TAPPETS
Very little wear takes place on tappet diameters
or
in
tappet bores.
If
the clearance between tappet and bore
in cylinder block exceeds specifications, replace the
tappet. Oversize tappets are available.
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
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 the valve 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 sur-
faces. 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.
around the whole valve face, keep grinding until com-
plete 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 indicate whether valve head is
worn
or
warped.
WARPED
VALVE WITH
KNIFE
EDGE
KNIFE
EDGE
GOOD
MARGIN
INCH
MINIMUM
(.E
MM)
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 manufacturer's
directions.
VT-1021
FIGURE
6.
VALVE INTERFERENCE ANGLE
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
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
(0.8
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
ail points. The
line
of
contact should
be
at the center of the valve face.
10-5
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