TM 5-4240-501-14&P
Valve seat burning is usually caused by an accumulation
of carbon or fuel lead either on the valve stem or on the
valve face, or from insufficient tappet clearance. These
deposits on the valve stem or on the face will hold the
valve open, allowing the hot flames of the burning fuel to
eat away the valve face and seat. A dished valve is one
that has a sunken head. This is caused by operating at
too high a temperature with too strong a spring, or the
head can be eroded away by highly leaded fuels. A
necked valve is one that has the stem directly beneath
the head eaten away badly by heat or where the stem
has been stretched.
Valve sticking is caused by fuel lead, gum or varnish
forming on the valve stem and in the valve guide. We
believe that most of the deposits formed are caused by
carbon, fuel lead, or gum. Since the amount of lead in
different fuels varies, the rate of deposit build-up
naturally will vary. When an exhaust valve no longer
closes properly, due to excess deposits, the hot gases
escaping from the combustion chamber heat up the
valve stem and guide excessively. This causes the oil on
the valve stem to oxidize into varnish which holds the
valve partially open and causes burning. Intake valve
sticking may be caused by the use of fuels having an
excessively high gum content. Fuels that are stored for
too long a period of time may contain high amounts of
gum. -
If burning occurs in a rather limited area on the valve
face, it indicates that something may have caused the
valve to tip. This could be due to a bent valve stem or a,
deposit on one side of the valve seat or stem.
Such a condition would leave an opening for the passage
of hot exhaust gases which could burn the valve so badly
that it could not be refaced. These valves must be
discarded.
THEORIES OF OPERATION
Compression
Figure 9.
The important parts of a valve are the head, the margin,
face, and stem. They make contact with the seat and
the valve guide in the cylinder. The margin is the edge of
the valve head. As a general rule, the valve should be
discarded when the margin becomes less than one-half
of the original thickness.
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