TOHATSU BFT 250A, Service Manual

Page: 403 / 639

14-35
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CYLINDER HEAD
VALVE STEM-TO-VALVE GUIDE
CLEARANCE
Subtract each valve stem O.D. from the corresponding
valve guide I.D.
If the stem-to-guide clearance exceeds the service limit,
determine if the new guide with standard dimensions
would bring the clearance within tolerance. If so,
replace any guide as necessary and ream to fit. If the
stem-to-guide clearance exceeds the service limit with
new guides, replace the valves as well.
• Recondition the valve seats whenever the valve
guides are replaced (page 14-42).
VALVE SEAT WIDTH
Thoroughly clean the combustion chambers and valve
seats to remove carbon deposits. Apply a light coat of
Prussian Blue compound or erasable felt-tipped marker
ink to the valve faces.
Insert the valves, and then lift them and snap them
closed against their seats several times. Be sure the
valve does not rotate on the seat. The transferred
marking compound will show any area of the seat that
is not concentric.
Measure the valve seat width.
If the valve seat width is under the standard, or over the
service limit, or if the valve seat is too high/low,
recondition the valve seat (page 14-44).
VALVE INSTALLATION HEIGHT
Measure the valve installation height.
Replace the cylinder head if the measurement exceeds
the service limit.
STANDARD:
IN: 0.020 – 0.045 mm (0.0008 – 0.0018 in)
EX: 0.055 – 0.080 mm (0.0022 – 0.0032 in)
SERVICE LIMIT:
IN: 0.080 mm (0.0032 in)
EX: 0.120 mm (0.0047 in)
STANDARD:
IN/EX: 1.25 – 1.55 mm (0.049 – 0.061 in)
SERVICE LIMIT:
IN/EX: 2.0 mm (0.08 in)
VALVE SEAT
TOO LOW
VALVE SEAT
TOO HIGH
STANDARD:
IN: 46.75 – 47.55 mm (1.841 – 1.872 in)
EX: 46.68 – 47.48 mm (1.838 – 1.869 in)
SERVICE LIMIT:
IN: 47.80 mm (1.882 in)
EX: 47.73 mm (1.879 in)
IN: INSTALLATION HEIGHT
EX: INSTALLATION HEIGHT