Yamaha 55B Owner's Manual page 148

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Standard spark plug:
Refer to "SPECIFICATIONS", page 4-1.
Before fitting the spark plug, measure the
electrode gap with a wire thickness
gauge; adjust the gap to specification if
necessary.
Spark plug gap:
Refer to "SPECIFICATIONS", page 4-1.
When fitting the plug, always clean the
gasket surface and use a new gasket.
Wipe off any dirt from the threads and
screw in the spark plug to the correct
torque.
Spark plug torque:
Refer to "SPECIFICATIONS", page 4-1.
NOTE:
If a torque-wrench is not available when
you are fitting a spark plug, a good esti-
mate of the correct torque is 1/4 to 1/2 a
turn past finger-tight. Have the spark plug
adjusted to the correct torque as soon as
possible with a torque-wrench.
Initial of spark plug
I.D. mark
B
C/BK
D
1 Spark plug gap
2 Spark plug I.D. mark (NGK)
q
903022
4-12
GB
Plug wrench size
21 mm (13/16 in.)
16 mm (5/8 in.)
18.3 mm (23/32 in.)