
Figure 10-1. Checking Cylinder Head Mounting Surface
2. Check cylinder bores for wear using a cylinder bore gauge
or a telescoping gauge and dial caliper.
Standard Bore Size: 90.00-90.025 mm (3.543-3.544")
a. Measure cylinder bore in 6 points at right angles as shown,
Figures 10-2 and 10-3.
b. If cylinder bore is worn more than 0.075 mm (.003")
or more than 0.035 mm (.0015") out of round, it must
be replaced.
NOTE: If cylinder bores are within specification and
show no signs of scoring or other damage, new piston
rings may be installed providing the cylinder bores are
reconditioned using a rigid hone with finishing stones,
to restore the proper cross hatch angle in the cylinder
bores. The proper cylinder cross hatch ensures proper
lubrication and piston ring break in.
Refer to "Cylinder Finish (Cross Hatch)" for correct procedure
for cross hatch honing.
RESIZING:
Note: Oversize rings and pistons are NOT available. DO
NOT bore or over hone cylinder.
SeCTIoN 10: CYlINder & CrANkCASe Cover
Figure 10-2. Check Cylinder Bore
Figure 10-3. Measure at Six Points

CYLINDER FINISH (CROSS HATCH):

It is recommended that the cylinder bores be recondi-
tioned to restore the cross hatch when new piston rings
are to be installed in a cylinder that is within specification.
Be careful not to hone oversize or it will be necessary to
replace the crankcase.
Special honing stones are required when when recondition-
ing a cylinder bore. Honing stones will produce the correct
cross hatch necessary for proper lubrication and ring seating.
Honing is accomplished in a two step process called plateau
honing. These steps must be followed carefully and should be
performed by a competent machine shop. The correct cross
hatch angle is approximately 45 degrees, Figure 10-4.
Honing is done with a variable speed 1/2", portable drill and
a honing fixture. See Figure 10-10 for dimensions to make a
honing fixture. Use two crankcase cover mounting screws to
fasten the crankcase to the honing fixture, Figure 10-5.
10-1
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