TM 5-4240-501-14&PTHEORIES OF OPERATIONCompressionFigure 6.Valves have to seal well enough to stand pressures up to500 pounds per square inch. Under full load, theexhaust valve is exposed to temperatures high enoughto cause it to operate at a red heat. The temperature ofthe valve under these conditions may be 12000 F. ormore. The intake valve is cooled by the incomingmixture. The exhaust valve is subjected to hightemperature exhaust gases passing over it on their wayout of the cylinder. It is, therefore, very difficult to coolthe head of the exhaust valve. The cylinder head, thecylinder, and the top of the piston are exposed to thissame heat, but these parts are cooled by air from theflywheel fan and oil from the crankcase. Very specialsteel is required in the exhaust valve to enable it towithstand the corrosive action of the high temperatureexhaust gases.Figure 7.Remember again that the Briggs & Stratton engine is asingle cylinder engine with two (2) valves as compared tothe customary 12 or 16 valves in an automotive engine.The fewer the valves, the more important they become.In a 1 cylinder engine one bad valve can cause a greatdrop in horsepower or cause the engine to stop entirely.In a multicylinder engine, one valve may fail and only1/6th or 1/8th of the power is affected as the bad cylindermay be motorized by the other good cylinders. Hence,good valve condition is even more important in 1 cylinderengines than it is in multicylinder engines.Now if the valves and seats are so important, how do wedo a good valve job on a Briggs & Stratton engine?The first requirement is good equipment. A valverefacer and valve seat grinders are necessary. If you donot have them, arrangement should be made with yourlocal Briggs & Stratton dealer.After the valves are removed, they should be thoroughlycleaned on a wire hrush wheel to remove all carbondeposits. You will find sometimes it is easier to polishcarbon than to remove it, but it must come off. Also,remove carbon from valve guides. When the valves areclean, they should be visually inspected.Figure 8.As mentioned above, when a valve becomes defective ina multicylinder engine, the bad cylinder is motorized bythe other cylinders. This may cause serious damage tothe valve and seat. Briggs & Stratton engine valves areseldom subjected to the extremes of abuse thatautomotive valves are. While valves may burn to someextent, it is very seldom that a valve seat or face is verybadly burned. Dished or necked valves are almost neverfound.414
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