Cylinder And Piston - Honda CB350 Repair Manual

60 ENGINE AND TRANSMISSION
are stripped or appear to be damaged the
cam should be replaced.
8. Check the condition of the cam
sprocket. Replace the sprocket if there is
any wear or damage to the teeth. Check
that the cam bolts' heads and threads are
in good shape.
NOTE: On 350 models it is recom—
mended that the cambolts be replaced
with new ones upon assembly.
9. Check the operation ofthe ignition
timing advance assembly. Assuming that
the advance assemblyis in place, turn the
breaker cam so that the advance weights
are fully open. Release the breaker cam
and the weights should return to the fully
closed position. If they do not, suspect
weak springs or sticking weights.
Cylinder and Piston
Under ideal conditions an internal
combustion engine would have a per—
fectly cylindrical bore and "perfect cir—
cle" pistons with a diameter of a few
thousandths of an inch smaller than the
bore to allowfor expansion. However, in
actual practice, the bore is never per—
fectly circular due to machining toler—
ances.
To further complicate matters, the pis—
tons do not expand symmetrically when
they are heated. The piston crown is sub—
ject to the greatest heat and, therefore,
expands to a greater degree than the skirt.
Further, the area of the piston in the vi—
cinity of the wrist pin holes has thicker
walls to support the pin and will show a
greater expansion because of this than
the front and rear of the piston.
To compensate for this, pistons are ta—
pered inward from bottom to top and are
also "cam turned," that is, made slightly
ovoid in diameter.
When a piston expands during opera—
tion, it will assume a more cylindrical
shape.
As might be expected, the cylinder
bore is subject to uneven wear. After
many miles, it may assume a tapered
shape (tapering in from top to bottom)
and may also become elliptical. The wear
on the bore is usually greatest just below
the top of the cylinder at the highest
point the top compression ring reaches.
1. Make a visual inspection ofthe cyl—
inder bore, noting any imperfections.
After boring and honing, the bores should have a
smooth, even finish
The cylinder walls should be uniformly
smooth.
2. With an inside micrometer, mea—
sure the diameter of the bore at the top,
middle, and bottom parallel to the wrist
pin. Then turn the micrometer 90° and
measure the bore at the top, middle, and
bottom perpendicular to the wrist pin
(total of six measurements). If the cylin—
——— 1 & 2 ———»
3 & 4 ———>
e— 5 & 6 ———»
Bore measurement points