O.S. Engines FS-91S II (P) - Manual (Page 17)

Note:
Valve clearances of all O.S. four-stroke-cycle engines
must be checked and reset ONLY WHEN THE ENGINE IS
COLD. Procedure is as follows.
Remove the cover from the rocker-box on top of the
cylinder-head, using the correct size Allen hex key. 1.
( 1 )
Turn the propeller counter-clockwise until compression is
first felt, then turn it a futher 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.) 2.
The standard valve clearance, on both inlet and exhaust
valves, is between 0.04mm and 0.10mm(0.0015-0.004
inch), measured between valve stem and rocker arm. Use
the 0.04mm and 0.10mm feeler gauges to check
clearances. (See Fig.1.) 3.
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.
Fig.1
( 2 ) If a clearance is found to be outside either of these limits,
it should be reset as follows.
Carefully loosen the locknut
on rocker-arm 1/4-1/2 turn
with 5mm wrench.
(Fig.2.) 1.
2.
3.
Locknut
Wrench
Loosen approx.
1/4 to 1/2 turn.
Turn adjusting-screw approx.
1/2 turn counter-clockwise to
open gap, using appropriate
tool -i.e. Allen hex key.
(Fig.3.)
Fig.2
Fig.3
Fig.4
Adjusting
Screw Turn approx.
1/2 turn.
Allen Key
Insert 0.04mm feeler gauge
between valve stem and
rocker-arm and gently turn
adjusting screw clockwise
until it stops.(Fig.4.)
0.04mm Feeler
Gauge
Turn with fingers
until it stops.
0.04mm
Feeler Gauge
Rocker Arm
Valve
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