TM 5-3895-346-14
PISTON AND PISTON RINGS
ENGINE OVERHAUL
the tin-plating on the piston skirt. Clean the ring grooves with a suitable tool or a piece of an old compression
ring that has been ground to a bevel edge.
Clean the inside surfaces of the piston and the oil drain holes in the piston skirt. Exercise care to avoid
enlarging the holes while cleaning them.
Figure 3. Cleaning Piston
Inspection
If the tinplate on the piston and the original grooves in the piston rings are intact, it is an indication of very little
wear.
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 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.
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.
Keep the lubricating oil and engine coolant at the proper levels to prevent overheating of the engine.
Examine the piston for score marks, cracks, damaged ring-groove lands or indications of overheating. A piston
with light score marks which may be cleaned up may be reused (Fig. 4). Any piston that has been severely
scored or overheated must be replaced. Indications of overheating or burned spots on the piston may be the
result of an obstruction in the connecting rod oil passage.
Replace the piston if cracks are found across the internal struts.
Check the cylinder liner and block bore for excessive out-of-round, taper, or high spots which could cause
failure of the piston (refer to Specifications).
Inspection of the connecting rod and piston pin are covered in Connecting Rod.
Other factors that may contribute to piston failure include oil leakage into the air box, oil pullover from the air
cleaner, dribbling injectors, combustion blow-by, and low oil pressure (dilution of the lubricating oil).
Inspect and measure the piston pin bush- ings. The piston pin-to-bushing clearance with new parts is 0.0025 to
0.0034 inch. A maximum clearance of 0.010 inch is allowable with worn parts. The piston pin bushings in the
connecting rod are covered in Connecting Rod.
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