O.S. engine FS-52S Owner's Instruction Manual page 20

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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
1.
turn with 5mm wrench. (Fig.2.)
Locknut
2. Turn adjusting-screw approx. 1/2 turn counter-clockwise
to open gap, using appropriate tool -i.e. Allen hex key.
(Fig.3.)
Adjusting
Screw
Slacken approx.
1/4 to 1/2 turn.
Wrench
Fig.2
Turn approx.
1/2 turn.
Allen Key
Fig.3
19
Insert 0.04mm feeler gauge between valve stem
3.
and rocker-arm and gently turn adjusting screw
clockwise until it stops.(Fig.4.)
Turn with fingers
until it stops.
0.04mm Feeler
Gauge
4. Re-tighten locknut while holding adjusting screw
stationary. (Fig.5.)
Fig.5
Fig.4
Hold at the screw head.
Tighten Locknut.