9.9
Section 9
Inspection and Reconditioning
9
Valve Guides
If a valve guide is worn beyond specifications, it will
not guide the valve in a straight line. This may result
in burned valve faces or seats, loss of compression,
and excessive oil consumption.
To check valve guide-to-valve stem clearance,
thoroughly clean the valve guide and, using a split-
ball gauge, measure the inside diameter. Then, using
an outside micrometer, measure the diameter of
the valve stem at several points on the stem where
it moves in the valve guide. Use the largest stem
diameter to calculate the clearance. If the intake
clearance exceeds 0.038/0.076 mm (0.0015/0.0030 in.) or
the exhaust clearance exceeds 0.050/0.088 mm
(0.0020/0.0035 in.), determine whether the valve stem
or guide is responsible for the excessive clearance.
Maximum (I.D.) wear on the intake valve guide is
6.135 mm (0.2415 in.) while 6.160 mm (0.2425 in.)
is the maximum allowed on the exhaust guide. The
guides are not removable. If the guides are within
limits but the valve stems are worn beyond limits,
replace the valves.
Valve Seat Inserts
Hardened steel alloy intake and exhaust valve
seat inserts are press fied into the cylinder head.
The inserts are not replaceable, but they can be
reconditioned if not too badly pied or distorted. If
the seats are cracked or badly warped, the cylinder
head should be replaced.
Recondition the valve seat inserts following the
instructions provided with the valve seat cuer being
used. A typical cuer is shown in Figure 9-11. The final
cut should be made with an 89° cuer as specified for
the valve seat angle in Figure 9-10. With the proper 45°
valve face angle, and the valve seat cut properly (44.5°
as measured from centerline when cut 89°) this would
result in the desired 0.5° (1.0° full cut) interference
angle where the maximum pressure occurs on the
valve face and seat.
Figure 9-11. Typical Valve Seat Cutter.
Lapping Valves
Reground or new valves must be lapped in, to provide
a good seal. Use a hand valve grinder with suction
cup for final lapping. Lightly coat valve face with fine
grade of grinding compound, then rotate valve on seat
with grinder. Continue grinding until smooth surface
is obtained on seat and on valve face. Thoroughly
clean cylinder head in soap and hot water to remove
all traces of grinding compound. Aer drying cylinder
head, apply a light coating of engine oil to prevent
rusting.
Pistons and Rings
Inspection
Scuffing and scoring of pistons and cylinder walls
occurs when internal temperatures approach the
welding point of the piston. Temperatures high
enough to do this are created by friction, which is
usually aributed to improper lubrication, and/or
overheating of the engine.
Normally, very lile wear takes place in the piston
boss-piston pin area. If the original piston and
connecting rod can be reused aer new rings are
installed, the original pin can also be reused, but new
piston pin retainers are required. The piston pin is part
of the piston assembly; if the pin boss or the pin are
worn or damaged, a new piston assembly is required.
Ring failure is usually indicated by excessive oil
consumption and blue exhaust smoke. When rings
fail, oil is allowed to enter the combustion chamber
where it is burned along with the fuel. High oil
consumption can also occur when the piston ring
end gap is incorrect because the ring cannot properly
conform to the cylinder wall under this condition. Oil
control is also lost when ring gaps are not staggered
during installation.
Valve Seat Cutter
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