(See fig. 3-35.) Measure the fit at the point of
greatest piston size. Check the amount of force
required to pull out the feeler stock on the spring
gauge. If the feeler stock pulls out too easily, the
fit is too loose. If it pulls out too hard, the fit is
too tight. Check the manufacturers maintenance
manual for the correct amount of clearance.
PISTON PINS FITTING
If the piston-pin bushings are worn, they
should be reamed or honed oversize and oversize
pins installed. The pins should also be replaced
if they are worn, pitted, or otherwise defective.
Where the pin is of the type that floats or turns
in the piston-pin bushing, the fit is correct if the
pin will pass through with a light thumb pressure
when the piston and the pin are at room
temperature. Where the pin is of the type that does
NOT turn in the piston-pin bushing, the pin is
forced in place under pressure. Check the
manufacturers maintenance manual for the
correct pressure. If the pressure is too low, the
fit is too loose and will result in noise. Excessive
Figure 3-35.Checking piston fit in sleeve.
pressure indicates that the fit is too tight and may
fracture the piston-pin bosses.
PISTON RINGS FITTING
Piston rings must be fitted to their cylinder
and to their grooves on the piston. First, check
the gap or space between the ends of each ring.
To do so, push a ring down into the cylinder with
a piston, and measure the ring gap with a feeler
gauge (fig. 3-36). If the ring gap is too small, try
a slightly smaller ring, which will have a larger
gap. If the cylinder is worn tapered, the diameter
at the lower limit of ring travel (in the assembled
engine) will be smaller than the diameter at the
top. In this type of cylinder, the ring must be fitted
to the diameter at the lower limit of ring travel.
If the piston ring is fitted to the upper part of the
cylinder, the ring gap will NOT be great enough
as the ring is moved down to its lower limit of
travel. This means that ring ends will come
together and the ring will be broken or the cylinder
walls scuffed. In tapered cylinders, make sure that
the ring fits the cylinder at the point of minimum
diameter or at the lower limit of ring travel.
After the ring gap has been corrected, install
the ring in the proper ring groove on the piston
and roll it around in the ring groove to be sure
that the ring has a free fit around the entire
circumference of the piston. An excessively tight
fit means the ring groove is dirty and should be
cleaned. After the rings are installed in the ring
groove, test each ring for clearance by inserting
Figure 3-36.Measuring ring gap clearance in cylinder bore.