Onan T260G Service Manual page 39

Warpage occurs chiefly in the upper stem due to its
exposure to intense heat. Out-of-round wear follows
when the seat is pounded by a valve whose head is
not in line with the stem and guide. If a valve face is
b u r n e d or warped, or if the stem is worn, install a new
'valve.
Too much clearance in the intake guide admits air
and oil into the combustion chamber, upsetting
carburetion, increasing oil consumption, and making
heavy carbon deposits. Carbon reduces heat dissi-
pation. Clean metal is a good heat conductor but
carbon insulates and retains heat. This increases
combustion chamber temperatures which causes
warping and burning.
Unburned carbon residue gums valve stems and
causes them to stick in the guide. Deposits of hard
carbon with sharp points projecting become white
hot and cause pre-ignition and pinging.
Refinish valves that are slightly pitted or burned on an
accurate valve grinder. If valves are badly pitted or
have a thin margin when refacing, replace them.
VT-1020
FIGURE 2. VALVE STEM AND VALVE GUIDE INSPECTION
Stems And Guides: Always check valve stems and
guides for wear (Figure 2). Use a hole gauge to measure
the valve guide. When clearance with stem exceeds that
specific in DlMENSlONS AND CLEARANCES, replace
either valve or guide or both, as may be necessary.
Always regrind seat to make concentric with the newly
installed guide.
Worn valve stem guides can be replaced from inside
the valve chamber (a seal is provided behind the
intake valve guides only). The smaller diameter of the
CAUTION
the guide or tappet bores. Be careful
Driving out old guides can damage
not to strike guide bores with driver or allow guide to
strike tappet bores during removal.
Valve Guide Installation: Run a small polishing rod
covered with crocus cloth through valve guide holes
to clean out carbon and other foreign materials. Place
a new gasket on the intake valve guide, and coat the
outer edge of each new guide with oil. Place guide,
notch-up, in cylinder block and press in until guide
stops or protrudes 11/32 inch (8.7 mm) from rocker
box side of block. A suggested method of installation
is shown in Figure 3.
ROD
NUT
5/16-18 HEX, NUT
(2 REQUIRED) \
(2 REQUIRED)
(60
mm)
2-3/8"
5/8" (16 (mm)
FIGURE
3.
VALVE GUIDE INSTALLATION
Vl-1023
tapered valve guides must face toward the valve head.
after first removing the valve assemblies.
Valve Stem Seals: Do not reuse valve stem seals. Each
new seal must be used when valve is reinstalled.
Tappets are also replaceable from the valve chamber time the valves are removed from cylinder block, a
Valve Guide Removal: Before removing valve guides,
use an electric drill with a wire brush to remove
CAUTION
not remove valve after seal is
carbon and other foreign material from top surface of
CAUTION
installed Valve can be withdrawn
guides. Failure to perform this operation may result in
only as far as the groove in valve stem. Do not allow
damage to the guide bores. Drive the guides out with
valve stem seal to come in contact with groove or seal
a hammer and valve guide driver.
damage will result.
10-3