Yanmar 4TNV98 Service Manual page 124

Inspection of Pistons, Piston Rings and Wrist
Pin
• On an engine with low hours, the pistons, piston
rings may be reused if they are found to be
within specifications. The pistons and piston
rings must be reinstalled in the same cylinders
from which they were originally removed.
• On an engine with high hours, the pistons rings
should be replaced and the cylinder honed (See
Honing and Boring on page 126) or replaced.
The piston should be replaced as necessary.
1. Clean piston ring grooves using a piston ring
groove cleaning tool. Follow manufacturer's
instructions for correct operation.
2. Wash the pistons in an appropriate solvent using
a soft brush.
3. Visually inspect each piston for cracks. Pay
particular attention to the ring lands between the
piston ring grooves.
4. Measure the diameter of the piston skirt at 90° to
the wrist pin bore as shown (Figure 6-111).
Measurements must be taken at a specified
distance (Figure 6-111, (1)) from the bottom of the
piston, based on engine model. Record the
measurements. See Piston on page 71 for
specifications.
Figure 6-111
4TNV98 & 4TNE98 Diesel Engine
NOTES
5. Subtract the piston measurement from the
greatest measurement acquired during cylinder
inspection (see Inspection of Cylinder Block on
page 109) to obtain piston-to-cylinder clearance.
Record the measurements. See Piston on page
71 for specifications.
6. Measure the diameter of the wrist pin bore on
both sides of the piston (Figure 6-112). See
Piston on page 71 for specifications. Record the
measurements.
7. Measure the outside diameter of the wrist pin in
three places and at 90° (Figure 6-113). See
Piston on page 71 for specifications. Record the
measurements.
8. Using a micrometer, measure the thickness of
each piston ring. See Piston on page 71 for
specifications. Record the measurements.
122
Figure 6-112
Figure 6-113
Section 3. Engine