6-18
1968
PONTIAC
SERVICE
MANUAL
Fig.
6-29
Reaming
Valve
Guide
Bore
clearance
will
result in noisy and sticky function-
ing
of the valve and disturb engine smoothness of
operation*
Intake valve stem-to-bore clearance
should
be .0016" to .0033" while exhaust stem
clearance
should be
.0021"
to .0038". By using a
micrometer
and suitable telescope hole gauge,
check
the diameter of valve stem in three
places;
top,
center and bottom.
Insert
telescope hole gauge
in
valve guide
bore,
measuring at center. Subtract
highest
reading of valve stem diameter from valve
guide
bore center diameter to obtain valve to valve
guide
clearance.
If clearance is not within limits,
use
next oversize valve and ream bore to fit using
suitable
reamer.
FITTING
VALVE
STEMS
TO
GUIDES
Correct
valve stem clearance for valve guides is
.0016" to .0033" for the intake valve and .0021" to
.0038" for the exhaust valve.
Valves
with oversize stems are available in .001",
.003" and .005" larger than standard. The same
valve stem to guide clearance applies for oversize
stems.
Oversize
reamers are required to enlarge valve
guide
holes
to fit the oversize stems. When reamer
is turned through valve guide, it
will
size the hole to
fit valve stem according to above limits.
Carefully
ream the valve guide using valve guide
reamer
J
5830-1
for .003" oversize stems and valve
guide reamer J 6621 for .005" oversize stems (Fig.
6-29). For best results when installing .005" over-
size valve stem use the .003 oversize reamer first
and
then ream the .005" oversize. Always reface the
valve seat after reaming valve guide.
NOTE:
Valves are marked .001, .003 or .005 with
colored
ink.
VALVES
AND
SEATS-RECONDITION
1. Reface valves and
seats
as follows:
Valves
should be ground on a special bench grinder
designed specifically for this purpose and built by a
reputable manufacturer. Valve
seats
should be
ground with reputable power grinding equipment hav-
ing
stones
of the correct seat angle and a suitable
pilot which pilots in the valve stem guide. To ensure
positive sealing of the valve face to its seat, the
grinding
stones
should be carefully refaced before
any grinding is done.
The
intake valve seat angle is 30° with a face angle
of 29°. The exhaust valve has a seat angle of 45°
with
a face angle of 44° (Fig. 6-30).
This
will
pro-
vide hairline contact between valve and seat to pro-
vide positive sealing and reduce build-up of deposits
on seating surfaces.
DO
NOT USE
REFACING
EQUIPMENT
EXCES-
S/Viixir;
only enough material should be removed
to true up surfaces and remove
pits.
The valve
head
will
run hotter as its thickness is diminished;
therefore,
if valve face cannot be cleaned up with-
out
grinding
to point where outside diameter of
valve
has a sharp edge, the valve should be re-
placed.
Whenever it is necessary to
replace
a
valve,
the new valve should be of same stem diam-
eter
as valve removed (unless the valve guide is
reamed
to provide proper fit).
Width
of exhaust valve
seats
should be 1/16" to
3/32". Intake valve
seats
should be 1/32" to 1/16"
wide. If seat width is excessive, it should be
nar-
rowed by grinding with a flat
stone.
This
is the only
method that should be used to narrow seat.
2.
Check
concentricity of valve seat and valve
guide. Concentricity of valve seat and valve guide
A-SEAT
WIDTH
Fig.
6-30
Valve
Seat
and
Face
Angle
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