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 TM 55-1905-219-14-8
4-7.10A. ENGINE - MAINTENANCE INSTRUCTIONS (Continued).
ENGINE OVERHAUL (Cont)
b. Piston Burn-thru.
If a hole is burned right through the piston, look for an improperly adjusted carburetor or incorrect timing. If
these items check out OK, check use of wrong oil or improperly mixed fuel. The wrong type of spark plug for
conditions involved may also be the cause of abnormally high combustion temperatures which cause burn-thru.
Use Champion RN-3 spark plugs.
c.
Foreign Material.
When inspecting spark plugs, clean out around spark plug
openings before removing plugs. Otherwise, damage can
result from foreign objects dropping into the engine. If the
damage is on the piston crown, something probably fell into the
opening when the plug was removed. Damage on the sides of
piston may indicate that an object was drawn into the intake.
d
Loose Needle, Retainers, Pins.
Piston and cylinder damage caused by loose needle bearings, pin retainers, or piston pins is usually
quite easy to diagnose. A needle bearing is hard enough to cut right through piston rings without leaving
jagged edges - a loose retainer will not cut but will break the ring. If retainer falls out or is not installed in the
first place, the pin may rub a regular groove in the cylinder wall. Often damage such as this goes by unnoticed
until power becomes extremely poor due to lack of compression.
e. Ring Breakage.
If a ring breaks, the engine may continue running; however, it will probably back-fire badly and it cannot
be restarted after it is shut down. Ring breakage occurs from overheating due to lack of proper lubrication. A
poor quality or wrong type of oil may have been used or the oil may have been poorly mixed or mixed in
incorrect proportion with the gasoline. A ring that has been subjected to overheating often becomes soft and is
easily bent.
f.
Rings Sticking.
If there has been repeated occurrence of spark plug fouling, the rings might be stuck in the grooves due to
carbon fouling. This usually occurs from an "over-rich" fuel mixture. If the build-up is varnish rather than
carbon, this probably indicates use of an unauthorized lubricating oil.
4-389
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