Honda PCX150i Owner's Manual page 115

6.
Make sure the plug washer is in good
condition.
7. With the plug washer attached, thread
the spark plug in by hand to prevent
cross-threading.
8. Tighten the spark plug:
If the old plug is good:
1/8 turn after it seats.
If installing a new plug, tighten it twice
to prevent loosening:
a) First, tighten the plug:
NGK: 1/2 turn after it seats.
b) Then loosen the plug.
c) Next, tighten the plug again:
1/8 turn after it seats.
An improperly tightened spark plug can
damage the engine. If a plug is too loose, a
piston may be damaged. If a plug is too tight,
the threads may be damaged.
9. Reinstall the spark plug cap. Take care to
avoid pinching any cables or wires.
10. Install the removed parts in the reverse
order of removal.
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