Honda ST50 Shop Manual page 20

18
If the valve seat has been repaired or the valve replaced, the valve must
be
lapped to the seat. To lap the valve,
apply a small amount of lapping compound to the valve contact surface and rotate the valve back and forth
against the seat with a surction cup tool: lifting the valve off the valve seat occasionally. After valve lapping
is completed, wash the lapping compound off thoroughly from both the valve scat and face. Check the valve
seating with prussian blue or red lead to assure that a good seat has been obtained.
Note :
I. Apply a small am ounl of oil to the vah e stem when inserting the va lve into lhe guide.
2. After lhe ,·ah e has been assembled into the cylinder head, check lhe va h e sealing by pouring a small quanty-
ly of engine oil into the combustion chamber until the va lve heads are covered and then apply co mpressed
a ir a t 2 kg/cm
1
(28.4psi) altcmately into intakea nd ex haust portsand checkro rany bubblesaround the
va lveseats. lfthcreare no bubbtes, the va lvesa reseatin g properly,
Fig. 21
Fig.
22
Fig. 23
3.
Measuring 11al11e stem and 11al11e guide wear.
Place a dia! gauge against the 11a!11e stem and
mo11e the 11al11e sideways and fore and aft.
The amoun t of wea r in any direction will be
indicated on the gauge(Fig. 21 ).
hem
ln1ake
Exhaust
Standard Value
0.01---0.03
{0.0004--0.0012)
0.03---0.0S
{0.0012---0.002\
m m
(inch)
rv1ccable
Limn
Replace
if
over
0.08(0. 0032)
Replace
I
over
0. 10 (0.004
4.
Measuring the valve dimension. The va lve
stem diameter i~ measured with a micrometer.
(Fig. 22).
Item
Intake
Exhaust
mm(i neh)
Standard Value Serviceable
Li mit
S. 4SS~S.46S
Rcplace1fu nder
(0.2148---0.2187) S.40(0.2126)
S.OS-5.445
Rcplace1/ under
(0.2070---0.2109) 5. 38 (0.2048)
10
S.
Replacing the valve guide
If the valve guide
is
worn excessively and
requires replacement, follow the procedure
below:
a.
Remo11e the valve guide from the cylinder
j
head with the 11alve guide remover (Tool
, 1
No. 07942- 3290 100) (Fig. 23).
b. Install the new valve guide with theva11le
guide driver (Tool No. 07942- 1180100),
and carefully drive the guide into the
head. The replacement vavle guide should
be oversized.