DETROIT DIESEL 53
1.6
PISTON AND PISTON RINGS
The trunk type malleable iron piston (Fig. 1) is plated
Inspect Piston Rings
with a protective coating of tin which permits close
fitting, reduces scuffing and prolongs piston life. The
When an engine is hard to start, runs uneven or lacks
top of the piston forms the combustion chamber bowl
power, the cause may be worn or sticking compression
and is designed to compress the air into close proximity
rings which must be replaced to restore uniform
to the fuel spray.
compression pressure in the cylinders.
The piston is cooled by a spray of lubricating oil directed
The compression rings may be inspected through the
at the underside of the piston head from a nozzle in the
ports in the cylinder liners after removing the air box
top of the connecting rod, by fresh air from the blower to
covers. If the rings are free and are not worn to the
the top of the piston and indirectly by the water jacket
extent that the plating or grooves have disappeared, the
around the cylinder.
compression should be within operating specifications.
Refer to Section 15.2 for the procedure for checking the
Each piston is balanced to close limits by machining at a
compression pressure.
balancing rib, provided on the inside at the bottom of the
piston skirt.
However, if excessive wear on any part of the piston
assembly is indicated by inspection through the cylinder
Two bushings, with helical grooved oil passages, are
liner ports, the piston and connecting rod must be
pressed into the piston to provide a bearing for the
removed in the following manner:
hardened, floating piston pin. After the piston pin has
been installed, the hole in the piston at each end of the
pin is sealed with a steel retainer. Thus, lubricating oil
returning from the underside of the piston head and
working through the grooves in the piston pin bushings
is prevented from reaching the cylinder walls.
The piston pin is subject to downward loading only since
the piston is at all times under pressures of compression
or expansion in the two-stroke cycle.
Consequently, free movement of the piston pin is
desirable to secure perfect alignment and uniform wear.
The piston pin is therefore assembled with a full floating
fit in both the connecting rod and the piston bushings.
Rotation of the pin and positive lubrication through the
helical bushing grooves reduce wear to a minimum.
Moreover, worn clearances can be comparatively large
and still be satisfactory.
Each piston is fitted with six piston rings.
Four
compression rings are placed above the piston pin and
two oil control rings are placed below the pin to scrape
off the excess lubricating oil thrown onto the cylinder
liner by the crankshaft and the lower end of the
connecting rod. Two piece oil control rings are used in
both the upper and lower positions on the piston, Equally
spaced holes are drilled just below each oil control ring
land to permit the excess oil that is scraped off the
cylinder walls to return to the crankcase.
Fig. 1. - Typical Piston Assembly
1972 General Motors Corp.
February, 1972
SEC. 1.6
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