Yamaha BR250F Service Manual page 19

e. Wash the piston in solvent, and dry it
with compressed air.
f. Measure the outside diameter of the
piston at the piston skirt.
The measurement shou Id be made at a
point
10
mm
(0.39
in) above the bottom
edge of the piston. Place the micrometer
at a right angle to the piston pin.
The maximum piston diameter subtract-
ed from the minimum cylinder diameter
gives the piston clearance. If the clear-
ance is beyond tolerance, hone the
cylinder to tolerance or bore it to the
next oversize and install an oversized
piston.
Piston clearance:
Minimum 0.045 mm (0.0018 in)
Maximum
0.050
mm
(0.0020
in)
Piston oversize:
70.25 mm (2.766 in)
70.50
mm
(2.776
in)
1. 10 mm (0.39 in)
2-8
3. Piston rings
a. Check the rings for scoring.
If
any severe
scratches are noticed, replace the rings as
a set.
b. Measure the ring end gap in the un-
installed rings. If it is beyond tolerance,
replace the rings as a set.
Ring end gap, free:
Approx. 6.5 mm (0.26 in)
c. Push the ring into the cylinder, and
measure the ring end gap with a feeler
gauge. Push the ring into the cylinder
with the piston crown so the ring will be
at a right angle to the cylinder bore. If
the end gap is not within specification,
replace the rings as a set.
Ring end gap, installed:
Minimum
0.20
mm
(0.0079
in)
Maximum
0040
mm
(0.0157
in)