Inspection And Repair; Cylinder Head; Cylinder; Piston Pin And Bearing - Yamaha QT 50F Service Manual

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3-3. INSPECTION AND REPAIR
V. Cylinder Head
1. Remove spark plug.
2. Using a rounded scraper, remove car-
bon deposits from combustion chamber.
Take care to avoid damaging spark plug
threads. Do not use a sharp instrument.
Avoid scratching the combustion cham-
ber surface.
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3. Place on a surface plate. There should
be no warpage. Correct by re-surfacing
as follows:
Place 400 ~ 600 grit wet sandpaper
on surface plate and re-surface head us-
ing a figure-eight sanding pattern.
Rotate head several times to avoid re-
moving too much material from one
side.
B. Cylinder
1. Hone cylinder bore using a hone with
fine stones. Hone no more than required
to remove all wear marks.
2. Using a Cylinder Gauge set to standard
bore size, measure the cylinder. Mea-
sure front-to-rear and side-to-side at
top, center and bottom just above ex-
haust port. Compare minimum and
maximum measurements. If over toler-
ance and not correctable by honing,
rebore to next over-size.
Max. allowable taper (d — a):
0.05 mm (0.0020 in)
Max. allowable out-of-round
( B - A o r A - B):
0.01 mm (0.0004 in)
Wear limit: 40.10 mm (1.579 in)
(at "a" position)
C. Piston Pin and Bearing
Apply a light film of oil to pin and bearing
surfaces. Install in connecting rod small end.
Check for play. There should be no noticeable
vertical play. If play exists, check connecting
rod small end for wear. (Replace pin and
bearing or all as required.)
D. Piston
1. Remove carbon deposit from ring
grooves using old piston ring as shown.
2. Using an outside micrometer, measure
piston diameter. The piston is cam-
ground and tapered. The only measuring
point is at right angles to the piston pin
holes about 5 mm (0.20 in) from bot-
tom of piston. Compare piston diameter
to cylinder bore measurements.
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