Yamaha MA190, Service Manual

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Piston check
Check the piston and cylinder for contacting,
and check the groove for defects, piston top
for burn and cracks. If damaged, replace.
Clean the carbon deposit
Clean the deposit round the piston top and
cylinder neck before checking, first soak the
carbon deposit with kerosene, and then clean
with meter scraper or metal brush.
a. Piston skirt O.D.
Measure the piston skirt O.D. with outside
micrometer, if out of the service limit,
replace it.
Standard Service limit
67.989–67.971 mm 67.850 mm
b. Piston pin bore to piston clearance
Separately measure the piston pin bore
I.D. and O.D. with inside micrometer and
outside micrometer. Then calculate clear-
ance by measuring results.
Standard Service limit
0.004–0.016 mm 0.060 mm
c. Piston-cylinder clearance
Difference between cylinder maximum
diameter and piston skirt should be con-
sidered as piston-cylinder clearance.
Check with piston converting in the cylin-
der, and inserting feeler between piston
skirt bearing face and wall, then pull the
feeler out, if feeling resistance and
smoothly out, the thickness of the feeler
shall be considered as piston-cylinder
clearance.
Standard Service limit
0.011–0.049 mm 0.120 mm
4-3 Engine
10 mm