TM 5-3895-346-14
PISTON AND PISTON RINGS
ENGINE OVERHAUL
PISTON AND PISTON RINGS
The trunk-type malleable iron piston (Fig. 1) is plated with a protective coating of tin which permits close fitting,
reduces scuffing, and prolongs piston life. The top of the piston forms the combustion chamber bowl and is
designed to compress the air into close proximity to the fuel spray.
Each piston is internally braced with fin-shaped ribs and circular struts, scientifically designed to draw heat
rapidly from the piston crown and transfer it to the lubricating oil spray to ensure better control of piston ring
temperature.
The piston is cooled by a spray of lubricating oil directed at the under- side of the piston head from a nozzle in
the top of the connecting rod, by fresh air from the blower to the top of the piston, and indirectly by the water
jacket around the cylinder.
Figure 1. Typical Piston Assembly
Each piston is balanced to close limits by machining a balancing rib, provided on the inside at the bottom of the
piston skirt.
Two bushings, with helical grooved oil passages, are pressed into the piston to provide a bearing for the
hardened, floating piston pin (1.375 inches diameter). 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.
The current piston pin retainer for the 1.375-inch diameter piston pin has a greater outside diameter (1.6110
inches) and is color-coded black for identification. The former and new retainers are interchangeable in an
engine.
Each piston is fitted with compression rings and oil control rings (Fig. 1). Equally spaced drilled holes just below
each oil control ring groove permits excess oil, scraped from the cylinder walls, to return to the crankcase.
The piston bushings are vapor blasted and the piston pins are polished and drilled for positive piston pin
bushing lubrication.
Inspect Piston Rings
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.
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