Kohler CH18-CH26 Service Manual page 204

Section 10
Inspection and Reconditioning
Valve Guides
If a valve guide is worn beyond specifications, it will
not guide the valve in a straight line. This may result in
burnt 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 of the guide. 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 by subtracting the stem
diameter from the guide diameter. 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.
The maximum (I.D.) wear on the intake valve guide is
7.134 mm (0.2809 in.) while 7.159 mm (0.2819 in.) is the
maximum allowed on the exhaust guide. The guides
are not removable but can be reamed 0.25 mm
(0.010 in.) oversize. Valves with 0.25 mm oversize
stems must then be used.
If the guides are within limits but the valve stems are
worn beyond limits, install new valves.
Valve Seat Inserts
Hardened steel alloy intake and exhaust valve seat
inserts are press fitted into the cylinder head. The
inserts are not replaceable but can be reconditioned if
not too badly pitted or distorted. If cracked or badly
warped, the cylinder head should be replaced.
Recondition the valve seat inserts following the
instructions provided with the valve seat cutter being
used. A typical cutter is shown in Figure 10-7. The final
cut should be made with an 89° cutter as specified for
the valve seat angle in Figure 10-6. Cutting the proper
45° valve face angle as specified in Figure 10-6, and the
proper valve seat angle (44.5°, half of the full 89° angle),
will achieve the desired 0.5° (1.0° full cut) interference
angle where the maximum pressure occurs on the
outside diameters of the valve face and seat.
10.8
Valve Seat Cutter
Pilot
Figure 10-7. Typical Valve Seat Cutter.
Lapping Valves
Reground or new valves must be lapped in, to provide
proper fit. Use a hand valve grinder with a suction
cup for final lapping. Lightly coat the valve face with a
"fine" grade of grinding compound, then rotate the
valve on its seat with the grinder. Continue grinding
until a smooth surface is obtained on the seat and on
the valve face. Thoroughly clean the cylinder head in
soap and hot water to remove all traces of grinding
compound. After drying the cylinder head, apply a
light coating of SAE 10 oil to prevent rusting.
Intake Valve Stem Seal
These engines use valve stem seals on the intake
valves. Always use a new seal when the valves are
removed from the cylinder head. The seals should also
be replaced if deteriorated or damaged in any way.
Never reuse an old seal.
Pistons and Rings
Inspection
Scuffing and scoring of pistons and cylinder walls
occurs when internal engine temperatures approach
the welding point of the piston. Temperatures high
enough to do this are created by friction, which is
usually attributed to improper lubrication and/or
overheating of the engine.
Normally, very little wear takes place in the piston
boss-piston pin area. If the original piston and
connecting rod can be reused after new rings are
installed, the original pin can also be reused but new
piston pin retainers are required. The piston pin is
included as part of the piston assembly – if the pin
boss in the piston or the pin are worn or damaged, a
new piston assembly is required.