O.S. Engines FS-40S - Manual (Page 29)

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
necessary to readjust the clearance if the engine
runs satisfactorily.
Fig.1
0.04mm
Feeler Gauge
Rocker Arm
Valve
( 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.
Wrench
Slacken 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
Adjusting
Screw
Turn approx.
1/2 turn.
Allen Key
Locknut
28