1975 Thunderjet - Vintage Snow

10
0 .01 - 0.0 19 in.
(0 .0 3 - 0 .05 mm)
Figure 35
TJ35
Honi ng or rebori ng shou ld be do ne if t hi s tol erance is
exceeded. " Taper" of t he cylinder bo re is measu red by
mov ing t he micrometer from the top to th e bottom of
the cy linder bei ng care f ul to miss th e port o penings . (Fi g­
ure 39) The difference in these meas ureme nts (between
top and bottom) should not exceed 0.002 in. (0.05 mm .).
If upon inspect ion of th e cy linder bore , sco ring of the
cyl inder wa ll is observed , hone or rebore unti l these
scores d isappear.
Before assembly , always clean the carbo n f rom th e exhaust
port. Also was h out the cy linder with so lven t or
soap and water to insure that all hone material has been
removed .
Failure to do this may resu lt in premature wearing or
scoring of the cylinder and piston.
Installing the piston
The piston shou ld be install ed so tha t the arrow mark on the
piston crown po ints to the exhaust port (toward the front of
the engine). (Fi gure 36)
Remedies:
1) If any piston ri ng is found stuck, poor sealing will result.
2) If t he cyl inder wall is scuffed, poo r seali ng will resu lt as
in th e case of t he pis ton ring being stuck. In t he above two
cases, the pis ton ring should be replaced w ith a new one,
and at the same t ime, t he cy linder wall should be checked
for scratches.
3) Whe ther t he contact between t he pi ston ring and the
cylinder wa ll is good or not w ill have a great effect on
sealing. The entire area of the piston ri ng face must be
in ti ght contact with th e cy linder wall. If excess wear is
found on the upp er or the lower half of th e pis to n ring
face, t he pis to n ri ng groove is considered to be worn
unevenly , caus ing the ring to twist.
If imi tat io n pisto n rin gs (other th an genuine Yamaha
part s) are used , defect s may eas ily occ ur, and in the
worst cas e, serious engin e troubles may result.
Figure 37
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TJ38
Figure 38
2. Since the for mat ion of carbon is a prod uc t of two-stroke
operat io n, decarbonizati on at regular intervals is esse n­
tial. Removing th e cy linder head and sc raping the carbon
from pis ton crow n and cylin der head should be done at
every tune-up or every 40 hours of op eration. (F igure 37)
During every other decarbonization, the cylinder shou ld be
removed and th e exha ust port decarbonized.
Figure 36
TJ36
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C. CYLINDER MAINTENANCE
1. Wh en t he cylinder and pis ton are removed, the re are
several checks which should be performed. Fi rst of all
the cylinder bo re should be checked for wear or scori ng
each time it is removed. The " o ut of round" and " taper"
can be checked with an inside m icrometer. The "out of
round" of the cylinder bore is checked by moving the
micrometer in a circular direction 90' at a time. The difference
in measurements should not exceed 0.04 in.
(0.10 mm.). (F igure 38)
,. .
1.. ...J
Figure 39
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TJ39