| In conventional
four-cycle engines the valves are opened by mechanical action
of the cams on the camshaft. The cams do this while working
against the pressures exerted by the valve springs, the latter
closing the valves when the lifters ride the heels of the cams.
But, the valve springs must do a lot more than merely close
the valves.
While the pressure
of the springs must be sufficient to close the valves, it
also must be ample to assure that the valves and lifters will
follow the contour of their cams at high speed engine operation.
A weak valve spring naturally may allow
its valve to be bounced from its cam at high engine rpm. Thus,
with insufficient pressure to allow the valve and its lifter
to follow the cam, the valve flutters or bounces. Such a condition
does not allow the valve to close at the right time, throws
off the timing and in general upsets engine performance.
Car and engine manufacturers provide
tolerances on valve spring tensions primarily for checking
previously installed springs to see if such springs meet specifications
for further use. In some cases, for example, a 10% pressure
loss is allowed before replacement of a spring is recommended.
In any case, the mechanic is urged
to test every spring set-new or old-before installing the
springs. It enables him to match springs and thereby give
the valve train full opportunity to function smoothly.
Valve springs should be closely examined
for evidence of surface damage by corrosion, for distortion,
length and pressure. The spring testing devices sometimes
have built in indicators, others make use of a torque indicating
wrench. Testing should be done through the distance of normal
travel, never forcing the spring completely closed. The little
time necessary for this inspection and test is a small fraction
of the time spent in designing the spring to do the job on
an engine.
Below: Checking valve spring drop and
squareness. This should preferably be done on a surface plate.
The valves in an eight cylinder engine
may open and close some 840,000 times at 40 mph. So, the function
of the valve springs in controlling the valve operation is
really something to contemplate.
Every mechanic has at some time experienced
the poor performance and damage to valve and seat caused by
springs having either too great, or too little tension. Weak
springs allow the valve to bounce and flutter; too-strong
springs cause stretched valves, both causing parts damage
and poor performance.
Valve seat refacing in the block or
head permits the valve to sink deeper into the block or head.
Obviously, this reduces the amount to which the valve springs
will be compressed, thus reducing their tension.
A valve spring tester (various types
are available) should always be used on a valve job.
Realizing that valve spring installed
height and pressure is critical in modern engines, car manufacturers
prescribe procedures for checking and installing spacers to
compensate for loss of tension from change in assembled spring
height due to grind-ins and wear. |
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The spacers
known as "valve spring inserts" require no gauges
or measuring. They are made in three types-Type "A"
(.060" thick) for use with springs in service and-Type
"B" (.030" thick) for use with new springs. (This
makes an economical installation from a labor and profit standpoint.)
-Type "C" (.015" thick) for use when springs,
valves, and seats are replaced, or for spring balancing when
"blueprinting" engine.
In
practice it has been found that valve springs which have been
in service often encounter a tension drop, said to normally
take place during the break-in period of the engine. This
tension drop, also called "torsion relax" or "metal
set" is due to heat and flexing generated at the time
of break-in and subsequent operation.
The use of the type "A" insert
(.060") substantially corrects the installed height as
well as the tension drop. This brings the installed spring
with its minimum and maximum specified pressure limits.
The type "B" insert (.030")
with a new valve spring also results in an installed assembly
within specified limits. Thus, the desired results are accomplished
without the use of gauges or measuring tools.
The type "C" insert (.015")
allows precise balancing of valve spring tension where the
utmost perfection in engine balance is desired.
Valve spring inserts should always
be installed with the serrated side next to the cylinder block
or head. The serrated louvers retard heat transfer to the
spring, thereby overcoming torsion relax. Overheated springs
collapse and usually cause valve bounce.
All authorities on valves and springs
agree that valve springs should always be checked for free
height by standing along-side each other. Springs with a drop
of over 1/16 in. should be replaced, as well as those over
3/16 in. out of square.
Excessive use of spacers or inserts
is not recommended, as too much stress is placed on the springs
and valve parts. Usually one insert is sufficient to make
up for the metal that has been removed from the head or block.
Above: To maintain correct spring height
and pressure. loss of metal at E is replaced at H by an insert.
INSPECTION
Grinding Valve Stems beyond recommended
limits creates fast stem wear and change in Valve Lash. It
is customary to remove only enough metal to correct wear irregularities
within 0.010 inch.
The critical inspection points and
tolerances of the valves are illustrated here.
Critical Valve Tolerances
Assembled Valve Spring should not turn
under finger grip pressure with valves in closed position.
Low spring pressure allows Hydraulic Lifter pump up holding
valves open.
Valve Spring Looseness |
