Tappets; Valve Face And Seat Grinding - Onan T260G Service Manual

TAPPETS
Very little wear takes place on tappet diameters or in
tappet bores. If the clearance between tappet and
in cylinder block exceeds specifications, replace
he tappet.
Inspect the tappet faces which contact camshaft
lobes for roughness, scuffing, or concave wear.
Replace any worn tappets. If tappets are worn,
inspect camshaft for wear.
VALVE FACE AND SEAT GRINDING
Some engines are equipped with a premium valve
package (aluminized intake valves). If thealuminized
valve face does not clean-up using a wire brush, the
valve face may be refinished. This removes the
aluminized coating from thevalve face. An aluminized
valve that has been refinished will normally have a life
expectancy equal to a standard valve. If longer valve
life is required, worn or damaged valves should be
replaced with new premium valves.
Before installing new valves or previously used valves,
inspect valve seats for proper valve seating. If used
valves are reinstalled, the valve stems should be
cleaned and valve faces ground to their specified
angles of 44". Refinish valve seats to a 45" angle.
When refacing valves and seats, remove all evidence
of pitting and grooving. If end of valve stem is pitted
or worn, true it and clean it up on the refacer wheel. A
very light grind is usually enough to square stem and
remove any pits or burrs. The valve guide should be
'thoroughly cleaned. If valve guide is worn, or valve is
warped, the necessary parts must be replaced.
By grinding thevalve face and seat at slightly different
angles, a fine line of contact on face and seat is
obtained, eliminating the need to lap the seating
surfaces. The one degree difference in angles is
defined as the interference angle (Figure 6). The
seat angle is greater than that of the valve face. This
assures contact at the maximum diameter on valve
seat seating surface.
FIGURE 6. VALVE INTERFERENCE ANGLE
VT-1021
10-5
Refinish valve faces to a 44" angle on a valve refacing
machine. The first cut from valve face must be a light
grinding. Check if there is an unevenness of metal
being removed. If only part of valve's face has been
touched, check to see if valve is properly seated in
machine or if valve is warped, worn, or distorted.
When cut is even around the whole valve face, keep
grinding until complete face is-ground clean. Be sure
the correct valve face angle is maintained. When
valve head is warped, a knife edge will be ground
(Figure,7) on part or all of the head due to the large
amount of metal that must be removed to completely
reface valve. Heavy valve heads are required for
strength and good heat dissipation. Knife edges lead
to breakage, burning, and pre-ignition due to heat
localizing on the edge.
Replace any valve that cannot be entirely refaced
while keeping a good valve margin (Figure 7) or is
warped, worn, or damaged in any way. The amount of
grinding' necessary to true a valve indicates whether
valve head is worn or warped.
VALVE WITH
WARPED
KNIFE EDGE
KNIFE EDGE
GOOD MARGIN
1/16
INCH
(1.6 rnm) MINIMUM
M-1184
FIGURE 7. VALVE HEAD MARGIN
When new valve seats are installed, or previously
used seats reground, refinishing must be done with a
valve seat grinder used according to the manufac-
turer's directions.
Valve seats should be ground with a 45 degree stone
and the width of the seat band should be 1/32 inch to
3/64 inch (0.79 to 1.2 mm) wide. Grind only enough
to assure proper seating.
Place each valve in its proper location. Check each
valve for a tight seat. Make several marks at regular
intervals across the valve face using machinist's
bluing.' Observe if the marks rub off uniformly when
the valve is rotated part of a turn against the seat. The
valve seat should contact the valve face evenly at all
points. The line of contact should be at the center of
the valve face.