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 TM 55-1905-221-14-3
6-27.1. PISTON - MAINTENANCE INSTRUCTIONS (Continued).
e. Two bushings, with helical grooved oil passages, are pressed into the piston to provide a bearing for the
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 sprayed underside of the piston head and working
through the grooves in the piston pin bushings is prevented from reaching the cylinder walls.
f. Each piston is fitted with compression rings and oil control rings. Eight equally spaced drilled holes just below
each oil control ring groove permit excess oil, scraped from the cylinder walls, to return to the crankcase.
g. When an engine is hard to start, runs rough or lacks power, worn or sticking compression rings may be the
cause. Replacing the rings will aid in restoring engine operation to normal.
h. The compression rings may be inspected through the ports in the cylinder liners after the air box covers have
been removed. If the rings are free and are not worn to the extent that the plating or grooves are gone, compression
should be within operating specifications.
i. Excessively worn or scored pistons, rings or cylinder liners may be an indication of abnormal maintenance or
operating conditions which should be corrected to avoid a recurrence of the failure. The use of the correct types and
proper maintenance of the lubricating oil filters and air cleaners will reduce to a minimum the amount of abrasive dust
and foreign material introduced into the cylinders and will reduce the rate of wear.
j. Long periods of operation at idle speed and the use of improper lubricating oil or fuel must be avoided, otherwise
a heavy formation of carbon may result and cause the rings to stick.
k.
Keep the lubricating oil and engine coolant at the proper levels to prevent overheating of the engine.
6-419
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