TM 5-4240-501-14&PValve seat burning is usually caused by an accumulationof carbon or fuel lead either on the valve stem or on thevalve face, or from insufficient tappet clearance. Thesedeposits on the valve stem or on the face will hold thevalve open, allowing the hot flames of the burning fuel toeat away the valve face and seat. A dished valve is onethat has a sunken head. This is caused by operating attoo high a temperature with too strong a spring, or thehead can be eroded away by highly leaded fuels. Anecked valve is one that has the stem directly beneaththe head eaten away badly by heat or where the stemhas been stretched.Valve sticking is caused by fuel lead, gum or varnishforming on the valve stem and in the valve guide. Webelieve that most of the deposits formed are caused bycarbon, fuel lead, or gum. Since the amount of lead indifferent fuels varies, the rate of deposit build-upnaturally will vary. When an exhaust valve no longercloses properly, due to excess deposits, the hot gasesescaping from the combustion chamber heat up thevalve stem and guide excessively. This causes the oil onthe valve stem to oxidize into varnish which holds thevalve partially open and causes burning. Intake valvesticking may be caused by the use of fuels having anexcessively high gum content. Fuels that are stored fortoo long a period of time may contain high amounts ofgum. -If burning occurs in a rather limited area on the valveface, it indicates that something may have caused thevalve 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 passageof hot exhaust gases which could burn the valve so badlythat it could not be refaced. These valves must bediscarded.THEORIES OF OPERATIONCompressionFigure 9.The important parts of a valve are the head, the margin,face, and stem. They make contact with the seat andthe valve guide in the cylinder. The margin is the edge ofthe valve head. As a general rule, the valve should bediscarded when the margin becomes less than one-halfof the original thickness.514
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