Generac Power Systems GTH1000, Service Manual

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SECTION 10: CYLINDER & CRANKCASE COVER
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.
Figure 10-2. Check Cylinder Bore
TOP
CENTER
BOTTOM
Figure 10-3. Measure at Six Points
CYLINDER FINISH (CROSS HATCH):
It is recommended that the cylinder bores be reconditioned 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 reconditioning 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.
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