Yamaha YFM660FS Owner's Manual page 144

Installation
1. Measure the electrode gap with a wire
thickness gauge and, if necessary, adjust
the gap to specification.
Spark plug gap:
0.8–0.9 mm (0.031–0.035 in)
a. Spark plug gap
2. Clean the gasket surface. Wipe off any
grime from the threads.
3. Install the spark plug and tighten it to the
specified torque.
Tightening torque:
Spark plug:
17.5 Nm (1.75 m·kgf, 12.5 ft·lbf)
NOTE:
_
If a torque wrench is not available when you
are installing a spark plug, a good estimate of
the correct torque is 1/4 to 1/2 turn past finger
tight. Have the spark plug tightened to the
specified torque as soon as possible.
_
4. Install the spark plug cap.
5. Install the panel.
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