Yamaha F6A Owner's Manual page 170

When fitting the plug, always clean the gas-
ket 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 estimate 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)
4-33
GB
Plug wrench size
21 mm (13/16 in.)
16 mm (5/8 in.)
18.3 mm (23/32 in.)