THEORIES OF OPERATION
Valves have to seal well enough to stand pressures up to
500 pounds per square inch. Under full load, the
exhaust valve is exposed to temperatures high enough
to cause it to operate at a red heat. The temperature of
the valve under these conditions may be 12000 F. or
more. The intake valve is cooled by the incoming
mixture. The exhaust valve is subjected to high
temperature exhaust gases passing over it on their way
out of the cylinder. It is, therefore, very difficult to cool
the head of the exhaust valve. The cylinder head, the
cylinder, and the top of the piston are exposed to this
same heat, but these parts are cooled by air from the
flywheel fan and oil from the crankcase. Very special
steel is required in the exhaust valve to enable it to
withstand the corrosive action of the high temperature
Remember again that the Briggs & Stratton engine is a
single cylinder engine with two (2) valves as compared to
the 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 great
drop in horsepower or cause the engine to stop entirely.
In a multicylinder engine, one valve may fail and only
1/6th or 1/8th of the power is affected as the bad cylinder
may be motorized by the other good cylinders. Hence,
good valve condition is even more important in 1 cylinder
engines than it is in multicylinder engines.
Now if the valves and seats are so important, how do we
do a good valve job on a Briggs & Stratton engine?
The first requirement is good equipment. A valve
refacer and valve seat grinders are necessary. If you do
not have them, arrangement should be made with your
local Briggs & Stratton dealer.
After the valves are removed, they should be thoroughly
cleaned on a wire hrush wheel to remove all carbon
deposits. You will find sometimes it is easier to polish
carbon than to remove it, but it must come off. Also,
remove carbon from valve guides. When the valves are
clean, they should be visually inspected.
As mentioned above, when a valve becomes defective in
a multicylinder engine, the bad cylinder is motorized by
the other cylinders. This may cause serious damage to
the valve and seat. Briggs & Stratton engine valves are
seldom subjected to the extremes of abuse that
automotive valves are. While valves may burn to some
extent, it is very seldom that a valve seat or face is very
badly burned. Dished or necked valves are almost never