Valve Guide Bores - GMC 1976 ZEO 6083 Maintenance Manual

6A- 4 0 ENGINE
NOTE :
Exhaust valve seats are induction hard-
ened and must be ground, Not cut.
If valve guide bores are worn excessively, they
can be reamed oversize . This will require replace-
ment of the valves with oversize valves (stems). The
guide bores should be reamed before grinding the
valve seats . Valve clearance in guide bore should be
.001" to .004" .
Measuring Valve Stem Height
Whenever a new valve is installed, or after grind-
ing valves, it will be necessary to measure valve stem
height . Install Gauge BT-6428. There should be at
least .015" clearance on all valves between gauge
surface and end of valve stem . (Valve stem can be
gauged with or without the valve rotator on the
valve) . If clearance is less than .015", remove valve
and grind tip of valve stems as required on a valve
tachment to insure a smooth 90° end . Also be certain
to break sharp edge on ground valve tip. Observe an
original valve to determine chamfer.
After all valve keys have been installed on valves,
tap each valve stem end with a hammer to seat valve
rotators and keys . Re-gauge all valves between valve
stem and gauge ( .015" minimum) and valve rotator
and gauge ( .030" minimum) . If any valve stem end
is less than .005" above rotator, the valve is too short
and a new valve must be installed.
NOTE:
There must be a minimum of .030" clear-
ance between valve rotator and gauge. Failure to
maintain this clearance will cause rocker arm
and valve rotator interference . Example:
Valve Rotator to Gauge Clearance. . . . .. . . ... . . ... . .038"
Minus Valve Stem to Gauge Clearance .... . . . -.035"
.003"
This is less than .005" and a new valve should be
installed.
VALVE GUIDE BORES
As previously stated, if the valve guide bores are
worn excessively, they can be reamed oversize . The
following reamers are available:
.003" Oversize Valve Guide Reamer
.005" Oversize Reamer
.013" Oversize Valve Guide Reamer
If a standard valve guide bore is being reamed,
use the .003" or .005" oversize reamer . For the .010"
oversize valve guide bore, use the .013" oversize
Figure 32-Valve Guide Bore Marking
reamer. If too large a reamer is used and the spiraling
is removed, it is probable that the valve will not
receive the proper lubrication .
Occasionally a valve guide bore will be oversize
as manufactured . These are marked
on
the inboard
side of the cylinder heads on the machined surface
just above the intake manifold surface (figure 32).
These markings are visible without removing any
parts other than the air cleaner assembly. Before
removing the cylinder heads to perform service to
either the valves or valve guide bores, the cylinder
heads should be inspected to determine if these
markings are present. If no markings are present, the
guide bores are standard . If oversize markings are
present, any valve replacement will require an over-
size valve. If the oversize marking is present, only
Figure 33-Cleaning Valve Guide Bores