Fitting Valve Stems To Guides; Valves And Seats - Recondition - Pontiac tempest 1968 Service Manual

6-18
1968 P O N T I A C SERVICE MANUAL
Fig. 6 - 2 9 Reaming Valve G u i d e 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 S T E M S 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
i s 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 / V i i x i r ; 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 - S E A T WIDTH
F i g . 6-30 V a l v e Seat and Face Angle