Measure the outside
up from the bottom of the piston at a right angle to
the direction of the piston pin.
If the measurement
is under the service limit, replace the piston.
Abnormal wear such as a marked diagonal
pattern across the piston skirt may mean a bent con-
necting rod or cran
Piston Diameter Measurement
Table H28 applies only to a cyli nder that has not
been bored to oversize, and Table
applies only to
the standard size piston. For the service limit of a bored
cylinder and oversize piston, see the "Boring, honing"
Whenever a piston or cylinder block has been
replaced with a new one, the motorcycle must be broken
in the same as with a new
The piston-to-cylinder clearance is measured when
ever a piston or the cylinder block is replaced with a
new one, or whenever a cylinder is rebored and an over-
The standard piston-ta-cylinder
clearance must be adhered to whenever the cylinder
block is replaced or a cylinder rebored
If only a piston
is replaced, the clearance may exceed the standard slight-
But it must not be less than the minimum, in order
to avoid piston
The most accurate way to find the piston clearance
is by making separate piston and cylinder
measurements and then computing the difference be-
tween the two values. Measure the piston diameter as
just described, and measure the cylinder diameter at the
bottom of the cylinder.
When boring and honing a
note the follow-
Before boring a cylinder, first measure the exact
diameter of the oversize piston, and then, in accord-
ance with the standard clearance given in Table H30,
determine the diameter of the
Never separate the liners from the cylinder when
boring and honing the liners, because the 'top surface
of cylinder and liners is machined at the factory as
an assembly to get the proper surface.
3. Cylinder inside diameter must not vary more than
at any point.
4. 0.5 mm oversize pistons are
pistons require oversize rings.
5. Be wary of measurements taken immediately after
affects cylinder diameter.
6. In the
a rebored cylinder and oversize piston,
the service limit for the cylinder is the diameter that
the cylinder was bored to plus 0.1 mm and the
service limit for the piston is the oversize piston
original diameter minus 0.15 mm. If the exact figure
for the rebored
is unknown, it can
ly determined by measuring the diameter at the base
of the cylinder.
Remove the cylin·der block and pistons to check the
there is only slight damage, the piston may
be smoothed with #400 emery cloth, and any aluminum
removed from the cylinder with either #400
emery cloth or light
in most cases,
the cylinder will have to be bored to oversize and honed,
and an oversize piston installed.
Built-up carbon on the piston head reduces the cool-
ing capability of the piston and raises compression
leading to overheating which could possibly even melt
the top of the piston. To decarbonize the piston head,
remove the piston
77), scrape off the carbon, and
then lightly polish the piston
fine emery cloth
accumulated in the piston ring grooves can
cause the rings to stick. Remove the rings, and clean
out any carbon deposits using an end of a
some other suitable tool.