Yamaha XT550J Service Manual page 52

Valve seat
1. The valve seat is subject to severe wear.
Whenever the valve is replaced or the
valve face is resurfaced (see caution),
the valve seat should be resurfaced at a
45° angle. If a new valve guide has been
installed, the valve seat must be recut to
guarantee complete sealing between the
valve face and seat.
If the valve seat is obviously pitted or
worn, it should be cleaned with a valve
seat cutter. Use the 45° cutter. When
twisting the cutter, keep an even
downward pressure to prevent chatter
marks.
If cutting section A of the valve seat, use
a 30° cutter. If cutting section 8, use the
45° cutter. If cutting section C, use the
60° cutter.
2. Measure the valve seat width. Apply
mechanic's bluing dye (such as Dykem)
to the valve face and valve seat, apply a
very small amount of fine grinding com-
pound around the surface of the valve
face, insert the valve into position, and
spin the valve quickly back and forth.
Lift the valve, clean off all grinding com-
pound, and check valve seat width. The
valve seat and valve face will have
removed the bluing wherever they con-
tacted each other. Measure the seat
width with vernier calipers. It should
measure approximately 1.3 mm (0.051
in). The valve-seat contact area should
be one uniform width. If valve seat
width varies or if pits still exist, further
3-15
cutting will be necessary. Remove just
enough material to achieve a satisfac-
tory seat.
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Standard Width Wear Limit
Seat width
1.3 ±0.1 mm 2.0mm
(0.051 ± 0.0039 in) (0.080 in)
3. If the valve seat is uniform around the
perimeter of the valve face but is too
wide or not centered on the valve face, it
must be altered. Use either the 30°, 45°,
or 60° cutters to correct the improper
seat location in the manner described
below:
1.
2.
3.
• If the valve face shows that the valve
seat is centered on the valve face but is
too wide, then lightly use both the 30°
and the 60° cutters to reduce the seat
width to 1.3 mm (0.051 in).