Yamaha RS90NL Owner's Manual page 60

Specified spark plug:
· R · CR8E (NGK) or CR8E (NGK)
Spark plugs are produced in several different thread
lengths. The thread length or reach is the distance from
the spark plug gasket seat to the end of the threaded
portion. If the reach is too long, overheating and engine
damage may result. If the reach is too short, spark plug
fouling and poor performance may result. Also, if the
reach is too short, carbon will form on the exposed
threads resulting in combustion chamber hot spots and
thread damage. Always use a spark plug with the speci-
fied reach.
Spark plug reach a: 19.0 mm (0.75 in)
Before installing any spark plug, measure the electrode
gap with a wire thickness gauge and adjust to specifica-
tion.
Spark plug gap b: 0.7–0.8 mm (0.028–0.031 in)
When installing the spark plug, always clean the gasket
surface. Wipe off any grime from the threads and tighten
the spark plug to the specified torque.
Spark plug tightening torque:
12.5 Nm (1.25 m·kgf, 9 ft·lb)
CAUTION :
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Make sure that the spark plug caps are securely
installed. Otherwise the spark plug caps could be
damaged due to engine vibration.
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