O.S. engine FS-40S-C Owner's Instruction Manual page 19

( 1 )
Remove the cover from the rocker-box on
1.
top of the cylinder-head, using the correct
size Allen hex key.
2. Turn the flywheel counter-clockwise until
compression is first felt, then turn it a further
quarter revolution. At this point, both valves
should be closed. (If the prop driver ('drive
hub') of your engine is engraved with a letter
'T', this mark should now be at the top.)
3.
The standard valve clearance, on both inlet
and exhaust valves, is between 0.04mm and
0.14mm(0.0015-0.005 inch), measured
between valve stem and rocker arm. Use the
0.04mm and 0.10mm feeler gauges to check
clearances.
(See Fig.1.)
0.04mm Feeler Gauge
Rocker Arm
Valve
Fig.1
18
Note:
If the gap is found to be less than 0.04mm,
it is not necessary to readjust the clearance
if the engine has good compression and
starts easily. Equally, if the gap exceeds
0.10mm but is not more than 0.14mm (i.e.
the thickness of both feeler gauges inserted
together), it is not essential to readjust the
clearance if the engine runs satisfactorily.
( 2 )
If a clearance is found to be outside either of
these limits, it should be reset as follows.
1. Carefully loosen locknut on rocker-arm 1/4-
1/2 turn with a 5mm wrench. (Fig.2.)
Slacken approx. 1/4 to 1/2 turn.
Locknut
Wrench
Fig.2